atomic mass is the sum of the masses of all protons, neutrons and electrons that make up an atom or molecule. Compared to protons and neutrons, the mass of electrons is very small, so it is not taken into account in the calculations. Although it is incorrect from a formal point of view, this term is often used to refer to the average atomic mass of all isotopes of an element. In fact, this is the relative atomic mass, also called atomic weight element. Atomic weight is the average of the atomic masses of all naturally occurring isotopes of an element. Chemists must distinguish between these two types of atomic mass when doing their job - an incorrect value for atomic mass can, for example, lead to an incorrect result for the yield of a reaction product.

Steps

Finding the atomic mass according to the periodic table of elements

Learn how atomic mass is written. Atomic mass, that is, the mass of a given atom or molecule, can be expressed in standard SI units - grams, kilograms, and so on. However, due to the fact that atomic masses expressed in these units are extremely small, they are often written in unified atomic mass units, or a.u.m. for short. are atomic mass units. One atomic mass unit is equal to 1/12 the mass of the standard carbon-12 isotope.

• The atomic mass unit characterizes the mass one mole of the given element in grams. This value is very useful in practical calculations, since it can be used to easily convert the mass of a given number of atoms or molecules of a given substance into moles, and vice versa.

1. Find atomic mass V periodic table Mendeleev. Most standard periodic tables contain the atomic masses (atomic weights) of each element. As a rule, they are given as a number at the bottom of the cell with the element, under the letters denoting the chemical element. This is usually not an integer, but a decimal.

   Remember that the periodic table shows the average atomic masses of the elements. As noted earlier, the relative atomic masses given for each element in the periodic table are the averages of the masses of all the isotopes of an atom. This average value is valuable for many practical purposes: for example, it is used in calculating the molar mass of molecules consisting of several atoms. However, when you are dealing with individual atoms, this value is usually not enough.

   • Since the average atomic mass is an average of several isotopes, the value given in the periodic table is not accurate the value of the atomic mass of any single atom.
   • The atomic masses of individual atoms must be calculated taking into account the exact number of protons and neutrons in a single atom.

   Calculation of the atomic mass of an individual atom

   1. Find the atomic number of a given element or its isotope. The atomic number is the number of protons in an element's atoms and never changes. For example, all hydrogen atoms, and only they have one proton. Sodium has an atomic number of 11 because it has eleven protons, while oxygen has an atomic number of eight because it has eight protons. You can find the atomic number of any element in the periodic table of Mendeleev - in almost all of its standard versions, this number is indicated above the letter designation of the chemical element. The atomic number is always a positive integer.

      • Suppose we are interested in a carbon atom. There are always six protons in carbon atoms, so we know that its atomic number is 6. In addition, we see that in the periodic table, at the top of the cell with carbon (C) is the number "6", indicating that the atomic carbon number is six.
      • Note that the atomic number of an element is not uniquely related to its relative atomic mass in the periodic table. Although, especially for the elements at the top of the table, the atomic mass of an element may appear to be twice its atomic number, it is never calculated by multiplying the atomic number by two.

   2. Find the number of neutrons in the nucleus. The number of neutrons can be different for different atoms of the same element. When two atoms of the same element with the same number of protons have different amount neutrons, they are different isotopes of this element. Unlike the number of protons, which never changes, the number of neutrons in the atoms of a particular element can often change, so the average atomic mass of an element is written as a decimal fraction between two adjacent whole numbers.

      Add up the number of protons and neutrons. This will be the atomic mass of this atom. Ignore the number of electrons that surround the nucleus - their total mass is extremely small, so they have little to no effect on your calculations.

   Calculating the relative atomic mass (atomic weight) of an element

   1. Determine which isotopes are in the sample. Chemists often determine the ratio of isotopes in a particular sample using a special instrument called a mass spectrometer. However, during training, this data will be provided to you in the conditions of tasks, control, and so on in the form of values ​​taken from the scientific literature.

      • In our case, let's say that we are dealing with two isotopes: carbon-12 and carbon-13.

   2. Determine the relative abundance of each isotope in the sample. For each element, different isotopes occur in different ratios. These ratios are almost always expressed as a percentage. Some isotopes are very common, while others are very rare—sometimes so rare that they are difficult to detect. These values ​​can be determined using mass spectrometry or found in a reference book.

      • Assume that the concentration of carbon-12 is 99% and carbon-13 is 1%. Other isotopes of carbon really exist, but in quantities so small that in this case they can be neglected.

   3. Multiply the atomic mass of each isotope by its concentration in the sample. Multiply the atomic mass of each isotope by its percentage (expressed as a decimal). To convert percentages to decimal, just divide them by 100. The resulting concentrations should always add up to 1.

      • Our sample contains carbon-12 and carbon-13. If carbon-12 is 99% of the sample and carbon-13 is 1%, then multiply 12 (atomic mass of carbon-12) by 0.99 and 13 (atomic mass of carbon-13) by 0.01.
      • Reference books give percentages based on the known amounts of all the isotopes of an element. Most chemistry textbooks include this information in a table at the end of the book. For the sample under study, the relative concentrations of isotopes can also be determined using a mass spectrometer.

   4. Add up the results. Sum the multiplication results you got in the previous step. As a result of this operation, you will find the relative atomic mass of your element - the average value of the atomic masses of the isotopes of the element in question. When an element is considered as a whole, and not a specific isotope of a given element, it is this value that is used.

      • In our example, 12 x 0.99 = 11.88 for carbon-12, and 13 x 0.01 = 0.13 for carbon-13. The relative atomic mass in our case is 11.88 + 0.13 = 12,01 .
   • Some isotopes are less stable than others: they decay into atoms of elements with fewer protons and neutrons in the nucleus, releasing particles that make up the atomic nucleus. Such isotopes are called radioactive.

Isogony. The nucleus of the hydrogen atom - the proton (p) - is the simplest nucleus. Its positive charge is equal in absolute value to the electron charge. The proton mass is 1.6726-10'2 kg. The proton as a particle that is part of atomic nuclei was discovered by Rutherford in 1919.

For experimental definition masses of atomic nuclei were used and are being used mass spectrometers. The principle of mass spectrometry, first proposed by Thomson (1907), is to use the focusing properties of electric and magnetic fields with respect to charged particle beams. The first mass spectrometers with sufficiently high resolution were constructed in 1919 by F.U. Aston and A. Dempstrom. The principle of operation of the mass spectrometer is shown in Fig. 1.3.

Since atoms and molecules are electrically neutral, they must first be ionized. Ions are created in an ion source by bombarding vapors of the substance under study with fast electrons and then, after acceleration in an electric field (potential difference v) exit into the vacuum chamber, falling into the region of homogeneous magnetic field B. Under its action, the ions begin to move along a circle, the radius of which G can be found from the equality of the Lorentz force and the centrifugal force:

Where M- ion mass. The ion velocity v is determined by the relation

Rice. 1.3.

Accelerating potential difference Have or magnetic field strength IN can be chosen so that ions with the same masses fall into the same place on a photographic plate or other position-sensitive detector. Then, by finding the maximum of the mass-spring-stroke signal and using formula (1.7), we can also determine the mass of the ion M. 1

Excluding speed v from (1.5) and (1.6), we find that

The development of mass spectrometry techniques made it possible to confirm the assumption made back in 1910 by Frederick Soddy that fractional (in units of the mass of a hydrogen atom) atomic masses chemical elements explained by the existence isotopes- atoms with the same nuclear charge, but different masses. Thanks to Aston's pioneering research, it was found that most elements are indeed composed of a mixture of two or more naturally occurring isotopes. The exceptions are relatively few elements (F, Na, Al, P, Au, etc.), called monoisotopic. The number of natural isotopes in one element can reach 10 (Sn). In addition, as it turned out later, all elements without exception have isotopes that have the property of radioactivity. Most radioactive isotopes are not found in nature, they can only be obtained artificially. Elements with atomic numbers 43 (Tc), 61 (Pm), 84 (Po) and above have only radioactive isotopes.

The international atomic mass unit (a.m.u.) accepted today in physics and chemistry is 1/12 of the mass of the carbon isotope most common in nature: 1 a.m.u. = 1.66053873* 10" kg. It is close to the atomic mass of hydrogen, although not equal to it. The mass of an electron is approximately 1/1800 a.m.u. In modern mass spectrometers, the relative error in measuring the mass

AMfM= 10 -10 , which makes it possible to measure mass differences at the level of 10 -10 a.m.u.

The atomic masses of isotopes, expressed in amu, are almost exactly integer. Thus, each atomic nucleus can be assigned its mass number A(whole) e.g. H-1, H-2, H-3, C-12, 0-16, Cl-35, C1-37, etc. The latter circumstance revived new basis interest in the hypothesis of W. Prout (1816), according to which all elements are built from hydrogen.

The masses of atomic nuclei are of particular interest for identifying new nuclei, understanding their structure, predicting decay characteristics: lifetime, possible decay channels, etc.
For the first time, the description of the masses of atomic nuclei was given by Weizsäcker on the basis of the drop model. The Weizsäcker formula makes it possible to calculate the mass of the atomic nucleus M(A,Z) and the binding energy of the nucleus if the mass number A and the number of protons Z in the nucleus are known.
The Weizsacker formula for the masses of nuclei has the following form:

where m p = 938.28 MeV/c 2 , m n = 939.57 MeV/c 2 , a 1 = 15.75 MeV, a 2 = 17.8 MeV, a 3 = 0.71 MeV, a 4 = 23.7 MeV, a 5 = 34 MeV, = (+ 1, 0, -1), respectively, for odd-odd nuclei, nuclei with odd A, even-even nuclei.
The first two terms of the formula are the sums of the masses of free protons and neutrons. The remaining terms describe the binding energy of the nucleus:

• a 1 A takes into account the approximate constancy of the specific binding energy of the nucleus, i.e. reflects the saturation property nuclear forces;
• a 2 A 2/3 describes the surface energy and takes into account the fact that surface nucleons in the nucleus are weaker bound;
• a 3 Z 2 /A 1/3 describes the decrease in the nuclear binding energy due to the Coulomb interaction of protons;
• a 4 (A - 2Z) 2 /A takes into account the property of the charge independence of nuclear forces and the action of the Pauli principle;
• a 5 A -3/4 takes into account mating effects.

The parameters a 1 - a 5 included in the Weizsäcker formula are chosen in such a way as to optimally describe the masses of nuclei near the β-stability region.
However, it was clear from the very beginning that the Weizsacker formula did not take into account some specific details of the structure of atomic nuclei.
Thus, the Weizsäcker formula assumes a uniform distribution of nucleons in the phase space, i.e. essentially neglects the shell structure of the atomic nucleus. In fact, the shell structure leads to inhomogeneity in the distribution of nucleons in the nucleus. The resulting anisotropy of the mean field in the nucleus also leads to deformation of the nuclei in the ground state.

The accuracy with which the Weizsacker formula describes the masses of atomic nuclei can be estimated from Fig. 6.1, which shows the difference between the experimentally measured masses of atomic nuclei and calculations based on the Weizsäcker formula. The deviation reaches 9 MeV, which is about 1% of the total binding energy of the nucleus. At the same time, it is clearly seen that these deviations are systematic in nature, which is due to the shell structure of atomic nuclei.
The deviation of the nuclear binding energy from the smooth curve predicted by the liquid drop model was the first direct indication of the shell structure of the nucleus. The difference in binding energies between even and odd nuclei indicates the presence of pairing forces in atomic nuclei. The deviation from the "smooth" behavior of the separation energies of two nucleons in nuclei between filled shells is an indication of the deformation of atomic nuclei in the ground state.
Data on the masses of atomic nuclei underlie the verification of various models of atomic nuclei, therefore great importance has the accuracy of knowing the masses of the nuclei. The masses of atomic nuclei are calculated using various phenomenological or semi-empirical models using various approximations of macroscopic and microscopic theories. The currently existing mass formulas describe quite well the masses (binding energies) of nuclei near the -stability valley. (The accuracy of the binding energy estimate is ~100 keV). However, for nuclei far from the stability valley, the uncertainty in predicting the binding energy increases to several MeV. (Fig. 6.2). In Fig.6.2 you can find references to works in which various mass formulas are given and analyzed.

Comparison of the predictions of various models with measured nuclear masses indicates that preference should be given to models based on a microscopic description that takes into account the shell structure of nuclei. It should also be borne in mind that the accuracy of predicting the masses of nuclei in phenomenological models is often determined by the number of parameters used in them. Experimental data on the masses of atomic nuclei are given in the review. In addition, their constantly updated values ​​can be found in the reference materials of the international database system.
Behind last years Various methods have been developed for the experimental determination of the masses of atomic nuclei with a short lifetime.

Basic methods for determining the masses of atomic nuclei

We list, without going into details, the main methods for determining the masses of atomic nuclei.

• Measurement of the β-decay energy Q b is a fairly common method for determining the masses of nuclei far from the β-stability limit. To determine the unknown mass experiencing β-decay of the nucleus A

,

the ratio is used

M A \u003d M B + m e + Q b / c 2.

Therefore, knowing the mass of the final nucleus B, one can obtain the mass of the initial nucleus A. Beta decay often occurs in the excited state of the final nucleus, which must be taken into account.

This relation is written for α-decays from the ground state of the initial nucleus to the ground state of the final nucleus. The excitation energies can be easily taken into account. The accuracy with which the masses of atomic nuclei are determined from the decay energy is ~ 100 keV. This method is widely used to determine the masses of superheavy nuclei and their identification.

1. Measurement of the masses of atomic nuclei by the time-of-flight method

Determining the mass of the nucleus (A ~ 100) with an accuracy of ~ 100 keV is equivalent to the relative accuracy of mass measurement ΔM/M ~10 -6 . To achieve this accuracy, magnetic analysis is used in conjunction with the measurement of the time of flight. This technique is used in the spectrometer SPEG - GANIL (Fig. 6.3) and TOFI - Los Alamos. Magnetic rigidity Bρ, particle mass m, particle velocity v, and charge q are related by

Thus, knowing the magnetic rigidity of the spectrometer B, one can determine m/q for particles having the same velocity. This method makes it possible to determine the masses of nuclei with an accuracy of ~ 10 -4 . The accuracy of measurements of the masses of nuclei can be improved if the time of flight is measured simultaneously. In this case, the ion mass is determined from the relation

where L is the flight base, TOF is the time of flight. The span bases range from a few meters to 10 3 meters and make it possible to increase the accuracy of measuring the masses of nuclei to 10 -6 .
A significant increase in the accuracy of determining the masses of atomic nuclei is also facilitated by the fact that the masses of different nuclei are measured simultaneously, in one experiment, and the exact values ​​of the masses of individual nuclei can be used as reference points. The method does not allow separating the ground and isomeric states of atomic nuclei. A setup with a flight path of ~3.3 km is being created at GANIL, which will improve the accuracy of measuring the masses of nuclei to several units by 10 -7 .

1. Direct definition masses of nuclei by the method of measuring the cyclotron frequency

For a particle rotating in a constant magnetic field B, the frequency of rotation is related to its mass and charge by the relation

Despite the fact that methods 2 and 3 are based on the same ratio, the accuracy in method 3 of measuring the cyclotron frequency is higher (~ 10 -7), because it is equivalent to using a longer span base.

2. Measurement of the masses of atomic nuclei in a storage ring

   This method is used on the ESR storage ring at GSI (Darmstadt, Germany). The method uses a Schottky detector. It is applicable to determine the masses of nuclei with a lifetime > 1 min. The method of measuring the cyclotron frequency of ions in a storage ring is used in combination with on-the-fly ion pre-separation. On the FRS-ESR facility at GSI (Fig. 6.4), precision mass measurements were made a large number nuclei in a wide range of mass numbers.

   209 Bi nuclei accelerated to an energy of 930 MeV/nucleon were focused on a beryllium target 8 g/cm 2 thick located at the FRS entrance. As a result of 209 Bi fragmentation, a large number of secondary particles are formed in the range from 209 Bi to 1 H. The reaction products are separated on the fly according to their magnetic hardness. The target thickness is chosen so as to expand the range of nuclei simultaneously captured by the magnetic system. The expansion of the range of nuclei occurs due to the fact that particles with different charges are decelerated in a different way in a beryllium target. The FRS separator fragment is tuned for the passage of particles with a magnetic hardness of ~350 MeV/nucleon. Through the system at the chosen range of the charge of the detected nuclei (52 < Z < 83) can simultaneously pass fully ionized atoms (bare ions), hydrogen-like (hydrogen-like) ions having one electron or helium-like ions (helium-like) having two electrons. Since the velocity of particles during the passage of the FRS practically does not change, the selection of particles with the same magnetic rigidity selects particles with the M/Z value with an accuracy of ~ 2%. Therefore, the rotation frequency of each ion in the ESR storage ring is determined by the M/Z ratio. This underlies the precision method for measuring the masses of atomic nuclei. The ion revolution frequency is measured using the Schottky method. The use of the method of ion cooling in a storage ring additionally increases the accuracy of mass determination by an order of magnitude. On fig. 6.5 shows the plot of the masses of atomic nuclei separated by this method in the GSI. It should be borne in mind that nuclei with a half-life of more than 30 seconds can be identified using the described method, which is determined by the beam cooling time and the analysis time.

   On fig. 6.6 shows the results of determining the mass of the 171 Ta isotope in various charge states. Various reference isotopes were used in the analysis. The measured values ​​are compared with the table data (Wapstra).

3. Measuring Nucleus Masses Using the Penning Trap

   New experimental possibilities for precision measurements of the masses of atomic nuclei are opening up in a combination of the ISOL methods and ion traps. For ions that have very little kinetic energy and hence a small radius of rotation in a strong magnetic field, Penning traps are used. This method is based on the precise measurement of the particle rotation frequency

   ω = B(q/m),

   trapped in a strong magnetic field. The mass measurement accuracy for light ions can reach ~ 10 -9 . On fig. Figure 6.7 shows the ISOLTRAP spectrometer mounted on the ISOL - CERN separator.
   The main elements of this setup are the ion beam preparation sections and two Penning traps. The first Penning trap is a cylinder placed in a magnetic field of ~4 T. The ions in the first trap are additionally cooled due to collisions with the buffer gas. On fig. Figure 6.7 shows the mass distribution of ions with A = 138 in the first Penning trap as a function of rotational speed. After cooling and purification, the ion cloud from the first trap is injected into the second one. Here, the mass of the ion is measured by the resonant frequency of rotation. The resolution achievable in this method for short-lived heavy isotopes is the highest and amounts to ~ 10 -7 .

   Rice. 6.7 ISOLTRAP spectrometer

Many years ago, people wondered what all substances are made of. The first who tried to answer it was the ancient Greek scientist Democritus, who believed that all substances are composed of molecules. We now know that molecules are built from atoms. Atoms are made up of even smaller particles. At the center of an atom is the nucleus, which contains protons and neutrons. The smallest particles - electrons - move in orbits around the nucleus. Their mass is negligible compared to the mass of the nucleus. But how to find the mass of the nucleus, only calculations and knowledge of chemistry will help. To do this, you need to determine the number of protons and neutrons in the nucleus. View the tabular values ​​of the masses of one proton and one neutron and find their total mass. This will be the mass of the nucleus.

Often you can come across such a question, how to find the mass, knowing the speed. According to the classical laws of mechanics, the mass does not depend on the speed of the body. After all, if a car, moving away, begins to pick up its speed, this does not mean at all that its mass will increase. However, at the beginning of the twentieth century, Einstein presented a theory according to which this dependence exists. This effect is called the relativistic increase in body mass. And it manifests itself when the speeds of bodies approach the speed of light. Modern particle accelerators make it possible to accelerate protons and neutrons to such high speeds. And in fact, in this case, an increase in their masses was recorded.

But we still live in a world of high technology, but low speeds. Therefore, in order to know how to calculate the mass of a substance, it is not at all necessary to accelerate the body to the speed of light and learn Einstein's theory. Body weight can be measured on a scale. True, not every body can be put on the scales. Therefore, there is another way to calculate mass from its density.

The air around us, the air that is so necessary for mankind, also has its own mass. And, when solving the problem of how to determine the mass of air, for example, in a room, it is not necessary to count the number of air molecules and sum up the mass of their nuclei. You can simply determine the volume of the room and multiply it by the air density (1.9 kg / m3).

Scientists have now learned with great accuracy to calculate the masses of different bodies, from the nuclei of atoms to the mass the globe and even stars that are several hundred light-years away from us. Mass like physical quantity, is a measure of the body's inertia. More massive bodies, they say, are more inert, that is, they change their speed more slowly. Therefore, after all, speed and mass are interconnected. But main feature This value is that any body or substance has mass. There is no matter in the world that does not have mass!

atomic nucleus is the central part of the atom, made up of protons and neutrons (collectively called nucleons).

The nucleus was discovered by E. Rutherford in 1911 while studying the passage α -particles through matter. It turned out that almost the entire mass of an atom (99.95%) is concentrated in the nucleus. The size of the atomic nucleus is of the order of 10 -1 3 -10 - 12 cm, which is 10,000 times smaller than the size of the electron shell.

The planetary model of the atom proposed by E. Rutherford and his experimental observation of hydrogen nuclei knocked out α -particles from the nuclei of other elements (1919-1920), led the scientist to the idea of proton. The term proton was introduced in the early 20s of the XX century.

Proton (from Greek. protons- first, character p) is stable elementary particle, the nucleus of an atom of hydrogen.

Proton- a positively charged particle, the charge of which is equal in absolute value to the charge of an electron e\u003d 1.6 10 -1 9 Cl. The mass of a proton is 1836 times the mass of an electron. Rest mass of a proton m p= 1.6726231 10 -27 kg = 1.007276470 amu

The second particle in the nucleus is neutron.

Neutron (from lat. neuter- neither one nor the other, a symbol n) is an elementary particle that has no charge, i.e., neutral.

The mass of the neutron is 1839 times the mass of the electron. The mass of a neutron is almost equal to (slightly larger than) that of a proton: the rest mass of a free neutron m n= 1.6749286 10 -27 kg = 1.0008664902 amu and exceeds the proton mass by 2.5 electron masses. The neutron, along with the proton under common name nucleon is part of the atomic nucleus.

The neutron was discovered in 1932 by D. Chadwig, a student of E. Rutherford, during the bombardment of beryllium α -particles. The resulting radiation with high penetrating power (it overcame an obstacle made of a lead plate 10–20 cm thick) intensified its effect when passing through the paraffin plate (see figure). The Joliot-Curie estimates of the energy of these particles from the tracks in the cloud chamber, and additional observations made it possible to eliminate the initial assumption that this γ -quanta. The great penetrating power of new particles, called neutrons, was explained by their electrical neutrality. After all, charged particles actively interact with matter and quickly lose their energy. The existence of neutrons was predicted by E. Rutherford 10 years before the experiments of D. Chadwig. On hit α -particles in the nuclei of beryllium, the following reaction occurs:

Here is the symbol of the neutron; its charge is equal to zero, and the relative atomic mass is approximately equal to one. A neutron is an unstable particle: a free neutron in a time of ~ 15 min. decays into a proton, an electron and a neutrino - a particle devoid of rest mass.

After the discovery of the neutron by J. Chadwick in 1932, D. Ivanenko and W. Heisenberg independently proposed proton-neutron (nucleon) model of the nucleus. According to this model, the nucleus consists of protons and neutrons. Number of protons Z coincides with the serial number of the element in the table of D. I. Mendeleev.

Core charge Q determined by the number of protons Z, which are part of the nucleus, and is a multiple of the absolute value of the electron charge e:

Q = + Ze.

Number Z called nuclear charge number or atomic number.

Mass number of the nucleus A called total number nucleons, i.e., protons and neutrons contained in it. The number of neutrons in a nucleus is denoted by the letter N. So the mass number is:

A = Z + N.

The nucleons (proton and neutron) are assigned a mass number equal to one, and the electron is assigned a zero value.

The idea of ​​the composition of the nucleus was also facilitated by the discovery isotopes.

Isotopes (from the Greek. isos equal, same and topoa- place) - these are varieties of atoms of the same chemical element, the atomic nuclei of which have the same number of protons ( Z) and a different number of neutrons ( N).

The nuclei of such atoms are also called isotopes. Isotopes are nuclides one element. Nuclide (from lat. nucleus- nucleus) - any atomic nucleus (respectively, an atom) with given numbers Z And N. The general designation of nuclides is ……. Where X- symbol of a chemical element, A=Z+N- mass number.

Isotopes occupy the same place in the Periodic Table of the Elements, hence their name. As a rule, isotopes differ significantly in their nuclear properties (for example, in their ability to enter into nuclear reactions). The chemical (and almost equally physical) properties of isotopes are the same. This is explained by Chemical properties element are determined by the charge of the nucleus, since it is he who affects the structure of the electron shell of the atom.

The exception is isotopes of light elements. Isotopes of hydrogen 1 H — protium, 2 H— deuterium, 3 H — tritium they differ so much in mass that their physical and chemical properties are different. Deuterium is stable (i.e., not radioactive) and is included as a small impurity (1: 4500) in ordinary hydrogen. Deuterium combines with oxygen to form heavy water. It boils at normal atmospheric pressure at 101.2°C and freezes at +3.8°C. Tritium β is radioactive with a half-life of about 12 years.

All chemical elements have isotopes. Some elements have only unstable (radioactive) isotopes. For all elements, radioactive isotopes have been artificially obtained.

Isotopes of uranium. The element uranium has two isotopes - with mass numbers 235 and 238. The isotope is only 1/140 of the more common.