What is the mass number of an atomic nucleus? The mass number is numerically equal to the sum of neutrons and protons of the nucleus. It is denoted by the letter A. The concept of "mass number" appeared due to the fact that the mass of the nucleus is due to the number of nuclear particles. How are the mass of the nucleus and the number of particles related? Let's find out.
The structure of the atom
Any atom consists of a nucleus and electrons. Except for the hydrogen atom, since it has only one electron. The nucleus is positively charged. Negative charge is carried by electrons. The charge of each electron is taken as -1. The atom as a whole is electrically neutral, that is, it has no charge. This means that the number of particles that carry a negative charge, that is, electrons, is equal to the positive charge of the nucleus. For example, in an oxygen atom, the nuclear charge is +8 and electrons are 8, in a calcium atom, the nuclear charge is +20, electrons are 20.
Structure of the nucleus
The nucleus consists of two types of particles - protons and neutrons. Protons are positively charged, neutrons have no charge. Thus, protons give charge to the nucleus. The charge of each proton is taken as +1. That is, how many protonscontained in the nucleus, such will be the charge of the entire nucleus. For example, there are 6 protons in the carbon nucleus, the nuclear charge is +6.
In Mendeleev's periodic system of elements, all elements are arranged in order of increasing nuclear charge. Hydrogen has a nuclear charge of +1, it is located first; helium has +2, it is second in the table; lithium has +3, it is the third and so on. That is, the charge of the nucleus corresponds to the ordinal (atomic) number of the element in the table.
In general, any atom is electrically neutral. This means that the number of electrons is equal to the charge of the nucleus, that is, the number of protons. And since the number of protons determines the atomic number of an element, knowing this atomic number, we thus know the number of electrons, the number of protons, and the nuclear charge.
The mass of an atom
The mass of an atom (M) is determined by the mass of its constituent parts, that is, the electrons and the nucleus. Electrons are very light compared to the nucleus and contribute almost nothing to the mass of the entire atom. That is, the mass of an atom is determined by the mass of the nucleus. What is a mass number? The mass of the nucleus is determined by the number of particles that make up its composition - protons and neutrons. Thus, the mass number is the mass of the nucleus, expressed not in units of mass (grams), but in the number of particles. Of course, the absolute mass of nuclei (m), expressed in grams, is known. But these are very small numbers expressed in negative powers. For example, the mass of a carbon atom is m(C)=1.99 ∙ 10-23 g. Using such numbers is inconvenient. And if there is no need for absolute mass values, but you just need to comparemasses of elements or particles, then use the relative masses of atoms (Ar), expressed in amu. The relative mass of an atom is indicated in the periodic table, for example, nitrogen has 14.007. The relative mass of an atom, rounded to an integer, is the mass number of the element's nucleus (A). Mass numbers are such that they are convenient to use - they are always integers: 1, 2, 3, and so on. For example, nitrogen has 14, carbon has 12. They are written with the upper left index, for example, 14N or 12C.
When do you need to know the mass number?
Knowing the mass number (A) and the atomic number of an element in the periodic system (Z), you can determine the number of neutrons. To do this, subtract protons from the mass number.
Knowing the mass number, you can calculate the mass of the nucleus or the entire atom. Since the mass of the nucleus is determined by the mass of the particles that make up its composition, it is equal to the product of the number of these particles and the mass of these particles, that is, the product of the mass of the neutron and the mass number. The mass of a neutron is equal to the mass of a proton, in general they are denoted as the mass of a nucleon (nuclear particle).
M=A∙mN
For example, let's calculate the mass of an aluminum atom. As can be seen from the periodic system of elements of Mendeleev, the relative atomic mass of aluminum is 26.992. Rounding, we get the mass number of the aluminum nucleus 27. That is, its nucleus consists of 27 particles. The mass of one particle is a constant value equal to 1.67 ∙ 10-24 g. Then, the mass of the aluminum nucleus is: 27 ∙ 1.67 ∙ 10-24r=4, 5 ∙ 10-23 r.
What is the mass number of nuclei of elements you need to know when compiling radioactive decay reactions or nuclear reactions. For example, fission of a uranium nucleus 235U, capturing one neutron 1n, produces barium nuclei 141Ba and krypton 92Kr, as well as three free neutrons 1n. When compiling such reactions, the rule is used: the sum of the mass numbers on the right and left sides of the equation does not change. 235+1=92+141+3.
