What is the mass number of an atomic nucleus? The mass number is numerically equal to the sum of the 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 interconnected? Let's find out.
Atom structure
Any atom consists of a nucleus and electrons. In addition to the hydrogen atom, since it has only one electron. The core is positively charged. The 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 carrying a negative charge, that is, electrons, is equal to the positive charge of the nucleus. For example, in an oxygen atom the charge of the nucleus is +8 and electrons 8, in the calcium atom the charge of the nucleus is +20, electrons 20.
Core structure
The nucleus consists of two types of particles - protons and neutrons. Protons are positively charged, neutrons have no charge. Thus, protons give a charge to the nucleus. The charge of each proton is taken as +1. That is, how many protons are contained in the nucleus, such will be the charge of the whole nucleus. For example, in the carbon nucleus there are 6 protons, the charge of the nucleus is +6.
In the periodic system of Mendeleev’s elements, all elements are arranged in order of increasing precisely the nuclear charge. Hydrogen has a core charge of +1, it is located first; helium +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 charge of the nucleus.
Atom mass
The mass of an atom (M) is determined by the mass of its constituent parts, that is, electrons and nucleus. Electrons are very light in comparison with the nucleus and make almost no contribution to the mass of the whole atom. That is, the mass of the atom is due to the mass of the nucleus. What is a mass number? The mass of a 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 degrees. 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 compare the masses of elements or particles, then use the relative masses of atoms (A r ), expressed in amu The relative mass of the atom is indicated in the periodic table, for example, for nitrogen 14,007. The relative mass of an atom, rounded to an integer, is the mass number of the nucleus of the element (A). Mass numbers are such that they are convenient to use - they are always integers: 1, 2, 3, and so on. For example, for nitrogen - 14, for carbon - 12. They are written with the upper left index, for example, 14 N or 12 C.
When do you need to know the mass number?
Knowing the mass number (A) and atomic number of an element in the periodic system (Z), one can determine the number of neutrons. To do this, subtract the protons from the mass number.
Knowing the mass number, we can calculate the mass of the nucleus or the entire atom. Since the mass of the nucleus is determined by the mass of particles included in 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 the neutron is equal to the mass of the proton, in general, they are designated as the mass of the nucleon (nuclear particle).
M = A ∙ m N
For example, we calculate the mass of an aluminum atom. As can be seen from the periodic system of Mendeleev’s elements, the relative atomic mass of aluminum is 26.992. By rounding, we get the mass number of the aluminum core 27. That is, its core 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 core is equal to: 27 ∙ 1.67 ∙ 10 -24 g = 4.5 ∙ 10 -23 g.
What is the mass number of element nuclei you need to know when compiling radioactive decay reactions or nuclear reactions. For example, upon fission of a 235 U uranium nucleus that has captured one neutron 1 n, barium 141 Ba and krypton 92 Kr nuclei are formed, as well as three free 1 n neutrons. When compiling such reactions, they use the rule: the sum of the mass numbers in the right and left sides of the equation does not change. 235 + 1 = 92 + 141 + 3.