ATOMIC NUMBER AND MASS NUMBER
The atomic number of an element is equal to the number of protons in the nucleus of its atom. The atomic number is also known as proton number of the element. Further, in an atom, number of protons is equal to the number of electrons. Hence, atomic number is also equal to the number of electrons in an atom of the element. Thus,
Atomic number of an element is equal to the number of protons in the nucleus of its atom or the number of extra nuclear electrons.
Atomic number is denoted by the letter z.
Atomic Number (Z) = Number of protons =Number of electrons
For example, an atom of magnesium contains 12 protons, therefore, its atomic number is 12.
All the atoms of a particular element contain same number of protons in their nuclei. Therefore, all the atoms of an element have same atomic number. Atoms of different elements contain different number of protons and hence no two elements can have same atomic number. Thus, each element has its characteristic atomic number which can be used to identify the element.
For example, when we say an element with atomic number 13, we are referring to aluminum. No other element has atomic number 13.
It has already been stated that mass of an atom is mainly concentrated in the nucleus. In the nucleus, there are protons and neutrons. From this it follows that mass of an atom is mainly due to protons and neutrons. Protons and neutrons are collectively called nucleons as they are present in the nucleus of an atom.
The total number of protons and neutrons in the nucleus is called mass number of the atom.
It is generally represented by the letter A.
Mass Number (A) = Number of protons (Z)+ Number of neutrons (N)= Number of nucleons
The number of protons in the nucleus is equal to atomic number, Z while the number of neutrons in the nucleus is sometimes called neutron number, N.
Mass number gives the approximate atomic mass of the element. For example, mass number of magnesium is 24 as it contains 12 protons and 12 neutrons in its nucleus. Each proton
and neutron has relative mass approximately 1 u. Therefore, atomic mass of magnesium is 12 u + 12 u = 24 u.
Calculation of Number of Electrons, Protons and Neutrons
From the knowledge of atomic number and mass number of an element it is possible to calculate number of electrons, protons and neutrons in an atom of the element. For example,
atomic number and mass number of aluminum are 13 and 27 respectively. Number of electrons, protons and neutrons in an atom of it can be calculated as under:
Number of protons = Atomic number = 13
Number of electrons = Atomic number= 13
Number of neutrons = Mass number – Atomic number= 27-13 = 14.
Example 5.1. Calculate the number of electrons, protons and neutrons in the following species:
(i) Phosphorus atom (ii) Phosphide ion (P3-)
(iii) Magnesium ion (Mg2+)
Mass numbers: P = 31, Mg = 24
Atomic numbers: P = 15, Mg = 12.
Solution. (i) Phosphorus atom
Number of electrons = Atomic number= 15
Number of protons = Atomic number= 15
Number of neutrons = Mass number-Atomic number
= 31-15 = 16.
(ii) Phosphide ion (P3-)
Phosphide ion (P3-) = Phosphorus atom + 3 electrons
p3- = P+ 3e-
Thus, phosphide ion has same number of protons and neutrons as phosphorus atom but has three electrons more.
Number of electrons = 15 + 3 = 18
Number of protons = 15
Number of neutrons = 31- 15 = 16.
(iii) Magnesium ion (Mg2+)
Mg2+ ion is formed by loss of two electrons by Mg atom.
Therefore, it has two electrons less than the number of electrons in Mg atom.
Mg2+ = Mg – 2e
Number of electrons = 12-2 = 10
Number of protons = 12
Number of neutrons = (24- 12) = 12.
Example. 5.2. The number of protons in the nucleus of an atom of mass number 97 is 41. What will be the number of neutrons in its isotope of mass number 99?
Solution. The atomic number of isotopes is same. Therefore, the number of protons in both the atoms is same.
Mass number= Number of protons + Number of neutrons
. . Number of neutrons
= Mass number-Number of protons
Example 5.3. If the number of electrons in an atom is 8 and number of protons is also 8, then
(i) What is the atomic number of the element?
(ii) What is the charge on the atom?
Solution. (i) Atomic number of the element
= Number of protons in the atom = 8
(ii) There is no charge on the atom as the number of protons is
equal to the number of electrons.
Example 5.4 The nucleus of an atom of a certain isotope contains 6 protons and 7 neutrons. What is the atomic number and the mass number of this isotope? To which element does it belong?
Solution. We know that:
Atomic number = Number of protons
Atomic number = 6
Mass number = Number of protons + Number of neutrons =6 +7 = 13.
Since atomic number of t