Question

A nucleus of stable phosphorus has 15 protons and 16 neutrons. The nucleus of radio phosphorus has one neutron more than the stable nucleus. What will be the atomic number and mass number for the radio phosphorus?
A. 16,32
B. 15,32
C. 15,30
D. 16,30

Answer 1

Atomic mass number of an atom is defined as the sum of the protons and neutrons in the nucleus. It is expressed by the alphabet ‘A’. The atomic number of an atom is defined as the total number of protons present in the nucleus of an atom. It is expressed by the alphabet ‘Z’.

Complete answer:
As we know that, ‘Z’ is the atomic number and ‘A’ is the mass number of an atom, then the atom contains ‘Z’ number of electrons. So, ‘Z’ is the number of protons and ‘A-Z’ is the number of neutrons.
The number of neutrons of radio phosphorus is given as 16 + 1 = 17.
So, the atomic number which is equal to the number of protons = 15.
Mass number = 15 + 17= 32.
Thus, the atomic number and mass number for the radio phosphorus will be 15, 32 respectively.

Hence, option (B) is the correct answer.

Additional Information:
Atomic number and mass number are always whole numbers because they are obtained by counting the whole objects i.e. protons, neutrons, and electrons. The sum of the mass number and the atomic number for an atom i.e. A-Z equivalent to the total number of subatomic particles which is present in the atom. The mass number describes the mass of the atom’s nucleus in atomic mass units. (amu).

Note:
As students are aware that both protons and neutrons are present in the nucleus of an atom, it should be known that they are together called nucleons. Also, the weight of an electron is almost negligible. So, the atomic mass of the atom is almost the same as its mass number.