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Question
(a) What is meant by (i) atomic number, and (ii) mass number, of an element? Explain with the help of an example.
(b) What is the relation between the atomic number and mass number of an element?
(c) If an element M has mass number 24 and atomic number 12, how many neutrons does its atom contain?
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Solution
(a)(i) Atomic number is the number of protons in the nucleus of an atom. In other words, each element has a unique number that identifies how many protons or electrons are present in one atom of that element. For example, all hydrogen atoms contain one proton and have an atomic number of 1. All carbon atoms contain six protons or electrons and have an atomic number of 6.
(ii) The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. The mass number is different for each isotope of a chemical element. This is not the same as the atomic number (Z), which denotes the number of protons in a nucleus. Thus, it uniquely identifies an element. Therefore, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N = A − Z.
Example- One atom of Na contains 11 protons and 12 neutrons; so its mass no. is 23.
(b) Relation between mass number and atomic number:
Mass No. = Atomic no. + No. of neutron
(c) No. of neutrons = Mass No. - Atomic no.
= 24 – 12 = 12
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