Question

A nucleus $Na_{11}^{24}$is radioactive. What are the numbers 24 and 11?
A- 24- atomic number, 11- mass number
B- 24- mass number, 11- atomic number
C- 24- electron number, 11- atomic number
D- 24- mass number, 11- neutron number

Answer 1

Radioactivity refers to the phenomenon in which the substance decays by emission of radiation. Half-life is defined as the time taken by the material in which the number of undecayed atoms becomes half. A material containing unstable nuclei is considered radioactive

Complete step by step answer:
Given sodium nucleus, and its symbol is represented as $Na_{11}^{24}$. We need to find what the numbers 11 & 24 represent. The number written above is the mass number and it represents the total mass of that atom. The number below is called atomic number and for a neutral atom, the number of electrons is equal to the number of protons and is equal to the atomic number.

So, 24 is the mass number and 11 is the atomic number. Hence, the correct option is (B).

Additional Information:
Half-life is the time for half the radioactive nuclei in any sample to undergo radioactive decay. For example, after 2 half-lives, there will be one fourth the original material remains, after three half-lives one eight the original material remains, and so on. Half-life is a convenient way to assess the rapidity of a decay.

Note:
Also, inside the atom there are neutrons which are inside the nucleus and the mass number subtracted from the atomic number gives us the total number of neutrons inside the nucleus. Protons and neutrons are inside the nucleus.