Question

The mass number of an element is 18. It contains 7 electrons. What is the number of protons and neutrons in it?

Answer 1

The elements have equal number of neutrons and protons to be electrically neutral. As given in the question, electrons are $7$.

Therefore, protons must be also $7$.

Mass number($\mathrm{A}$) is equal to the total sum of protons and neutrons in the element. So, total protons and neutrons are $18$.

According to the mass number, protons + neutrons $=$ $18$

Given, as number of protons are $7$

Therefore, $7+$neutrons $=$$18$

Neutrons $=18-7$ $=11$

Therefore, Number of neutrons are $11$.