Question

An atom of a certain element has 19 protons, 19 electrons, and a mass number of 39. This element is very reactive. How many neutrons are in this atom?

Answer 1

There would be
`20` neutrons in that atom.

Step-by-step explanation:

The mass number of an atom is the sum of:

• the number of neutrons in this atom, and
• the number of protons in this atom.

Electrons do not contribute to the mass number of an atom.

From the question:

• Mass number of this atom:
  `39`.
• Number of protons in this atom:
  `19`.

`(\text{Mass Number}) = (\text{Number of protons}) + (\text{Number of neutrons})`.

In this atom:

`39 = 19 + (\text{Number of neutrons})`.

Hence, the number of neutrons would be
`39 - 19 = 20`.