# The element calcium has the atomic number of 20 and the mass number of 40, how many neutrons does it have?

Nov 2, 2015

20

#### Explanation:

There are few simple rules to follow.

(1) The atomic number is equal to the number of protons.

$Z$ = number of protons

(2) In neutrally charged elements, the number of electrons is the same as the number of protons.

$Z$ = number of protons = number of electrons (no charge)

Otherwise, positive charge means that the element lost an electron and negative charge means it gained an electron.

(3) The atomic mass is equal to the sum of the number of protons and number of neutrons.

$A$ = number of protons + number neutrons

or

$A$ = $Z$ + number neutrons

So if you say that $Z$ = 20 and $A$ = 40, then

$A$ = $Z$ + number neutrons

40 = 20 + number of neutrons

40 - 20 = number of neutrons

Therefore,

number of neutrons = 20

Nov 2, 2015

An atom of the calcium-40 isotope has $20$ neutrons in its nucleus.

#### Explanation:

Isotopes are named for their mass numbers, for example, carbon-14. The mass number of an isotope is the sum of the number of protons and neutrons in the atomic nucleus. The atomic number is the number of protons.

The calcium-40 isotope has a mass number of $40$, and an atomic number of $20$, therefore it has $20$ protons.

$\text{Number of neutrons"="(mass number)"-"(atomic number)}$

$\text{Number of neutrons} = 40 - 20 = 20$