Question

A neutral atom of potassium (K) has an average mass of 39 amu and 19 electrons. How many neutrons does it have?

A. 10
B. 19
C. 20
D. 58

Answer 1

20

Explanation:

In a neutral atom, the number of protons (p) equals the number of

electrons (e).

p = e

e = 19, so

p = 19

=====

The mass number (A) is the number of protons plus the number of

neutrons (n).

A = p + n

A = 39, so

39 = 19 + n Subtract 20 from each side.

n = 20

The atom has 20 neutrons.

Answer 2

20

Explanation:

In a neutral atom, the number of protons (p) equals the number of

electrons (e).

p = e

e = 19, so

p = 19

=====

The mass number (A) is the number of protons plus the number of

neutrons (n).

A = p + n

A = 39, so

39 = 19 + n Subtract 20 from each side.

n = 20

The atom has 20 neutrons.