Question

What is the ratio of the number of neutrons present in potassium and magnesium atoms with mass number $39$ and $24$ respectively?

• A. $19:12$
• B. $5:3$
• C. $5:6$
• D. $4:3$

Answer 1

Answer: (B)

$5:3$

$\left(B\right)$ $5:3$

$Given$ - Mass number of potassium = $39$

Mass number of magnesium = $24$

mass number = number of protons + number of neutrons

As we know, number of protons in an element is it's elemental number or atomic number .So,

Potassium has $19$ protons and magnesium has $12$ protons.

Now,

In potassium,

mass number = no. of protons + no. of neutrons

$39$ = $19$ + no. of neutrons

number of neutrons = $39-19$ = $20$

In magnesium,

mass number = no. of protons + no. of neutrons

$24$ = $12$ + no. of neutrons

no. of neutrons = $24-12$ = $12$

Therefore, the ratio of neutrons in $K$ and $Mg$ is $20:12$ = $5:3$.