An atom has 18 protons, 18 electrons, and 20 neutrons.
this atom has ?
a negative charge
an atomic number of 20
a mass number of 38
a positive charge

a mass number of 38

Explanation:

X This atom has no a negative charge .

because, number of  protons (18) & electrons (18) are same. so,This atom is neutral.

X This atom has an atomic number of 20.

Because, it has 18 protons. so, This atom has an atomic number of 18.

X  This atom has a positive charge

because, number of  protons (18) & electrons (18) are same.so,This atom is neutral.