Answer:
d = 21 amu
Step-by-step explanation:
An atom consist of electron, protons and neutrons. Protons and neutrons are present with in nucleus while the electrons are present out side the nucleus.
All these three subatomic particles construct an atom. A neutral atom have equal number of proton and electron. In other words we can say that negative and positive charges are equal in magnitude and cancel the each other. For example if neutral atom has 6 protons than it must have 6 electrons. The sum of neutrons and protons is the mass number of an atom while the number of protons are number of electrons is the atomic number of an atom.
For example:
In given shown atom there are 12 neutrons and 9 protons are present with in nucleus thus its mass number will be 12+9 = 21 amu
while there are 9 electrons present out side the nucleus. There are equal number of protons and electrons thus atom will be neutral and atomic number is 9.