Question

How many protons, electrons, and neutrons does an atom with atomic number 50 and mass number 125 contain?

Answer 1

protons=50
electrons=50
neutrons=75

Explanation:

The atomic number tells us how many protons and electrons are in the element.

To work out the number of neutrons you do mass number take away atomic number.

125-50 = 75

Answer 2

Number of protons= 50
Number of electrons= 50
Number of neutrons= 75

Explanation:

In a neutral atom,
number of electrons= number of protons= atomic number of the element. ( in this question= 50)

however, we cannot state this for an ion ( charged atom) as their number of electrons is not equal to the number of protons:
In a positively charged ion: number of electrons < number of protons
In a negatively charged ion: number of electrons > number of protons.

But, number of protons is always equal to the atomic number of the element.

Now,
Mass number is basically the mass of an atom of the element when expressed in a.m.u.

Now, mass of atom should be equal to total mass of electons present in atom + total mass of proton present in atom + total mass of neutron present in atom

but, mass of electron <<< mass of proton and mass of a neutron

So, we ignore the mass of electron and state that the entire atomic mass of an atom resides in its nucleus, which constitutes of protons and neutrons.

therefore,
mass number= mass of total neutrons present in the atom + mass of total protons present in the atom
125 = x(1) + 50(1) ( as no. of protons= 50)
x= 125-50
x=75