How many neutrons are there in the nucleus of an atom of lithium if the atomic number is 7?

The amount of neutrons can change among elements atoms of the same type element. These are called isotopes. Lithium actually has an atomic number of 3. An atomic number means how many protons it has. The problem most likely means that it has an Atomic Mass of 7. If that is true, it becomes easy. Atomic mass means the total weight of the element. A proton and a neutron each weigh '1.' All you have to do is look and see that lithium has an atomic number of '3' which means that it has 3 protons. You subtract that from the mass to get an answer of 4. This means that lithium in this example has 4 neutrons.

Electrons aren't counted in the mass of the element because they weigh so little. That is the same reason why a neutron (which is technically a proton and an electron together) has the same weight as a regular proton.