The first thing to do is identify the element and it's symbol. Since a specific element isn't listed in the question, we'll pick sodium as an example. The symbol for sodium is Na and it is element 11 on the periodic table.
The whole number (no decimal places, smaller of two numbers) is the atomic number and increases by one with each additional element.
The atomic number tells us the number of protons in one atom of sodium. Protons have positive charges and since we are talking about a neutral atom, the atom must have an equal number of electrons which are negatively charged.
That accounts for the protons and electrons. Now we need to figure out the number if neutrons. The protons and neutrons make up the vast majority of the mass of an atom. The electrons are so light that they do not contribute enough to the mass to chanGe the value.
To find the number of neutrons, we look at the atomic mass which is the other number given on the periodic table (usually has decimals). Round this to the nearest whole number. For sodium that will be 23.
Atomic mass= protons +neutrons
23 = 11 + neutrons
Neutrons = 12
Sodium has 11 protons, 11 electrons, and 12neutrons.