Effective nuclear charge is the net positive nuclear charge experienced by a valence electron orbiting the positively charged nucleus in an atom.
We can calculate a numerical value for effective nuclear charge (Zeff) by subtracting the number of shielding, or inner, electrons from the atomic number. From left to right on the periodic table, Zeff increases because the number of shielding electrons within each row stays constant while the number of protons increases. Zeff is one of the factors in the size of an atom. If an atom has a strong pull on its outer electrons, the atom as a whole will be smaller.
Despite the fact that Zeff knows, it is very difficult to determine the size of the atom, which is the size of the electronic cloud around the nucleus of the atom. But since we can never know the exact location of an electron, we can not actually determine the size of an atom. One way to overcome this problem is to assume that the atom is an ideal field. Then we can determine the atomic radius by measuring the nucleus to the edge of the spherical cloud of electrons. There are many experimental and theoretical methods for determining or calculating the atomic radius for an element. Using these radii, moving from left to right through a periodic table, the atomic radius decreases due to an increase in the effective nuclear charge.