As you read from left to right across the periodic table atomic numbers and what by one of each element

As we read from left to right across the periodic table atomic numbers are increased by one each of element.

Explanation:

As we move from left to right across the periodic table the atomic number is increased by one and the number of valance electron in an atom increase. The atomic size tend to decrease in same period of periodic table because the electrons are added with in the same shell. When the electron are added, at the same time protons are also added in the nucleus. The positive charge is going to increase and this charge is greater in effect than the charge of electrons. This effect lead to the greater nuclear attraction. The electrons are pull towards the nucleus and valance shell get closer to the nucleus. As a result of this greater nuclear attraction atomic radius decreases and ionization energy increases because it is very difficult to remove the electron from atom and more energy is required.