Identify and explain the trend in atomic radius across a period

The atomic radius across a period in the periodic table tends to decrease from left to right. As you move across a period the number of electrons from one element to the next increases by one. The electron is added in the same valence shell which increases electron repulsion which slightly increases the atomic radius but this is a very small. As you move across a period an extra proton is aded from one element to the next, increasing the positive charge of the nucleus and therefore increasing the attractive force between the nucleus and the valence shell. This increased electrostatic attraction pulls the valence shell closer to the nucleus so the atomic radius decreases.