shielding effect

The screening effect, or shielding effect, is how electrons in the same atom interact with each other. In a single-electron atom (in isolation), the electron is only interacting with the proton; in a multielectron atom, the electrons are both interacting with the proton(s), but also with each other. While electrons are attracted to the protons in the nucleus, they are repelled by the other electrons. This electron-electron repulsion decreases the attractive force of the protons on the electrons.

The shielding effect changes the effective nuclear charge -- effectively decreasing the true nuclear charge. This effect causes atoms to get smaller as you across a period (row) of the periodic table, as well as many other periodic trends observed in the periodic table.