Periodic Trend Of Atomic Radii
In general, the size of atoms increases as a group is descended. For example, cesium is larger than lithium. As a period is transversed from left to right, the size of the atom decreases. For example, a fluorine atom is smaller than a lithium atom.
Addition Of Electrons And The Effect On The Size Of The Atom
An oxygen atom gains an electron, thereby providing more electron density to shield the electrons from the positively charged nucleus. Consequently, the electron cloud expands. The anion is larger than the atom from which it was derived.
The Removal Of An Electron And Its Effect On Atomic Size
An oxygen atom with its ground state electron configuration is shown losing an electron. The cation that results from this ionization has less electron density to shield the remaining electrons from the nucleus. Therefore, the remaining electrons are more strongly attracted to the positively charged nucleus and the atom shrinks in size.