When an electron is removed from a metal, it moves further away from the positively charged nucleus, increasing the likelihood of the metal losing an electron. In other words, unlike the electron that must be removed from a lithium atom to form Li+, the electron lost by Cs to form Cs+ lies at a much greater distance from the attractive positive nucleus and is thus easier to remove. Group 2 elements (the alkaline-earth metals) follow the same pattern; the lower the metal is in the group, the more likely it is to lose an electron.
Nonmetals exhibit a range of properties similar to that of metals. There is a general trend in the periodic table where the most reactive metals are found in the bottom left corner, and the most reactive nonmetals are found in the top right corner. Metalloids, or semimetals, are materials whose properties fall somewhere between those of metals and those of nonmetals.
Because metals typically have low ionization energies, metal atoms lose electrons to nonmetal atoms. It is easier for metals at the bottom of a group to lose electrons than those at the top. In other words, ionization energies typically decrease from most intense to least intense within a set. In the periodic table, nonmetals are found on the right side because their ionization energies are higher than those of metals.
For a given time period, ionization energies tend to rise as one reads from left to right. Consequently, the elements in the bottom left of the periodic table have the lowest ionization energies (and are the most chemically active metals) and the elements in the top right of the periodic table have the highest ionization energies (and are thus the most chemically active nonmetals).