Periodic Table Trends

(1) Atomic Radius As one goes from left to right across a period, the atomic radii decreases. Reason: All the elements in a period have the same principal quantum number. As atomic number increases, so does the attraction between the electrons and the nucleus, and the attraction pulls the electrons in.

Atomic Radius Continued As one goes down a group, the atomic radii increases. Reason: The principal quantum number increases (each new period represents a new energy level), therefore the size of the electron cloud increases.

(2) Ionization Energy The removal of an electron is known as ionization (the formation of a positive ion). The energy required to remove an electron from an atom is called ionization energy. The further the outermost electron is from the nucleus, the less energy it takes to remove it.

Ionization energy continued. From left to right across a period, ionization energy increases. Reason: As you go from left to right across a period, the atomic radius decreases as the nuclear charge increases. Thus, the valence electrons are more strongly attracted to the nucleus and more energy is needed to remove an electron from the atom.

Ionization energy continued From top to bottom of a group, the ionization energy decreases. Reason: As you go down a column, the atomic radius increases, therefore, the distance between the valence electrons and the nucleus increases. In addition, the shielding effect by the inner electrons increases (they block the attraction of the nucleus for outer electrons), therefore lowering ionization energy.

(3) Electron Affinity Electron affinity is a measure of the tendency of an atom to gain electrons (a measure of the attraction of an atom for electrons).

Electron affinity continued From left to right across a period, electron affinity increases. Reason: The increased nuclear charge of each successive element increases the attraction for electrons.

Electron affinity continued As you go down a column, electron affinity decreases. Reason: As you go down a column, the atoms become larger. As a result, the nucleus is farther away from the surface and attracts the outer electrons less strongly.

(4) Electronegativity Electronegativity is the tendency for an atom to attract electrons to itself when it is chemically combined with another element.

Electronegativity continued As one goes from left to right across a period, electronegativity increases. As one goes down a group, electronegativity decreases.

(5) Metallic Character Chemical properties associated with those elements classed as metals. These properties arise from the element's ability to lose electrons.

Metallic character continued As one goes from left to right across a period, metallic character decreases. As one goes down a group, metallic character increases.