Periodic Trends

Definition: Half of the distance between nuclei in covalently bonded diatomic molecule Radius decreases across a period

Increased Zeff due to decreased shielding thereby pulling e- cloud closer to the nucleus

Radius increases down a group Each row on the periodic table adds a “shell” or energy level to the atom increasing distance the nucleus must pull

Atomic Radius

Table of Atomic

Radii

Period Trend:Atomic Radius

Tends to increase across a period As radius decreases across a period, the electron you are removing is closer to the nucleus and harder to remove

Tends to decrease down a group Outer electrons are farther from the nucleus and easier to remove

Ionization Energy

Definition: the energy required to remove an electron from an atom

Periodic Trend:Ionization

Energy

Electronegativity

Definition: A measure of the ability of an atom in a chemical compound to attract electrons

o Electronegativity tends to increase across a periodo As radius decreases, electrons get closer to the bonding atom’s nucleus

o Electronegativity tends to decrease down a group or remain the sameo As radius increases, electrons are farther from the bonding atom’s nucleus

Periodic Table of Electronegativities

Periodic Trend:Electronegativi

ty

Ionic RadiusDefinition: distance from the nucleus to the outer edge of the electron cloud in a charged ion. (same general trend as atomic radius)Electron

AffinityDefinition: the energy gained or lost with the addition of an electron to a gaseous atom or ion.

(becomes more negative)

Decreasing electron affinity (becomes more positive)

Summary of Periodic Trends

Ionic Radii (same trend as atomic radii)

Cations

Positively charged ions formed when

an atom of a metal loses one or more electrons Smaller than the corresponding

atom

Anions

Negatively charged ions formed

when nonmetallic atoms gain one or more electrons Larger than the corresponding

atom

Graphic courtesy Wikimedia Commons user Popnose

VALENCE ELECTRONS

• Valence electrons – the electrons available to be lost, gained, or shared in the formation of chemical compounds.

• They are the electrons located in the outermost energy shell in an atom

• Atoms are most energetically stable when they have 8 valence electrons