Chemistry: Effective Nuclear Charge, Atomic Radius, and Periodic Trends

Effective Nuclear Charge (Z_eff)

The net positive charge experienced by an electron in an atom.

Formula for Z_eff

Z_eff = Z - S

Z

Number of protons in the nucleus.

S

Shielding constant (repulsion from other electrons).

Key Concept of Z_eff

Z_eff is always less than the total number of protons because inner electrons shield outer electrons from the full nuclear charge.

Atomic Radius

The distance from the nucleus to the outermost electron.

Trend in Atomic Radius Down a Group

Radius increases (higher quantum number, more electron shells).

Trend in Atomic Radius Across a Period

Radius decreases (Z_eff increases, electrons pulled closer to nucleus).

Isoelectronic Ions

Ions that have the same number of electrons.

Trend in Size of Ions

Cations < neutral atom < anions (positive charge pulls electrons closer, negative charge pushes them out).

Ionization Energy (IE)

The energy required to remove an electron from an atom or ion.

First Ionization Energy

Energy to remove the first electron.

Second Ionization Energy

Energy to remove the second electron (IE₂ > IE₁).

Third Ionization Energy

Energy to remove the third electron (IE₃ > IE₂ > IE₁).

Trend in Ionization Energy Across a Period

IE increases (Z_eff increases).

Trend in Ionization Energy Down a Group

IE decreases (more shielding, outer electrons feel less nuclear pull).

Metals

Shiny, malleable, good conductors of heat & electricity.

Nonmetals

Dull, brittle, poor conductors of heat & electricity.

Metalloids

Semi-conductors, moderate conductivity.

Common Metalloids

B, Si, Ge, As, Sb, Te, At.

Periodic Table Group 1

+1 charge, lose 1 electron.

Periodic Table Group 2

+2 charge, lose 2 electrons.

Periodic Table Group 13

+3 charge, lose 3 electrons.

Periodic Table Group 15

-3 charge, gain 3 electrons.

Periodic Table Group 16

-2 charge, gain 2 electrons.

Periodic Table Group 17

-1 charge, gain 1 electron.

Periodic Table Group 18

Mostly unreactive.

Monoatomic Ions

Formed from a single atom.

Polyatomic Ions

Formed from multiple atoms.

Ionic Compounds

Compounds held together by electrostatic forces (ionic bonds).

Cation

Positive, usually a metal.

Anion

Negative, usually a nonmetal.

Key Rule of Ionic Compounds

The total charges of the ions must balance to neutral.

group 1 metals are called

alkali metals

group 2 metals are called

alkaline earth metals

group 7 metals are called

halogens

group 18 has 

noble gases

ionic compounds are composed of 

cation(mental)+anion(nonmental)