Periodic Trends Unit 1

What is effective nuclear charge (Z_eff)?

The net positive charge felt by valence electrons: Z_eff = # of protons − # of core (shielding) electrons

How does Z_eff change across a period?

Z_eff increases across a period because protons increase while shielding stays the same.

How does Z_eff change down a group?

Z_eff slightly decreases or stays constant due to increased shielding by inner electrons.

How does atomic radius change across a period?

Atomic radius decreases across a period due to increasing Z_eff pulling electrons closer.

How does atomic radius change down a group?

Atomic radius increases down a group because more energy levels (shells) are added.

How do cations compare in size to their neutral atoms?

Cations are smaller than their atoms due to loss of electrons and reduced repulsion.

How do anions compare in size to their neutral atoms?

Anions are larger than their atoms due to added electrons and increased repulsion.

What is ionization energy (IE)?

The energy required to remove the outermost electron from a gaseous atom.

How does ionization energy change across a period?

IE increases across a period due to stronger attraction between nucleus and electrons.

How does ionization energy change down a group?

IE decreases down a group because outer electrons are farther from the nucleus.

What is electron affinity (EA)?

The energy released when a neutral atom in the gas phase gains an electron.

How does electron affinity change across a period?

EA becomes more negative (more exothermic) across a period as Z_eff increases.

How does electron affinity change down a group?

EA becomes less negative down a group because added electrons are farther from the nucleus.

What is an exception to the EA trend between F and Cl?

Chlorine has a more negative EA than fluorine due to less electron–electron repulsion.

What is electronegativity?

A measure of how strongly an atom attracts shared electrons in a bond.

How does electronegativity change across a period?

Electronegativity increases across a period (stronger attraction to bonding electrons).

How does electronegativity change down a group?

Electronegativity decreases down a group due to larger radius and shielding.

What is metallic character?

The tendency of an atom to lose electrons and behave like a metal.

How does metallic character change across a period?

Metallic character decreases across a period as atoms hold electrons more tightly.

How does metallic character change down a group?

Metallic character increases down a group since electrons are more easily lost.

What is nonmetallic character?

The tendency of an atom to gain electrons and form negative ions.

How does nonmetallic character change across a period?

Nonmetallic character increases across a period (atoms gain electrons more easily).

How does nonmetallic character change down a group?

Nonmetallic character decreases down a group due to weaker attraction to electrons.

Which element is the most electronegative?

Fluorine (F) – highest Z_eff and smallest atomic radius.

Why is the second ionization energy of Na so much higher than the first?

The second electron comes from a full inner shell, which is much more stable and tightly bound.