periodic table trends

A predictable pattern in element properties across the periodic table

periodic trend

What determines periodic trends?

Electron configuration and effective nuclear charge

The net positive charge felt by valence electrons

effective nuclear charge (zeff)

What causes Zeff to increase across a period?

Increasing protons with little additional shielding

Distance from the nucleus to the outermost electron

atomic radius

What happens to atomic radius across a period?

Decreases left to right

What happens to atomic radius down a group?

Increases

Why does atomic radius decrease across a period?

Increasing Zeff pulls electrons closer

Why does atomic radius increase down a group?

Additional electron shells

Size of an ion

ionic radius

How does cation size compare to its neutral atom?

Cations are smaller

How does anion size compare to its neutral atom?

Anions are larger

Why are cations smaller than neutral atoms?

Loss of electrons reduces repulsion

Why are anions larger than neutral atoms?

Gain of electrons increases repulsion

Energy required to remove an electron

ionization energy

What happens to ionization energy across a period?

Increases

What happens to ionization energy down a group?

Decreases

Why does ionization energy increase across a period?

Higher Zeff holds electrons more tightly

Why does ionization energy decrease down a group?

Electrons are farther from the nucleus

Which elements have the highest ionization energies?

Noble gases

Which elements have the lowest ionization energies?

Alkali metals

Energy change when an electron is added to an atom

electron affinity

Is electron affinity usually exothermic or endothermic?

Exothermic

What happens to electron affinity across a period?

Becomes more negative

What happens to electron affinity down a group?

Becomes less negative

Which group has the most negative electron affinity?

Halogens

Why do noble gases have low electron affinity?

Filled valence shells

Ability of an atom to attract electrons in a bond

electronegativity

What happens to electronegativity across a period?

Increases

What happens to electronegativity down a group?

Decreases

Which element is the most electronegative?

Fluorine

Tendency to lose electrons

metallic character

What happens to metallic character across a period?

Decreases

What happens to metallic character down a group?

Increases

Which elements are the most metallic?

Bottom-left of the periodic table

Inner electrons blocking nuclear charge

shielding

How does shielding change down a group?

Increases

How does shielding change across a period?

Stays roughly constant

What is valence electron count?

Number of electrons in the outer shell

How does valence electron count change across a period?

Increases

How does valence electron count change down a group?

Stays the same

What determines an element’s reactivity?

Valence electrons and electronegativity

Which metals are most reactive?

Alkali metals

Which nonmetals are most reactive?

Halogens

What happens to reactivity of metals down a group?

Increases

What happens to reactivity of nonmetals down a group?

Decreases

Atoms or ions with the same number of electrons

isoelectronic species

In an isoelectronic series, which species is smallest?

The one with the most protons

What is a common MCAT trap with periodic trends?

Forgetting trends reverse direction for metals vs nonmetals

Another common MCAT trap with trends?

Ignoring effective nuclear charge

What is the fastest way to remember periodic trends on the MCAT?

Top-right = small, electronegative, high IE; bottom-left = large, metallic, low IE