Periodic Trends in Properties: Quick Revision

A brief note on Periodic Trends in Properties. Read it before a test. Try each question at the end before you open its answer.

Size

• Covalent radius: half the distance between two bonded nuclei. Metallic radius: half the distance between neighbouring nuclei in a metal.
• Across a period atoms shrink; down a group they grow.
• Cations are smaller than their atoms, anions larger.
• Isoelectronic species: same electrons; the more protons, the smaller the particle.

Ionisation and electron gain

• First ionisation enthalpy: $\text{X(g)} \rightarrow \text{X}^+\text{(g)} + e^-$. Always positive.
• $Z_{\text{eff}} = Z - S$. It rises across a period.
• Ionisation enthalpy rises across a period and falls down a group, with dips at groups 13 and 16.
• Electron gain enthalpy: $\text{X(g)} + e^- \rightarrow \text{X}^-\text{(g)}$. Most negative for the halogens, with chlorine above fluorine. Positive for the noble gases.

Electronegativity and its consequences

• Electronegativity: the pull of an atom in a compound on a shared pair. Fluorine is $4.0$ on the Pauling scale.
• It rises across a period and falls down a group. Metallic character goes the other way.
• Valence: outer electrons, or eight minus them.
• Second-period elements: covalency at most 4. Diagonal pairs: Li and Mg, Be and Al.
• Oxides: basic, then amphoteric, then acidic across a period.

A worked example

Arrange $\text{Mg}^{2+}$, $\text{O}^{2-}$, $\text{Na}^+$ and $\text{F}^-$ by size, from the largest.

1. Electrons. All four have $10$ electrons, so they are isoelectronic.
2. Protons. $\text{O}^{2-}$ has $8$, $\text{F}^-$ $9$, $\text{Na}^+$ $11$ and $\text{Mg}^{2+}$ $12$.
3. Order. More protons pull the same electrons closer: $\text{O}^{2-} > \text{F}^- > \text{Na}^+ > \text{Mg}^{2+}$.

Practice with new numbers

A program to try

The program counts how often radius, ionisation enthalpy and electronegativity rise or fall across period 2.

Mistakes to avoid

• More electrons, bigger atom. Not across a period: the nuclear charge wins.
• A smooth rise in ionisation enthalpy. Remember the dips at groups 13 and 16.
• Fluorine first for electron gain. Chlorine releases more energy.
• Electronegativity as an energy. It is a scale value for an atom in a compound.
• Boron with a covalency of 6. Second-period atoms stop at 4.

Check yourself

1. Which is larger: the chlorine atom or the chloride ion?

2. Why is the first ionisation enthalpy of aluminium below that of magnesium?

3. Which element has the highest electronegativity of all?

4. What kind of oxide does aluminium form?

5. Which element of group 2 does lithium resemble?