- Identify periods and groups as directions on the periodic table.
- Describe the directional trends in atomic radius, ionization energy, and electronegativity.
- Distinguish the differently qualified electron-affinity statements presented in the lesson.
- Apply stated periodic trends to comparisons between elements.
Periodic Table Organization
The periodic table shows patterns in how elements behave, and periodic trends are used to predict how elements will react.
- Moving horizontally from left to right follows a period.
- Moving vertically from top to bottom follows a group.
- Elements are arranged according to atomic number.
The four main properties considered are atomic radius, ionization energy, electronegativity, and electron affinity.
Which direction follows a period, and which direction follows a group?
A period is followed horizontally from left to right. A group is followed vertically from top to bottom.
Atomic Radius
- Atomic radius increases from top to bottom within a group.
- Atomic radius decreases from left to right across a period.
Adding new electron shells makes an atom larger. Across a period, increasing positive nuclear charge pulls electrons closer to the nucleus, while electron shells are not added.
When moving from left to right across a period, how does atomic radius change, and what happens to the electrons as positive nuclear charge increases?
Atomic radius decreases, and increasing positive nuclear charge pulls electrons closer to the nucleus.
Ionization Energy
Ionization energy decreases down a group. Outer electrons become farther from the nucleus down a group, and an outer electron farther from the nucleus is easier to remove.
Ionization energy increases across a period. Across a period where no significant new shells are added, a stronger nuclear charge holds electrons more tightly.
If an outer electron becomes farther from the nucleus down a group, how do ease of removal and ionization energy change?
The outer electron becomes easier to remove, and ionization energy decreases down the group.
Electronegativity
- Electronegativity decreases down a group.
- Electronegativity increases across a period.
Generally, nonmetals accept electrons and are electronegative.
How does electronegativity change across a period and down a group?
Electronegativity increases across a period and decreases down a group.
Electron Affinity
Electron affinity generally becomes more negative across a period.
Metals such as lithium are described as likely to give out an electron rather than gain one.
What does the initial down-group explanation say about electron affinity, and what does the later conclusion and final summary say?
The initial explanation says electron affinity decreases down a group. The later conclusion and final summary say electron affinity increases down a group.
Applying Periodic Trends
Oxygen is farther across the same period than carbon and has a smaller atomic radius than carbon.
Oxygen is more electronegative than carbon and attracts electrons more strongly than carbon.
Compare oxygen with carbon in position across their period, atomic radius, and electronegativity.
Oxygen is farther across the same period, has a smaller atomic radius, and is more electronegative than carbon.
Common misconceptions
Watch out: A period runs from top to bottom, while a group runs from left to right.
Instead: A period is followed horizontally from left to right, while a group is followed vertically from top to bottom.
Watch out: Atomic radius increases from left to right across a period.
Instead: Atomic radius decreases from left to right across a period.
Watch out: The lesson presents only one down-group statement for electron affinity.
Instead: The initial down-group explanation says electron affinity decreases, while the later conclusion and final summary say it increases down a group.
Lesson recap
- Atomic radius increases down a group and decreases across a period.
- Ionization energy decreases down a group and increases across a period.
- Electronegativity decreases down a group and increases across a period.
- For electron affinity, retain the stated qualifiers: it generally becomes more negative across a period, while the final summary says it decreases across a period.