Unit 1 · Atomic Structure and Properties

AP Chemistry Periodic Table Groups

The names of the periodic table groups, how a group differs from a period, where metals, nonmetals, and metalloids sit, and how the main families behave, in four worked examples.

An element’s position on the periodic table is the fastest way to predict how it behaves. Its group tells you its valence electrons and usual ion charge, and which side of the staircase line it sits on tells you whether it is a metal or a nonmetal. The AP exam gives you a table with symbols, atomic numbers, and masses but no family names or colors, so this information has to come from you; what the exam’s table does and does not show is covered in the AP Chemistry periodic table.

How a Group Differs From a Period

• Group: a vertical column, numbered 1 to 18. Elements in a group have the same number of valence electrons, so they have similar chemical properties.
• Period: a horizontal row, numbered 1 to 7. The period number equals the highest occupied shell (n) of the atom’s electrons, and properties change steadily across a period.

Group similarity comes straight from electron configurations: lithium is [He]2s¹, sodium is [Ne]3s¹, and potassium is [Ar]4s¹. Each has one valence electron in an s subshell, which is why they react alike. The full method is in how to write electron configurations, and counting valence electrons from group number is covered in AP Chemistry valence electrons.

The Names of the Periodic Table Groups

The two rows usually printed below the table are the lanthanides (elements 57 to 71) and actinides (89 to 103), together called the inner transition metals. Why these charges appear, and how cations and anions differ, is explained in AP Chemistry ions.

Where Metals, Nonmetals, and Metalloids Sit

Position also predicts bonding: a metal with a nonmetal usually forms an ionic compound, two nonmetals form covalent bonds, and metal atoms bond to each other through metallic bonding. The comparison is worked through in AP Chemistry ionic vs covalent bonds.

How the Main Families Behave

• Alkali metals: soft, very reactive metals that react with water to form a hydroxide and hydrogen gas. Reactivity increases down the group.
• Alkaline earth metals: reactive, but less so than the alkali metal beside them, because two electrons must be removed to form the 2+ ion.
• Transition metals: often form more than one ion, and they lose their outer s electrons before their d electrons.
• Halogens: diatomic molecules (F₂, Cl₂, Br₂, I₂). They are the most reactive nonmetals; fluorine is the most reactive, and reactivity decreases down the group.
• Noble gases: single atoms with a filled valence shell, so they are very unreactive. Only a few compounds are known, mostly of xenon, such as XeF₄.

The direction of each change follows from the same three factors (nuclear charge, shielding, and distance) explained in AP Chemistry periodic trends.

Worked Example 1: Classifying Six Elements by Family

For Si, Br, Ca, Fe, Xe, and S, name the family or category, say whether each is a metal, nonmetal, or metalloid, and give its usual ion.

Find the column first, then check which side of the staircase line the element is on. Silicon is the one to watch: it sits right on the line.

Worked Example 2: Comparing Sodium and Potassium in Water

Sodium and potassium both react with water. Write the equation for potassium, and explain which metal reacts more vigorously.

Same group, same reaction: both have one valence s electron, so both form a +1 ion, a hydroxide, and hydrogen gas: 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g).
Which is faster: potassium’s valence electron is in the n = 4 shell, farther from the nucleus than sodium’s n = 3 electron, and the effective nuclear charge is about the same. The electron is held less tightly, so it is lost more easily.
Evidence: first ionization energy falls from 496 kJ/mol for Na to 419 kJ/mol for K, and potassium reacts more vigorously.

The group tells you what kind of reaction happens; the period tells you how easily it happens. Reading ionization energy data like this is covered in AP Chemistry ionization energy.

Worked Example 3: Explaining the States of the Halogens

At 25°C, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Explain this trend using the boiling points: F₂ −188°C, Cl₂ −34°C, Br₂ 59°C, I₂ 184°C.

Type of attraction: halogen molecules are nonpolar, so the only forces between them are London dispersion forces.
Trend: going down the group, the molecules have more electrons in larger, more polarizable electron clouds, so the dispersion forces get stronger.
Result: more energy is needed to separate the molecules, so the boiling point rises, from below room temperature (F₂, Cl₂) to above it (Br₂, I₂).

The noble gases follow the same pattern, with boiling points rising from helium (−269°C) to radon (−62°C). The mechanism is covered in AP Chemistry London dispersion forces.

Worked Example 4: Identifying an Element From Its Group and Period

An element is in period 4 and group 16. Identify it, give its valence electron count and configuration, classify it, and predict its usual ion.

Element: period 4, group 16 is selenium (Se), atomic number 34.
Valence electrons: group 16 means 6 valence electrons (4s²4p⁴).
Configuration: [Ar]4s²3d¹⁰4p⁴.
Type and ion: a nonmetal to the right of the staircase line, so it gains 2 electrons to form Se²⁻, isoelectronic with krypton.

The period sets the shell (n = 4), and the group sets how many electrons are in it. Together they pin down the element and most of its chemistry.

Five Periodic Table Group Practice Questions With Answers

1. Which family does strontium belong to, and what ion does it form?
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   The alkaline earth metals (group 2). Strontium forms Sr²⁺.
2. Is hydrogen a metal? Why is it placed in group 1?
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   No, hydrogen is a nonmetal. It sits in group 1 because its configuration, 1s¹, has one valence electron, like the alkali metals.
3. Which halogen is a liquid at room temperature?
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   Bromine, Br₂, which boils at 59°C.
4. Which element is in period 3, group 15, and how many valence electrons does it have?
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   Phosphorus (P), a nonmetal with 5 valence electrons.
5. Which reacts more vigorously with water, lithium or cesium? Explain.
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   Cesium. Its valence electron is in the n = 6 shell, much farther from the nucleus, so it has the lowest ionization energy of the two and loses its electron most easily.

Common Periodic Table Group Mistakes

• Mixing up groups and periods. Groups are columns; periods are rows.
• Counting hydrogen as an alkali metal. It is in group 1 but is a nonmetal.
• Confusing alkali metals (group 1) with alkaline earth metals (group 2).
• Writing halogens as single atoms. The elements are diatomic: Cl₂, not Cl.
• Assuming reactivity always increases down a group. It does for metals but decreases for the halogens.
• Using the group number for transition-metal charges. Their charges vary, and the name tells you, as in iron(III).

Reading the periodic table quickly saves time on every unit of the exam. To see how stronger multiple-choice answers would move your overall estimate, try the AP Chem Score Calculator.

Related Resources

• AP Chem Score Calculator
• Unit 1 Review: Atomic Structure and Properties
• AP Chemistry Periodic Table
• AP Chemistry Periodic Trends
• AP Chemistry Valence Electrons
• AP Chemistry Ions
• How to Write Electron Configurations
• AP Chemistry Ionic vs Covalent Bonds
• AP Chemistry Study Guide

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