Atoms & the periodic table · Topic 4 of 15

The Periodic Table & Its Trends

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Chemistry · periodic table

The Periodic Table & Its Trends

Elements sit in order of atomic number in periods (rows) and groups (columns); a group shares valence electrons, and size and electronegativity trend opposite ways.

01 · The layoutRows = periods · columns = groups

02 · Where they sitMetals left, nonmetals right, metalloids between

03 · ElectronegativityElectronegativity: up and right (F highest)

What it is

Ordered by atomic number: 7 rows = periods, 18 columns = groups must know

Same group → same valence electrons → similar chemistry and ion charge must know

Period number = number of electron shells

Parts & jobs

Transition metals: groups 3–12 · lanthanides and actinides: the 2 bottom rows

Know by heart

Group 1 alkali metals: 1 valence e⁻, +1 ions, the most reactive metals

Group 2 alkaline earth metals: 2 valence e⁻, +2 ions (Mg²⁺, Ca²⁺)

Group 17 halogens: 7 valence e⁻, gain 1 → −1 ions, very reactive

Group 18 noble gases: full outer shell, almost unreactive (inert)

Latin symbols: Na sodium, K potassium, Fe iron, Ag silver, Au gold, Pb lead

The rules

Atom size grows down a group and shrinks left → right across a period must know

Electronegativity rises up a group and to the right; fluorine is highest must know

Reactivity: metals ↑ down and left · nonmetals ↑ up and right (fluorine)

Worked examples

Largest → smallest: K > Na > Mg > Cl (down = bigger, right = smaller)

Don't confuse

Metals left and center · nonmetals upper right · metalloids on the staircase must know

Metal or nonmetal?

Commonly confused

	Metals	Nonmetals
Where	Left and center	Upper right
Look and feel	Shiny, malleable	Dull
Heat and electricity	Good conductors	Poor conductors
In reactions	Lose e⁻ → cations	Gain e⁻ → anions
Examples	Sodium, magnesium, iron	Oxygen, chlorine, fluorine

Common mistakes

Trap: hydrogen sits in group 1 but is a nonmetal

More detail

Show 5 more facts

Ionization energy rises across a period and falls down a group

Metalloids (boron, silicon) are typically semiconductors

Group 15 = nitrogen family · group 16 = oxygen family (chalcogens)

Main-group elements: groups 1, 2 and 13–18

Metals lose e⁻ (cations); nonmetals gain e⁻ (anions) or share them

Pearl

Atoms get bigger going down and to the left; electronegativity gets bigger going up and to the right.

Sources · OpenStax Chemistry 2e §2.5 The Periodic Table · OpenStax Chemistry 2e §6.5 Periodic Variations in Element Properties · OpenStax Chemistry 2e §7.2 Covalent Bonding (electronegativity trend) · OpenStax Chemistry 2e §18.1 Periodicity (alkali metals, metalloids) · OpenStax Chemistry 2e §18.11 Occurrence, Preparation, and Properties of Halogens · OpenStax Chemistry 2e §2.3 Atomic Structure and Symbolism (element symbols, Table 2.3) · OpenStax Chemistry 2e §18.3 Structure and General Properties of the Metalloids · Union Test Prep — HESI Chemistry study guide, page 1 (the periodic table, periods, groups, valence electrons) · NurseHub — HESI A2 Chemistry review course (unit: Atomic Structure and The Periodic Table) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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