The Periodic Table and Periodic Trends

Development of the Periodic Table

The modern periodic table arranges elements in order of increasing atomic number into rows called periods and columns called groups. Elements in the same group have similar chemical properties because they have the same number of electrons in their outermost (valence) shell.

Structure of the Periodic Table

• Periods (1–7) — horizontal rows; elements across a period have the same number of electron shells.
• Groups (1–18) — vertical columns; elements in a group have the same number of valence electrons.
• Elements are broadly classified as metals, non-metals and metalloids.

Periodic Trends

• Atomic radius decreases across a period (left to right) and increases down a group.
• Ionisation energy generally increases across a period and decreases down a group.
• Electronegativity increases across a period and decreases down a group.
• Metallic character decreases across a period and increases down a group.

Why These Trends Occur

Across a period, protons are added to the nucleus, increasing nuclear charge and pulling electrons closer, so atoms get smaller and hold their electrons more tightly. Down a group, extra electron shells are added, so outer electrons are farther from the nucleus and less tightly held.

Exam Relevance

• Periodic trends explain why sodium (Group 1) is highly reactive while chlorine (Group 17) readily gains an electron.
• Understanding trends helps predict reactivity in WAEC, NECO and JAMB objective and theory questions.