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MDCAT Prep · Chemistry · Quick Reference
Lesson 8 — Atomic Structure Cheat Sheet
In one line: Across a period atoms shrink and hold electrons tighter; down a group they grow and lose electrons more easily.
Key Ideas
1Proton. +1 charge, in the nucleus. Count = atomic number.
2Neutron. 0 charge, in the nucleus. Protons + neutrons = mass number.
3Electron. −1 charge, in shells around the nucleus.
4Across a period (L→R). Atomic radius ↓, ionisation energy ↑, electronegativity ↑.
5Down a group. Atomic radius ↑, ionisation energy ↓, electronegativity ↓, metallic character ↑.
Trend Directions
ACROSS a period (Na → Cl):
radius ↓ IE ↑ EN ↑
DOWN a group (Li → Cs):
radius ↑ IE ↓ EN ↓ metallic ↑
Why radius shrinks across a period
Rising nuclear charge pulls electrons in
Why radius grows down a group
Extra shells + shielding push electrons out