Atomic structure basics
Every solid, liquid, gas, and living thing is built from the same three particles — and how they arrange themselves explains almost everything else in chemistry.
The three particles
An atom is mostly empty space. The nucleus holds protons (positive charge) and neutrons (no charge), packed into a tiny volume at the center. Circling far outside are electrons — lightweight, negatively charged, and the particles that actually do chemistry.
Protons and neutrons each weigh about 1 atomic mass unit (amu). An electron is roughly 1,836 times lighter, so almost all of an atom's mass sits in the nucleus.
Atomic number & mass number
The atomic number (Z) is the count of protons. It defines the element — change the proton count and the element changes. Carbon has Z = 6, gold has Z = 79, uranium has Z = 92.
The mass number (A) is protons + neutrons. Isotopes of an element have the same Z but different A (more or fewer neutrons). Carbon-12 and carbon-14 are both carbon; only the neutron count differs.
Reading the periodic table
Elements are arranged left-to-right by increasing atomic number. The period (row) tells you the highest electron shell in use. The group (column) tells you how many electrons are in the outermost shell — which controls bonding behavior.
Group 1 (alkali metals) all have one outer electron and react similarly. Group 18 (noble gases) all have full outer shells and barely react at all. The table is a map of recurring behavior, period after period.
Flashcard · Unit 1
The number of protons in the nucleus. Defines the element.
Check your understanding
An atom has 17 protons and 18 neutrons. Which element is it, and what is its mass number?