Atomic Structure
Atoms are made of three subatomic particles: protons (positive, in nucleus), neutrons (neutral, in nucleus), and electrons (negative, orbiting in shells).
Atomic number = number of protons (defines the element). Mass number = protons + neutrons. In a neutral atom: protons = electrons.
The Periodic Table
Elements are organized by increasing atomic number. Elements in the same column (group) have similar chemical properties. Elements in the same row (period) have the same number of electron shells.
Metals (left/center): shiny, conduct electricity, malleable. Nonmetals (right): poor conductors, brittle. Metalloids (staircase): properties of both.