The Atom
An atom is the smallest particle of an element that retains the chemical properties of that element.
Sub-Atomic Particles
| Particle | Symbol | Charge | Mass | Location |
|---|---|---|---|---|
| Proton | p⁺ | +1 | 1 amu | Nucleus |
| Neutron | n⁰ | 0 | 1 amu | Nucleus |
| Electron | e⁻ | −1 | ~0 | Electron shells |
Key Terms
- Atomic number (Z): number of protons in the nucleus
- Mass number (A): protons + neutrons
- Electron configuration: arrangement of electrons in shells (2, 8, 8, 2, ...)
- Isotopes: atoms with the same atomic number but different mass numbers (different neutron count)
The Periodic Table
Elements are arranged in order of increasing atomic number. The table has periods (horizontal rows) and groups (vertical columns).
- Group I — Alkali metals (Na, K, Li): very reactive
- Group VII — Halogens (F, Cl, Br): reactive non-metals
- Group VIII/0 — Noble gases (He, Ne, Ar): inert/stable
- Transition metals (Fe, Cu, Zn): in the middle block
Carbon example: ¹²C → Z=6 (6 protons, 6 electrons), A=12 (6 neutrons)