Atoms

Definition

An atom is the smallest unit of ordinary matter that retains the chemical properties of an element. Atoms consist of a nucleus (protons and neutrons) surrounded by electrons in orbitals.

Subatomic particles

• Proton: positively charged particle in the nucleus (defines atomic number).
• Neutron: neutral particle in the nucleus (contributes to mass and isotopes).
• Electron: negatively charged particle in orbitals (governs chemical behavior).

Structure

• Nucleus: dense core containing protons and neutrons.
• Electron cloud: region around the nucleus with electrons arranged in shells and orbitals.
• Atomic number (Z): number of protons — identifies the element.
• Mass number (A): total protons + neutrons.

Isotopes

Atoms of the same element with different numbers of neutrons. Isotopes can be stable or radioactive.

Chemical behavior

Chemical reactions are driven primarily by interactions of electrons in the outermost shell (valence electrons). Bonding types include ionic, covalent, and metallic.

Typical properties

• Size: on the order of 0.1–0.5 nm (atomic radius varies by element).
• Mass: measured in atomic mass units (u), where 1 u ≈ 1/12 mass of 12C.

Examples

• Hydrogen (H): simplest atom, 1 proton, 0 neutrons (most common isotope).
• Carbon (C): common element in organic chemistry; isotopes include 12C and 14C.

Further reading / related notes

• Nuclear structure
• Electron configurations and the periodic table
• Chemical bonding basics