Early atomic theory began with Greek philosophers, who believed all matter was made of four elements; Democritus introduced the concept of the atom as an indivisible particle.

Key laws in atomic theory:

• Law of Conservation of Mass (Lavoisier): Mass is neither created nor destroyed in chemical reactions.

• Law of Definite Proportions (Proust): Compounds have constant composition by mass.

Dalton's Atomic Theory (1808) established that:

• All matter is made of solid, indivisible atoms.

• Atoms retain their identity in chemical reactions.

• Atoms of the same element are identical; atoms of different elements differ in mass and properties.

• Compounds are formed from atoms in small, whole-number ratios.

Atoms are composed of three subatomic particles:

• Protons (positive charge, in nucleus)

• Neutrons (neutral, in nucleus)

• Electrons (negative charge, outside nucleus)

Most of an atom's volume is empty space; the nucleus is extremely small but contains most of the atom's mass.

The overall charge of an atom is determined by the difference between the number of protons and electrons: Q=p−e

Each atom is defined by:

• Atomic number (Z): Number of protons

• Mass number (A): Number of protons + neutrons (nucleons)

A=p+n

Changing the number of protons changes the element; atoms with the same atomic number but different mass numbers are called isotopes.

Isotopes have important applications in science, including tracing and dating samples in biology, geology, and archaeology. For example, 14C in tooth enamel can be used to estimate year of birth.

Isotopes of an element differ in mass; this difference allows their identification and quantification using mass spectrometry, which produces a spectrum showing the relative abundance of each isotope.

Most elements exist as mixtures of isotopes; the atomic mass on the periodic table is a weighted average of all naturally occurring isotopes: ∑(fractional abundance×isotope mass)

Example: Silicon has three naturally occurring isotopes, and the average atomic mass is calculated using their masses and abundances.