Chemistry - Terminology

Atom: The smallest particle of an element that can take part in a chemical reaction. It consists of a nucleus containing protons and neutrons, surrounded by electrons in shells.

Element: A pure substance made of only one type of atom. For example, Iron ($Fe$) or Oxygen gas ($O_2$).

Compound: A substance made of two or more different elements chemically combined in a fixed ratio, such as Water ($H_2O$) or Sodium Chloride ($NaCl$).

Molecule: A group of two or more atoms held together by covalent bonds. This can be an element (e.g., $Cl_2$) or a compound (e.g., $CO_2$).

Mixture: A combination of two or more substances that are not chemically joined and can be separated by physical means.

Atomic Number ($Z$): The number of protons in the nucleus of an atom. It defines the identity of the element.

Mass Number ($A$): The total number of protons and neutrons in the nucleus of an atom.

Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons. For example, $^{35}_{17}Cl$ and $^{37}_{17}Cl$.

Relative Atomic Mass ($A_r$): The weighted average mass of naturally occurring atoms of an element on a scale where an atom of Carbon-12 has a mass of exactly $12$ units.

Relative Molecular Mass ($M_r$): The sum of the relative atomic masses of the elements in a chemical formula.

Valency: The combining power of an element, determined by the number of electrons an atom needs to lose, gain, or share to reach a stable electronic configuration.

State Symbols: Used in chemical equations to show the physical state: $(s)$ for solid, $(l)$ for liquid, $(g)$ for gas, and $(aq)$ for aqueous solution.