Science · Grades 8–12
Atomic structure and chemical language · quick reference
Distinguish mass number from average atomic mass. Symbols and subscripts identify atoms; coefficients count whole formula units.
Key terms at a glance
Atom
The smallest unit of an element that retains that element's chemical identity.
Element
A substance whose atoms all have the same atomic number.
Compound
A pure substance made from two or more elements chemically combined in a fixed ratio.
Molecule
An electrically neutral group of atoms bonded together as a discrete unit.
Proton
A positively charged particle found in an atomic nucleus.
Neutron
An electrically neutral particle found in an atomic nucleus.
Electron
A negatively charged particle associated with an atom's surrounding electron structure.
Atomic number
The number of protons in an atomic nucleus.
Mass number
The total number of protons and neutrons in one nucleus.
Isotope
An atom with the same proton number as another atom but a different neutron number.
Ion
An atom or bonded group with a net electric charge.
Cation
An ion with a positive charge.
Anion
An ion with a negative charge.
Ionic bond
Electrostatic attraction between oppositely charged ions.
Covalent bond
A chemical bond involving shared electron density between atoms.
Reactant
A starting substance in a chemical reaction.
Product
A substance formed by a chemical reaction.
Catalyst
A substance that increases reaction rate without being consumed overall.
Exothermic reaction
A reaction that releases heat to the surroundings under the specified conditions.
Endothermic reaction
A reaction that absorbs heat from the surroundings under the specified conditions.
Mole
The SI unit of amount of substance, containing exactly 6.02214076 × 10²³ specified entities.
Molar mass
Mass per mole of a specified substance.
Concentration
The quantity of a component relative to a stated amount of solution or mixture.
Electrolyte
A substance that produces mobile ions when dissolved or melted and can conduct electricity.
pH
A logarithmic measure related to hydrogen-ion activity in a solution.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Learning preview · teacher review required. Grade bands are approximate, not official curriculum alignment.