Structure of Matter
26 concepts in Chemistry
The structure of matter explores what everything around us is made of and how the arrangement of atoms and molecules determines the properties we observe. Students begin with atomic structure — protons, neutrons, and electrons — and learn how the periodic table organizes elements by their properties and electron configurations. They study how atoms bond together through ionic, covalent, and metallic bonding, and how molecular shape influences a substance's behavior. The topic extends to states of matter (solid, liquid, gas) and the kinetic molecular theory that explains phase changes at the particle level. Intermolecular forces such as hydrogen bonding and van der Waals forces explain why water behaves differently from oil or steel. This foundational knowledge is essential for understanding biology, materials science, environmental chemistry, and nanotechnology.
Suggested learning path: Start with atomic structure and the periodic table, then study chemical bonding and molecular geometry, and finally explore states of matter, phase changes, and intermolecular forces.
Atom
The smallest unit of an element that retains the chemical properties of that element.
Element
A pure substance made entirely of one type of atom, uniquely defined by its number of protons.
Proton
A positively charged subatomic particle found in the nucleus of every atom, whose count defines the element's identity.
Neutron
A neutral subatomic particle found in the nucleus of an atom that has no electric charge but contributes to the atom's mass.
Electron
A negatively charged particle found outside the nucleus, occupying energy shells around the atom.
Atomic Number
The number of protons in an atom's nucleus, which uniquely identifies the element.
Mass Number
The total count of protons and neutrons in an atom's nucleus, used to identify isotopes.
Isotope
Atoms of the same element that have the same number of protons but different numbers of neutrons, giving them different mass numbers.
Electron Shell
A discrete energy level around the nucleus where electrons reside, with each shell holding a set maximum.
Valence Electron
An electron in the outermost energy shell of an atom, available for chemical bonding.
Periodic Table
An arrangement of elements by atomic number, showing periodic patterns in properties.
Ion
An atom or group of atoms that has gained or lost one or more electrons, resulting in a net positive or negative electric charge.
Molecule
Two or more atoms held together by chemical bonds, acting as a single distinct unit.
Compound
A pure substance made of two or more different elements chemically bonded in fixed proportions.
Mixture
A combination of two or more substances that are physically combined but not chemically bonded and can be separated by physical means.
Chemical Bond
A force of attraction between atoms that holds them together in molecules or compounds, formed by sharing or transferring electrons.
Ionic Bond
A bond formed when one atom transfers electrons to another, creating oppositely charged ions that attract.
Covalent Bond
A bond formed when two atoms share one or more pairs of electrons, holding them together.
Octet Rule
Atoms tend to gain, lose, or share electrons to achieve 8 electrons in their outer shell.
Lewis Structure
A diagram showing atoms, bonds, and lone pairs of electrons in a molecule, following the octet rule.
Electronegativity
A measure of how strongly an atom attracts the shared electrons in a covalent bond toward itself.
Electron Configuration
The arrangement of electrons in an atom's orbitals, described using quantum numbers and subshell notation.
Molecular Geometry
The three-dimensional arrangement of atoms in a molecule, determined by electron pair repulsion.
Polar Covalent Bond
A covalent bond in which electrons are shared unequally, creating partial positive and negative charges.
Radioactivity
The spontaneous emission of particles or energy from an unstable atomic nucleus.
Molecular Polarity
The overall separation of charge in a molecule, determined by bond polarities and molecular geometry.