NCEA L1 ChemBio key terms

The smallest particle of an element that still has that element's chemical properties. It contains protons and neutrons in a central nucleus, with electrons arranged around the nucleus.

A pure substance made of only one kind of atom, so it cannot be split into simpler substances by chemical means. Each element has a unique number of protons.

A pure substance formed when two or more different elements are chemically bonded together in fixed proportions. Its properties differ from those of the elements it contains.

A combination of two or more substances that are not chemically joined, so each keeps its own properties. Mixtures can usually be separated by physical methods such as filtering or evaporation.

The number of protons in the nucleus of an atom, which identifies the element. In a neutral atom it also equals the number of electrons.

The total number of protons plus neutrons in the nucleus of an atom. Electrons are ignored because their mass is negligible.

Atoms of the same element that have the same number of protons but different numbers of neutrons. Isotopes have the same chemical properties but slightly different masses.

An atom or group of atoms that has gained or lost electrons, giving it an overall electric charge. Losing electrons makes a positive ion; gaining electrons makes a negative ion.

The arrangement of electrons in the shells or energy levels around an atom's nucleus. It explains an element's position in the periodic table and how it is likely to react.

An electron in the outermost shell of an atom. Valence electrons are the ones involved in bonding and largely determine an element's chemical behaviour.

A table organising elements by increasing atomic number so that elements with similar properties line up in columns. Its rows and columns reveal repeating patterns in chemical behaviour.

A vertical column in the periodic table. Elements in the same group have the same number of valence electrons and therefore similar chemical properties.

A horizontal row in the periodic table. Moving across a period, atoms gain protons and electrons, and the number of electron shells occupied stays the same.

A lasting attraction that holds atoms together in a compound or molecule. Bonds form when atoms share, gain or lose electrons to become more stable.

The strong attraction between positively and negatively charged ions, formed when electrons are transferred from one atom to another. Ionic compounds usually form lattices and conduct electricity when molten or dissolved.

A bond formed when two atoms share one or more pairs of electrons. Covalent bonds typically form between non-metal atoms and hold molecules together.

A particle made of two or more atoms joined by covalent bonds. A molecule can contain atoms of the same element or of different elements.

A shorthand using element symbols and small numbers to show the type and number of atoms in a substance. For example, H2O shows two hydrogen atoms and one oxygen atom.

A written statement using formulas to show the reactants and products of a chemical reaction. A balanced equation has the same number of each type of atom on both sides.

A starting substance that is used up during a chemical reaction. Reactants appear on the left-hand side of a chemical equation.

A new substance formed during a chemical reaction. Products appear on the right-hand side of a chemical equation and have different properties from the reactants.

A process in which atoms are rearranged to form new substances with different properties. Bonds are broken and new bonds are formed, but atoms are never created or destroyed.

The principle that the total mass of the reactants equals the total mass of the products in a closed system. It follows because atoms are only rearranged, not created or destroyed.

A substance that releases hydrogen ions (H+) when dissolved in water. Acids turn blue litmus red and have a pH below 7.

A substance that can neutralise an acid, often by releasing hydroxide ions (OH-) in water. Bases have a pH above 7 and turn red litmus blue.

A scale from 0 to 14 that measures how acidic or basic a solution is. Values below 7 are acidic, 7 is neutral, and values above 7 are basic.

The reaction between an acid and a base to produce a salt and water. It brings the pH of the mixture closer to neutral.

A separation method that uses a filter to trap solid particles while a liquid passes through. It separates an insoluble solid from a liquid or solution.

A separation method in which a liquid is boiled into vapour and then condensed back to liquid. It separates substances with different boiling points, such as water from dissolved salt.

The maximum amount of a substance that will dissolve in a given amount of solvent at a particular temperature. Substances that do not dissolve are described as insoluble.

The smallest unit of life that can carry out all the processes needed to survive. All living organisms are made of one or more cells.

A specialised structure inside a cell that performs a particular job. Examples include the nucleus, mitochondria and chloroplasts.

A thin layer surrounding a cell that controls which substances enter and leave. It is partly permeable, letting some molecules pass while blocking others.

The organelle that contains a cell's genetic material (DNA) and controls the cell's activities. It is found in plant and animal cells but not in bacterial cells.

Organelles where aerobic respiration takes place, releasing energy from food molecules. Cells that need lots of energy, such as muscle cells, contain many mitochondria.

The organelle in plant cells that contains the green pigment chlorophyll and carries out photosynthesis. It captures light energy to make food from carbon dioxide and water.

The movement of particles from an area of higher concentration to an area of lower concentration. It happens without energy input and is how many substances move into and out of cells.

The movement of water molecules across a partly permeable membrane from a region of higher water concentration to a region of lower water concentration. It is a type of diffusion.

The process in which green plants use light energy to convert carbon dioxide and water into glucose and oxygen. It takes place mainly in the leaves and requires chlorophyll.

The chemical process in cells that releases energy from glucose, usually using oxygen and producing carbon dioxide and water. It supplies the energy cells need to live and grow.

The molecule that carries genetic instructions for how a living thing develops and functions. It is found mainly in the nucleus and is passed from parents to offspring.

A section of DNA that holds the instructions for making a particular protein or controlling a particular trait. Genes are passed from parents to offspring.

A community of living organisms interacting with each other and with their non-living surroundings, such as soil, water and climate. Energy and matter flow through an ecosystem.