science key terms 2026 sem2

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electrically neutral

Having an equal number of protons and electrons.

electron

A subatomic particle that orbits the nucleus of an atom; it is negatively charged.

electron configuration

The number of electrons in each shell of an atom, starting with the smallest; for example, 2,8,8,2

electron shell

The space around an atom's nucleus, in which electrons circulate.

matter

Any substance that has mass and takes up space.

molecule

A unit made up of two or more atoms chemically bonded to each other

neutron

A subatomic particle located in the nucleus of an atom; it is neutrally charged.

nucleus

The centre of an atom, which contains protons and neutrons.

proton

A subatomic particle located in the nucleus of an atom; it is positively charged.

subatomic particles

Particles that make up atoms; protons, neutrons, and electrons.

valence shell

The electron shell furthest away from the nucleus of an atom; occupied by electrons.

alpha particle

A positively charged particle that is four times larger than a proton.

cathode ray

A stream of electrons observed in a high-vacuum tube.

spectral line

A dark or bright line in an otherwise uniform spectrum.

atomic mass

The average mass of atoms in a sample of an element.

atomic number

The number of protons in an atom.

chemical symbol

A symbol of one or two letters to represent an element.

periodic table

A table of all known elements and their chemical symbols.

group

A column in the periodic table.

period

A row in the periodic table.

reactive

Undergoes chemical reactions easily.

valence electron

An electron in the valence shell.

chemical formula

the expression of the elements that make up a chemical compound, usually presented as a ratio using chemical symbols and numbers.

energy

The ability to do work.

kinetic energy

The energy of movement.

mass

The amount of matter that a physical body contains.

motion

The change in position of an object over time.

potential energy

Stored energy; has the ability to cause movement.

transfer

To move from one place or object to another.

transform

To change from one type to another.

work

When a force applied to an object causes it to move.

conduction

The transfer of energy through a substance.

conductor

A substance that allows the transfer of energy.

convection

The transfer of energy by movement of a liquid or gas.

heat

The flow of thermal energy between objects.

insulator

A substance that resists the transfer of energy.

radiation

The transfer of energy that does not require contact with matter.

system

A set of simple things that work together to perform a function.

closed system

Energy can flow in and out, but matter cannot.

conservation of energy

Scientific law stating that energy cannot be created or destroyed.

isolated system

Neither energy nor matter can flow in or out.

open system

a system that allows energy and matter to transfer in and out

chemical change

A change in properties with a new substance formed.

physical change

A change in appearance with no new substance formed.

property

A characteristic of a substance affecting its look and behavior.

reversible

Can be changed back to its previous state.

chemical reaction

A process where substances are changed to form new substances.

conservation of mass

Scientific law stating that mass cannot be created or destroyed.

product

A substance formed in a chemical reaction.

reactant

A substance that undergoes a chemical change.

word equation

A representation of a chemical change in words.

combustion

The reaction of a substance with oxygen to give off heat.

corrosion

The reaction of a metal with oxygen.

endothermic

A reaction that absorbs energy in the form of heat.

exothermic

A reaction that releases energy in the form of heat.

asthenosphere

The portion of Earth's mantle underneath the lithosphere that can flow.

atmosphere

The layer of gas that surrounds Earth.

core

Earth's central layer with a liquid outer and solid inner core.

crust

Earth's thin outer layer of continental and oceanic crust.

density

How heavy something is for its size; mass divided by volume.

lithosphere

Earth's rigid outer zone made up of tectonic plates.

mantle

Earth's middle layer made up of an upper and lower mantle.

tectonic plate

A section of Earth's lithosphere.

continental drift

The theory that the continents have moved over time.

mid-ocean ridge

A long chain of underwater mountains formed by plate tectonics.

rift valley

A valley formed when a continent was pulled apart.

subduction

Where one tectonic plate moves underneath another.

convergent boundary

Where two tectonic plates move toward each other and collide.

divergent boundary

Where two tectonic plates move away from one another.