An atom is a building block of all matter (all things that are solid/liquid/gas). An atom the smallest single building block that makes up all mater. An atom is made of: protons, neutrons and electrons.


An element is 2 or more of the same atom.

E.g. – O3.

The periodic table of elements is a list of 118 elements known to science.

Atomic No: The number of protons in the nucleus.

The first letter is always uppercase. The second/third letter is always lowercase.


An atom is a single. An element is made of multiple atoms that are the same. A compound is made of multiple atoms that are different.


A compound is a special molecule that has 2 or more different atoms in a ratio.

E.g. – HNO3, CO2.


2 or more atoms in a special ratio.

E.g. – H2O.


An ion is a charged atom that has lost or gained an electron. Cation = lost electron. Anion = gained electron.

A cation is a positive ion it has LOST electrons.

Na+ – Sodium ion has lost 1 electron. Cation is a positive ion.

Mg2+ – Magnesium ion lost 2 electrons. 12+ 10-.

Li+ – Lithium ion 1 electron. 3+ 2-.

Ca2+ – Calcium lost 2 electron. 20+ 18-.

An anion is a negative ion it has GAINED electrons.

Cl – Chlorine has gained 1 electrons. 17+ 18-.

Br – Bromine has gained 1 electrons. 35+ 36-.

O2 – Oxygen has gained 2 electrons. 8+ 10-.


An isotope is an atom that has the same of the number of protons but different number of neutrons.

Atomic No: the number of PROTONS in the nucleus of an atom.

Mass No: the number of PROTONS + NEUTRONS in the nucleus of an atom.



A lens is a transparent piece of plastic or glass that is shaped to curve inwards or outwards. A lens refracts light.

There are two types of lenses: Convex and Concave


A convex lens bends light in, converges light to a point called the focal point. A convex lens is also called a converging lens.



A diverging lens spreads light out, diverges light out.

(light dies in a cave concave = diverging)



Light is a form of energy called electromagnetic radiation. Light travels as waves, e.g. UV rays, X-rays, infrared light, and microwaves. Light is the fastest thing in the universe. Light travels at a speed of 300,000 km per seconds.

  • Transparent: allowing light to pass through so that objects behind can be distinctly seen
  • Translucent: allowing light to pass through but not detailed shapes, to pass through
  • Opaque: not allowing light to pass through
High school Careers: Terms & Definitions


Radiation transmits heat as invisible waves that travels at the speed of light (around 300,000 km per second).

Forms of radiation:

  • Light
  • Infrared – night vision
  • Heat is radiated from fires
  • UV rays
  • Radiation for cancer kills cancer cells

Radiation can be:

  • Absorbed – taken into the object especially black and dark colours
  • Reflected – the heat energy bounces off as waves
  • Transmitted – transmit means ‘travels through’. The waves pass through glass


Regular reflection is when an image is seen in a flat surface. The light rays are reflected back.

Diffuse reflection is when no image is seen in a rough surface because the light is reflected back at all angles.



Refraction is the bending of the light. Refraction occur when it passes from a medium with a different density.



Solubility is how easily an ionic compound dissolves.


What is sound?

Sound is produced when something vibrates. E.g. speech – flaps of skin vibrate, and drum – strings inside the piano vibrate.

Sound is produced when something vibrates so it makes particles in the air vibrate.

A sound wave is the movement of alternating compressions and rarefactions.

Sound waves rely on particles to vibrate in: sold, liquid and air.


Compression is when vibrations creates regions of space in which the air particles are bunched together.


Rarefaction is when air particles are more spread out.


Radiation: is a form of heat transfer by invisible waves. Radiation does not transfer from particle to particle, instead it travels through vacuum.

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Cite this article as: William Anderson (Schoolworkhelper Editorial Team), "Grade 7 and 8 Science Definitions," in SchoolWorkHelper, 2019,
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