GRADE 8 Science MIXTURES,ELEMENTS AND COMPOUNDS – Structure of the atom Notes
Science — MIXTURES, ELEMENTS AND COMPOUNDS
Subtopic: Structure of the Atom (Age 13, Kenya)
What is an atom?
An atom is the smallest part of an element that still has the chemical properties of that element. Atoms are extremely small — too small to see with the eye or an ordinary microscope.
Main parts of an atom
- Nucleus — the centre of the atom. It contains protons and neutrons. It is very small but holds most of the atom’s mass.
- Electrons — tiny particles that move around the nucleus in energy levels or shells. Electrons have very little mass but affect chemical behaviour.
- Protons — positively charged particles in the nucleus. Each proton has one positive charge (+).
- Neutrons — neutral particles in the nucleus (no charge). They add mass and can make different isotopes.
Simple visual: a basic atom model
The nucleus (centre) has protons and neutrons. Electrons (e−) move around in shells.
Important terms
- Atomic number (Z) — number of protons in the nucleus. It defines the element. Example: Z of oxygen = 8.
- Mass number (A) — total number of protons + neutrons. Example: carbon commonly has A = 12 (6 protons + 6 neutrons).
- Isotopes — atoms of the same element with different numbers of neutrons (e.g., carbon-12 and carbon-14).
- Ion — an atom that has gained or lost electrons and so has a charge. A lost electron = positive ion (cation); a gained electron = negative ion (anion).
How atoms relate to elements, compounds and mixtures
Element — a substance made of only one type of atom. Examples: oxygen (O), iron (Fe), gold (Au). In Kenya: oxygen in air, iron in tools and nails.
Compound — a substance formed when atoms of different elements join chemically in fixed ratios. The properties change. Examples:
- Water (H2O): 2 hydrogen atoms + 1 oxygen atom.
- Sodium chloride (NaCl): sodium + chlorine (table salt).
- Sodium chloride (NaCl): sodium + chlorine (table salt).
Mixture — two or more substances mixed together but not chemically joined. Components keep their properties. Examples: air (mixture of gases), soil (mix of minerals and organic matter), tea (water + tea leaves).
Simple examples to remember
- Hydrogen atom: 1 proton, 0 neutrons, 1 electron (smallest atom).
- Helium atom: 2 protons, usually 2 neutrons, 2 electrons — noble gas, stable.
- Carbon in living things: carbon atoms combine to form many compounds (sugars, proteins).
- Air is a mixture: mostly nitrogen (N2) and oxygen (O2); each gas is made of atoms (or molecules) of elements.
Short activities (for practice)
- Draw an atom of lithium (3 protons). Show where the 3 electrons would be (use shells 2, 1).
- Give one example of an element, one of a compound and one of a mixture you see at home or school.
- Explain why salt dissolved in water is a mixture but when salt reacts chemically it forms a compound (think about what happens to particles).
Key points to remember
- Atoms = building blocks of matter.
- Protons define the element (atomic number).
- Electrons determine how atoms bond to form compounds.
- Mixtures are physical blends; compounds are chemical combinations.
Note: These ideas help you understand chemical reactions, why materials behave differently, and why some substances are useful (for example, iron for building, water for life).