2.6Chapter 2: Elements, Compounds and Mixtures

Compounds — Alhussan Chemistry - Stage 7 Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 13 4 quizzes

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2.6 Compounds — Chemical Bonding and New Properties — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define compound and molecule, explain how compounds form and why a compound's properties differ from those of the elements in it.Compound = a substance made up of …

Fixed ratio
Separation
Carbon compounds
Limitations
Chemical bonds
Hydrogen, oxygen and water

Section 1 of 9What a compound is

Compound

Compound a substance made up of atoms of two or more different elements strongly joined together, in fixed proportions.

An element has one type of atom, but a compound has two or more types.

Example

Example in tooth enamel the atoms of calcium, phosphorus and oxygen are not just mixed up but joined to make one substance, calcium phosphate.

Fixed ratio

Fixed ratio water, H₂O, always has two hydrogen atoms for every oxygen atom, and carbon dioxide always has one carbon atom for every two oxygen atoms.
This fixed ratio is why each compound has its own formula.

Separation

Separation a compound can only be split into elements by a chemical reaction, such as strong heating or electricity, and not by filtering.

Section 2 of 9Molecules

Molecule

Molecule a particle made up of two or more atoms strongly joined together.

Hydrogen

Hydrogen in hydrogen gas atoms join in pairs, so the two-atom particles are molecules of hydrogen.

Oxygen

Oxygen each molecule has two oxygen atoms.

Water

Water each molecule has three atoms: one oxygen atom joined to two hydrogen atoms.

Carbon compounds

Carbon compounds carbon and oxygen make two different compounds.
A carbon monoxide molecule has one carbon atom joined to one oxygen atom, and the gas is poisonous.
A carbon dioxide molecule has one carbon atom joined to two oxygen atoms,
- and the gas is not poisonous but is a greenhouse gas that makes the Earth's surface hotter.

Section 3 of 9Modelling molecules

Strength

Strength a model of molecules made from coloured balls shows the number and type of atom in each particle.

Limitations

Limitations real atoms are not coloured balls, and the model does not show that hydrogen and oxygen atoms are different sizes.

Section 4 of 9How compounds form

Chemical bonds

Chemical bonds atoms join by forming bonds that involve their outer electrons.
In covalent bonding atoms share electrons, which is typical of non-metals as in water and carbon dioxide.
In ionic bonding electrons transfer from a metal to a non-metal so charged ions form and attract each other, as in sodium chloride.

Reaction

Reaction a chemical reaction takes place and new substances with new properties are formed.

Section 5 of 9New properties

New properties

Reason

Reason the properties of a substance depend on its particles, and elements and compounds have different particles.

Hydrogen, oxygen and water

Hydrogen, oxygen and water hydrogen is a gas that burns easily, oxygen is a gas that helps other substances burn,
- and water is a liquid that puts out fires.

Tooth enamel

Tooth enamel calcium fizzes in water and phosphorus is poisonous and catches fire easily, yet the compound enamel protects teeth.

Compound	Elements	Properties of the compound
Sodium chloride	Sodium (soft, reactive metal) and chlorine (toxic gas)	Safe white solid used as food seasoning
Water	Hydrogen (flammable gas) and oxygen (supports burning)	Liquid that puts out fires
Iron sulfide	Iron (magnetic) and sulfur (yellow)	Grey-black and not magnetic

Section 6 of 9Compound or mixture

1Mixture of iron and sulfur

Mixture of iron and sulfur a magnet can separate it, because each substance keeps its own properties and they can be mixed in any ratio.

2Heating them strongly

Heating them strongly it makes iron sulfide, a compound, and energy is given out.
A magnet no longer works because the iron is chemically bonded to the sulfur.
The new substance cannot be separated by physical means.

Section 7 of 9Evidence of chemical change

Signs

Signs a change in colour, heat or light given out, a gas produced, a new solid appearing, or a change that cannot easily be reversed.

Magnesium

Magnesium burning it in air gives a bright white flame and white magnesium oxide, whose mass is greater than the magnesium because oxygen has joined it.
Atoms are conserved, and the extra mass is the oxygen bonded to the metal.

Section 8 of 9Key terms

Term	Meaning
Compound	A substance with atoms of two or more elements strongly joined together.
Molecule	A particle of two or more atoms strongly joined together.
Chemical bond	A force of attraction that holds atoms together.
Mixture	Substances together that are not chemically joined.
Element	A substance with only one type of atom.
Formula	Symbols and numbers showing the atoms in a substance.