SS1 First Term Chemistry Lesson Note – Atoms; Molecules and Atomicity

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PERIOD 3: ATOMS, MOLECULES AND ATOMICITY

ATOMS

DEFINITION: An atom is the smallest particle of an element which can take part in a chemical reaction.

STRUCTURE AND CONSITITUENTS OF AN ATOM

 An atom is made up of the three sub-particles known as protons, neutrons and electrons as shown in the diagram below. Their characteristics are also summarized in the table below:

CHARACTERISTICS OF THE SUB-PARTICLES

SUB-PARTICLE	LOCATION	RELATIVE CHARGE	RELATIVE MASS	SYMBOL
Proton	Nucleus	+	1	p
Electron	Outside nucleus	_	0.005	e–
Neutron	Nucleus	Zero	1	n

MOLECULES

DEFINITION: A molecule is the smallest particle of a substance that normally exist alone and still retain the chemical properties of that substance be it an element or a compound.

Some molecules can exist independently as single atoms e.g. He, Ne, Ar, Xe etc. Some molecules may be made up of atoms of the same element e.g., a molecule of hydrogen is H_2, that of chlorine is Cl₂, Oxygen is O_2, phosphorus is P₄, Sulphur is S_8. Some molecules may be made up of different elements e.g., a molecule of water is H₂O, Methane is CH₄, ammonia is NH₃, carbon (iv) oxide is CO₂ etc.

ATOMICITY: Atomicity is the number of atoms in each molecule of an element.

Atomicity of some molecules

Element	Formula of molecule	Atomicity
Hydrogen	H2	2
Oxygen	O2	2
Ozone	O3	3
Phosphorus	P4	4
Sulphur	S8	8

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