THIS TOPIC DEALS WITH THE FIRST TWENTY ELEMENTS ON THE PERIODIC TABLE, THE STRUCTURE OF THE ATOM, PROPERTIES OF COMPOUNDS AND HOW THEY ARE FORMED, MIXTURES AND TYPES.
ELEMENTS
An element is a simple chemical substance which is made up of only one kind of atom. All the elements in have been arranged on a special table in groups and periods known as the Periodic Table. There are over 105 elements in the world. Examples include hydrogen, helium , lithium, etc.
Emphasis is placed on the firs twenty elements and the very common elements we use in everyday life. These include:
Iron
Mercury
Silver
Copper
The Atomic Structure
The atom has a central part called nucleus which is made up of two sub-atomic particles called protons and neutrons. Protons are positively charged and the neutrons are neutral or carry no charge. Around the nucleus are shells which carry electrons. The electrons are negatively charged. The number of protons is always equal to the number of electrons and this is called atomic number or proton number. Atomic number is represented by the letter ‘z’ and written at the left sub-script of the element’s chemical symbol. The number of protons and neutrons in the nucleus of the atom is called Mass number. The mass number is represented by the letter ‘A’ and written at the left super-script of the chemical symbol of the element.
COMPOUNDS
It is the substance formed when two or more elements combine chemically.
Properties of compounds
- They cannot be separated through physical means
- They have different properties from their constituents
- Elements combine in fixed ratios to form compounds.
- Constituents of compounds are not seen lying side by side
Examples of compounds are:
water
starch
sand
wood
petrol
kerosene etc
MIXTURES
They are substances formed when two or more substances combine physically. Mixtures can be categorized under:
Solid-solid
Solid-liquid
Solid- gas
Liquid-liquid
Liquid-gas
Gas-gas
A mixture is either homogenous or heterogenous depending on the level of variation.
A homogenous mixture is the one in which the particles are evenly mixed. An example is a solution.
A heterogenous mixture has its particles not evenly mixed up. Examples include suspension and colloid.
SOLUTION
It is a homogenous mixture of a solute and solvent.
Salt and sugar solutions are examples.
Solute
A solute is a substance that dissolves in a solution. Examples are
- Salt
- Sugar
- Monosodium glutamate
- Iodine crystals
- Ink
- Pigment
- Poster colours
- Lacquer
- Enamel paint etc
Solvent
A solvent is a substance that dissolves the solute in a solution. Examples of solvents include:
- water ( the universal solvent )
- Petrol
- Kerosene
- Methylated spirit
- Thinner
- Turpentine
- Vanish
- Nail remover etc