Chapter 4- Elements, Compounds and Mixtures
- Atom: the smallest particle of an element that has the same chemical properties as that element/monoatomic element, e.g. xenon, argon.
- Molecule: group of two or more atoms that are chemically combined together, e.g. iodine, ozone.
- Element: a pure substance made of atoms that cannot be broken down into simpler substances by chemical processes, e.g. carbon, sodium.
- Compound: a pure substance made of two or more elements chemically combined in a fixed ratio, e.g. water, ammonia.
- Mixtures: two or more substances that are not chemically combined and are in an unfixed ratio, e.g. air, blood.
ELEMENTS:
- Chemical symbols are used to represent elements. The periodic table consists of all elements arranged in order of increasing proton number.
- Elements may be monoatomic, e.g. Ne, C, K, diatomic, e.g. H
2
, N2
, Cl2
, or polyatomic, e.g. O3
, P4
, S8
- Elements are characterised as either metallic or non-metallic.
Metals | Non-metals |
---|
Lusturous | Dull appearance |
Usually solids at rtp | Liquids, gases or solids with low boiling points at rtp. |
Malleable, ductile, and sonorous | Brittle |
Typically high melting and boiling points. | Typically low melting and boiling points. |
Good conductors of heat and electricity | Poor conductors of heat and electricity |
NAMING COMPOUNDS
- The name of the metal element is always written first.
- A compound made up of two elements has a name that ends with -ide, e.g. Sodium Chloride.
- A compound with hydroxide ions is named a hydroxide, e.g. Sodium hydroxide.
- A compound with a polyatomic anion containing oxygen ends with -oate, e.g. Sodium nitrate.
- For compounds containing metallic and non-metallic elements, write the symbol of the metallic element first.
- Number of atoms of each element is written as a subscript to the right of each element. 1 is not written.
- Oxygen atom is written at the end of the formula.
- Some chemical formulae contain brackets if the number of atoms is common and the compound contains more than two elements.
DIFFERENCES BETWEEN COMPOUNDS AND MIXTURES
Compounds | Mixtures |
---|
Physical and chemical properties are different from its components. | Chemical properties are the same as its components. |
Chemical reactions take place when a compound is formed and there is energy change. | No chemical reactions take place when a mixture is formed and there is little or no energy change. |
The components have to be mixed in a fixed ratio. | The components can be mixed in any ratio. |
Only separated by chemical reactions (e.g. decomposition) or by electrolysis. | Separated by physical processes such as filtration and crystallisation. |
Examples include ammonia, water and salt. | Examples include alloys, blood and air. |