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. H2, N2, Cl2, or polyatomic, e.g. O3, P4, S8
• Elements are characterised as either metallic or non-metallic.

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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.

STEPS FOR WRITING CHEMICAL FORMULAE

1. For compounds containing metallic and non-metallic elements, write the symbol of the metallic element first.
2. Number of atoms of each element is written as a subscript to the right of each element. 1 is not written.
3. Oxygen atom is written at the end of the formula.
4. 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

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