Atoms, Molecules and Compounds Atom -an atom is the smallest part of an element -matter is made up of tiny particles called atoms. This means that everything is made from atoms. An atom is the smallest particle that can exist. John Dalton proposed the Atomic theory, which states that Matter consists of very small particles ( atoms) Atoms cannot be broken into smaller parts Atoms of one element are the same. They have same size , mass and properties Atoms of different elements have different size, mass and properties When atoms combine as small whole numbers they form compounds Atoms are shown in diagrams as small circles. Different colours or sizes of spheres are used to show different atoms. Fig 1.1 shows Oxygen and Hydrogen atoms: Fig 1.1 Oxygen and Hydrogen atoms Note that each atom of Hydrogen and Oxygen is shown as a single sphere. Element An element is made up of only one type of atom. 1|Page An example of an element is Copper. A piece of pure Copper is made up of only of Copper atoms only. There are 118 known elements on Earth and they are all listed in the Periodic table Each element has its own name and symbol. Symbols are used to represent elements and each element from the Periodic table has a symbol. This symbol can be made up of one or two letters but it always starts with a capital letter. For example, the symbol for Nitrogen is N and the symbol for Magnesium is Mg. Table 1.1 shows the common elements and their symbols Table 1.1: Common elements and their symbols Element Carbon Nitrogen Oxygen Hydrogen Sulphur Chlorine Magnesium Copper Iron Symbol C N O H S Cl Mg Cu Fe Molecule atoms can combine to form molecules a molecule is the smallest particle of a compound A molecule describes two or more atoms bonded together (all compounds are molecules The atoms of elements like Hydrogen join as pairs, making a molecule. 2|Page Molecules are shown in diagrams by drawing two or more spheres joined which are joined together. Fig 1.2 and 1.3 shows the molecules of Hydrogen, Nitrogen, Oxygen , Fluorine , Chlorine, Bromine and Iodine Fig 1.2 : Molecules of Elements Fig 1.3: Molecules of elements Compound Elements can combine in a chemical reaction and form a compound. 3|Page A compound is a substance made up of two or more atoms of different elements chemically joined (or bonded) together. For example, carbon dioxide gas (CO2) consists of one carbon atom and two oxygen atoms bonded together. Fig 1.4 shows a Water molecule. Water is formed when one atom of oxygen combines with two atoms of hydrogen Fig 1.4: Molecule of a Compound (Water) SUMMARY OF ATOMS, MOLECULES AND COMPOUNDS 4|Page 5|Page