C1.2 Atoms, elements and compounds · KS3 Science

Atoms and elements

Everything is made of atoms. An element is a substance made of only one type of atom. There are about 118 elements.
Each element has a symbol: O (oxygen), H (hydrogen), C (carbon), N (nitrogen), Na (sodium), Fe (iron), Cu (copper), Au (gold), Mg (magnesium), Cl (chlorine), S (sulfur), K (potassium).
A symbol has one or two letters: the first is a capital and the second is lower case (Co is cobalt, but CO is carbon monoxide).

The elements are arranged in the periodic table. Rows are called periods and columns are called groups.
Elements in the same group have similar properties.
Metals are on the left and in the middle; non-metals are on the right.

Metals are shiny, good conductors of heat and electricity, malleable (can be hammered into shape), ductile (can be pulled into wires) and sonorous (ring when hit). Most are solids with high melting points; mercury is a liquid.
Iron, nickel and cobalt are magnetic.
Non-metals are usually dull and are poor conductors (insulators); graphite (carbon) is an exception. Solid non-metals are brittle. Many are gases at room temperature; bromine is a liquid.
Some metals react more vigorously than others. From most to least reactive: potassium, sodium, calcium, magnesium, zinc, iron, copper, gold.

A compound is two or more elements chemically joined. Its properties are different from the elements it is made from: sodium (a reactive metal) and chlorine (a toxic gas) make sodium chloride (table salt).
Heating iron and sulfur makes iron sulfide. The mixture could be separated with a magnet, but the compound can't, because it is a new substance.
Naming: the metal comes first, and the non-metal ends in -ide (oxygen → oxide, chlorine → chloride, sulfur → sulfide). The ending -ate means oxygen is also present (copper sulfate contains copper, sulfur and oxygen).
A molecule is two or more atoms joined together. A formula shows the atoms: H₂O has 2 hydrogen atoms and 1 oxygen atom; CO₂ has 1 carbon atom and 2 oxygen atoms.

Mean = the total of the values ÷ the number of values. For 12, 14 and 16: 42 ÷ 3 = 14.
Range = the highest value − the lowest value. For 12, 14 and 16 the range is 4.

Atom: The smallest part of an element that can exist.
Element: A substance made of only one type of atom.
Compound: Two or more elements chemically joined together.
Molecule: Two or more atoms joined together.
Chemical symbol: One or two letters that stand for an element, such as Na for sodium.
Chemical formula: Symbols and numbers that show the atoms in a substance, such as H₂O.
Periodic table: A chart of all the elements, arranged in groups and periods.
Group: A column of the periodic table.
Period: A row of the periodic table.
Malleable: Can be hammered into shape without breaking.
Reactivity: How easily a substance takes part in chemical reactions.