Atoms, Elements And Compounds - AQA GCSE Chemistry

• All substances are made from tiny particles called atoms.
• In the periodic table, metals are located on the left of the stepped line, and non-metals are located on the right.
• An atom is the smallest part of an element that can exist.
• Atoms of each element are represented by their own chemical symbol.
• A chemical symbol consists of one or two letters and always starts with a capital letter, with any other letter in lower case.
• The symbol O represents an atom of oxygen, and Na represents an atom of sodium.
• Elements are arranged into groups with similar properties.
• The names and symbols of the elements are shown in the periodic table.
• There are over 100 different elements.
• Groups are numbered from 1 to 7, then 0.
• The chemical symbol of sodium is Na, not as NA, na or nA.
• A chemical formula is used to represent an element or compound in balanced chemical equations.
• The formula for most elements is just its chemical symbol.
• Some non-metal elements exist as diatomic molecules that are made up of two atoms joined together.
• The formulae of these elements are the element's symbol followed by a subscripted '2'.
• A compound is a substance that contains two or more elements that are chemically combined.
• The elements in a compound are present in fixed proportions.
• A chemical formula can be used to represent a compound.
• The formula show the symbols for each element in the compound and the number of atoms of each element in a unit of the compound.
• Many compounds exist naturally.
• Compounds can also be formed from their elements in chemical reactions.
• In a chemical reaction, one or more new substances are formed.
• Most chemical reactions involve energy changes.
• It is not easy to split up a compound into its elements - the only way to do this is in chemical reactions.
• An ion is a charged particle formed when an atom, or a group of atoms, loses or gains electrons.
• The number and sign of its electrical charges are shown in superscript text.
• The formula of an ionic compound can be predicted using the formulae of its ions.
• The numbers of ions in a formula must give an equal number of positive and negative charges.
• A word equation represents a chemical reaction using the names of the substances involved.
• Word equations do not show any chemical symbols or formulae.
• Reactants are substances that react together in a chemical reaction.
• In a chemical reaction, the atoms or ions in reactants separate from one another and join back together in a different way to form products.
• Word equations always take the form: reactants → products.
• A + sign separates two or more reactants, or two or more products in a word equation.
• Potassium hydroxide reacts with sulfuric acid.
• Potassium sulfate and water are formed in the reaction.
• The reactants in the chemical reaction are potassium hydroxide and sulfuric acid.
• The products in the chemical reaction are potassium sulfate and water.
• The word equation for the chemical reaction is: potassium hydroxide + sulfuric acid → potassium sulfate + water.
• A balanced chemical equation represents a chemical reaction using the formulae of the reactants and products, showing the number of units of each substance involved.