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Atoms, elements and compoundsThere are about 100 different elements from which all substances are made. The periodic table is a list of the elements.Each element is made of one type of atom. Atoms are represented by chemical symbols e.g. Na for an atom of sodium. O for an atom of oxygen.The elements in the periodic table are arranged in columns, called groups. The elements in a group usually have similar properties.Atoms have a tiny nucleus surrounded by electrons. When elements react, their atoms join with atoms of other elements. Compounds are formed when two or more elements combine together.

Atomic StructureThe nucleus at the centre of an atom contains two types of particle, called protons and neutrons. Protons have a positive charge and neutrons have no charge.Electrons are tiny negatively charged particles that move around the nucleus. An atom has no overall charge. That is because the number of protons is equal to the number of electrons and their charges are equal and opposite (proton +1 and electron -1).All atoms of an element contain the same number of protons. This number is called the atomic number (or proton number) of the element. Elements are arranged in order of their atomic numbers in the periodic table. The atomic number is also the number of electrons in an atom of the element.The mass number is the total number of particles in the nucleus of an atom, so it is the number of protons plus the number of neutrons.

The Arrangement of Electron AtomsEach electron in an atom is in an energy level. Energy levels can be represented as shells, with electrons in the lowest energy level closest to the nucleus.The lowest energy level or first shell can hold two electrons, and the second energy level can hold eight. Electrons occupy the lowest possible energy levels. The electronic structure of neon with 10 electrons is 2,8. Sodium with 11 electrons has the electronic structure 2,8,1.Elements in the same group of the periodic table have the same number of electrons in their highest energy level, e.g. Group 1 elements have one electron in their highest energy level.Group 1 elements include lithium, sodium and potassium. These elements react quickly with water and oxygen.The atoms of the unreactive noble gases (in Group 0) all have very stable arrangements of electrons.

Forming BondsWhen different elements combine they form compounds.When a metal reacts with a non metal, ions are formed. Metal atoms lose one or more electrons to form positively charged ions. Non-metal atoms gain electrons to form negatively charged ions. The oppositely charged ions attract each other strongly and the compound has ionic bonds.The chemical formula of an ionic compound tells us the simplest ration of ions in the compound. For example, NaCl shows that sodium chloride is made from equal numbers of sodium ions and chloride ions.When non-metals combine, their atoms share electrons to form covalent bonds and molecules are formed.The chemical formula of a molecule tells us the number of atoms that have bonded together in the molecule. For example H20 shows that a water molecule contains two hydrogen atoms and one oxygen atom. Covalent bonds can be shown as lines between the atoms that are bonded together. 

Chemical EquationsIn chemical reactions the atoms in the reactants re-arrange themselves to form new substances, the products.Atoms are neither created nor destroyed in a chemical reaction. So the number and type of atoms remains the same before and after the reaction.This means that the mass of the products equals the mass of reactants. It also means that we can write chemical equations to represent reactions.Word equations only give the names of the reactants and products. Symbol equations show the numbers and types of atoms in the reactants and products. When symbol equations are written they should always be balanced. This means that the numbers of each type of atom should be the same on both sides of a symbol equation.

Fundamental Ideas