Scientific ideas, concepts and processes
Elements, Compounds and Mixtures
(Almost) everything is made of atoms.
Atoms bond together to make molecules.
Molecules of elements are made of just one kind of atom.
Molecules of compounds are made up of more than one kind of atom chemically bonded together. The atoms are joined in a fixed ratio.
Elements can be represented by symbols found on the periodic table.
The formula of a compound tells you how many of each kind of atom there are in its molecules or the ratio of the different atoms in the compound.
A mixture is more than one element and/or compound mixed together.
Chemical bonding
Atoms can be held together by bonds.
New substances are made in a chemical reaction when bonds are made and broken. Signs of a chemical reaction include a change in colour or smell, bubbles of gas given off, or energy given out or taken in.
In forming bonds, atoms can achieve a stable electron arrangement. They do this when they have a full outer shell of electrons, either by losing, gaining or sharing electrons.
Ionic bonding:
Ionic compounds are usually formed when metals combine with non-metals.
The elements which most readily form ions are found in groups 1 and 2 (Alkali metals and alkali earths) as well as groups 6 and 7 (chalcogens and halogens) in the period table. It’s possible to predict the charge on an ion using the periodic table.
Positive ions (cations) are formed by metal atoms losing electrons. Negative ions (anions) are formed by non-metal atoms gaining electrons.
Some transition metal ions have variable valencies, for example Fe+, Fe+2, Fe+3; Cu+, Cu+2
Compound ions are groups of atoms that form an ion. The most common compound ions are NO3– (Nitrate), SO4-2 (Sulphate), OH– (Hydroxide) and NH4+ (Ammonia).
Ionic bonding is the electrostatic force of attraction between oppositely charged ions.
Ionic compounds consist of giant lattices of positive and negative ions.
The formula of an ionic compound is given by calculating how many of each type of atom are needed for the overall charge on the compound to be zero.
Aqueous solutions of ionic compounds
Most ionic compounds are soluble in water.
When an ionic compound dissolves, the water separates the positive and negative ions. The separated positive and negative ions are free to move in the solution.
Ionic compounds can conduct electricity when dissolved because the ions are free to move.
The concentration of a solution is the number of grams of solute dissolved in 1 litre of solvent.
Solubility depends on temperature.
A saturated solution is one where the maximum possible solute has been dissolved.
Crystals are formed when the water is evaporated from a solution of an ionic compound and the ions line up to form a crystal lattice.
If the solubility of a particular ionic compound increases with temperature, crystals can also be made when a hot saturated solution of an ionic compound is cooled.
Atoms
Atoms are built from protons, neutrons and electrons.
The atomic number of an element is the number of protons in its nucleus.
The mass number of an element is the number of protons + the number of neutrons.
The electrons in an atom are arranged in shells (or energy levels) which hold 2, 8, 8, 18 electrons. The position of elements in the periodic table reflects the arrangement of electrons.
Atoms with a full outer shell (the noble gases) are chemically stable and unreactive.
A neutral atom has equal numbers of protons and electrons. When the numbers of protons and electrons are not equal, the atom has a positive or negative electric charge and is known as anion.
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