Chemistry - Topics from Year 9

Chemistry - Topics from Year 9

1. Atomic Structure & Periodic Table

   • Atoms, Elements, Compounds and Chemical Equations
     • Atoms
       • Atoms have a radius of about 0.1 nanometers.
       • Protons have a charge of +1 and a relative mass of 1. Neutrons have a charge of 0 and a relative mass of 1. Electrons have a charge of -1 and a very small relative mass.
       • Atoms are neutral - they have no charge. They have the same number of protons as neutrons. The charge of the electrons and protons are the same size but opposite, making it cancel out. In an ion, the number of protons and electrons aren't equal. So there is an overall charge.
       • Atomic number = The number of protons Mass number - Total number of protons and neutrons.
     • Elements
       • The number of protons decide what type of atom it is. If a substance only contains atoms with the same number of protons it's called an element. All atoms of a particular element have the the same number of protons and different elements have different numbers of protons.​ Isotopes are different forms of the same element, which have the same number of protons but a different number of neutrons. Isotopes jabe the same atomic number but different mass numbers.
       • As many elements can exist as a number of different isotopes, relative atomic mass is used instead when referring to the element as a whole. This is an average mass taking into account the different masses and abundances(amounts)of all the isotops that make up the element.​ ​
     • Compounds
       • When elements react, atoms combine with other atoms to form compounds. Compounds are substances formed from two or more elements, the atoms of each are in ficed proportions throughout the compound and they're held together by chemical bonds. Making bonds involves giving away, taking or sharing electrons. It is usually difficult to separate the original elements of a compound out again.
       • A compound which is formed from a metal and a non-metal consists of ions. Metal atoms – lose electrons forming positive ions. Non-metal atoms – gain electrons forming negative ions. The opposite charges mean that they're strongly attracted to each other(ionic bonding). Examples include: sodium chloride, magnesium oxide and chemical oxide.
       • A compound formed from non-metals consiss of molecules. Each atom shates an electron with another atom(convalent bonding). Examples include: hydrogen chloride gas, carbon monoxide, and water. The properties of a compound are usually totally different from the properties of the original elements. E.g If iron(a lustrous magnetic metal) and sulfur(a nice yellow powder)react, the conpound formed is a dull grey solid lump, and doesn't behave anything like iron or sulfur.​
       • Compounds can be represented by formulas. They are made up of elemental symbols in the same proportions that the elements can be found in the compound. Helpful examples

2. States of Matter

   • States of Matter
     • Materials come in three forms – solid, liquid and gas. These are the three states of matter. Which state something is at a certain temperature depends on how strong the forces of attraction are between the particles of the material. How strong the forces are depending of three things:
       • 1. The Material
       • 2. The Temperature
       • 3. The pressure
     • Solids:​ There are strong forces of attraction between particles, which holds them close together in fixed positions to form a very regular lattice arrangement. The particles don't move from their positions, so all solids keep a definite shape and volume. The particles vibrate about their positions – the hotter the solid becomes, the more they vibrate causing solids to slightly expand when heated.​
     • Liquids:​ There is a weak force of attraction between the particles. They are randomly arranged and free to move past each other, but they tend to stick closely together. They have a definite volume but don’t keep a definite shape and will flow to fill the bottom of a container. They are constantly moving with random motion. The hotter the liquid, the faster the movement. It causes liquids to expand slightly when heated.​
     • Gas:​ The force of attraction is very week. They are free to move and far apart. The particles travel in straight lines. Gases don't keep a definite shape or volume and will always fill any container. The particles move constantly with random motion. The hotter the gas, the faster they move. Gases either expand when heated, or their pressure increases.​
     • The particles aren't solife or inelastic and they are spheres – they are atoms, ions or molecules. Also, the model doesn't show the forces between the particles, so there's no way of knowing how strong they are.​
     • State symbols tell you the state of a substance in an equation.​
       • (s) - solid
       • (l) - liquid
       • (g) - gas
       • (aq) - aqueous - dissolved in water
   • Changing state
     • Physical changes don’t change the particles – just their arrangement or their energy.​
     • The stronger the forces, the more energy is needed to break them, and so the higher the melting and boiling points of a substance.​
     • How to predict what state a substance is in at a certain temperature.​ If the temperature is below the melting point of a substance, it's a solid. If it's above the boiling point, it's a gas and if it's in between the two points, it's a liquid.

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