C2.1.1 Chemical Bonding
- When two or more elements react together, compounds are formed. The atoms join together by either sharing electrons or by losing or gaining electrons to the other in order to have stable electronic structures. Atoms of noble gases have full outer shells which are stable.
- Bonding by sharing electrons is called covalent bonding.
- Ionic bonding occurs when metallic elements react with non-metallic elements. The metal atoms lose electrons to become positively charged ions, and the non-metallic atoms gain electrons to become negatively charged ions, and both have full outer electron shells, and the oppositely charged ions attract each other within the compound.
- Atoms of group 1 (in the periodic table) elements have one electron on its outer shell and atoms of group 7 have 7 electrons on their outer shell and when a reaction occurs, the group 1 element loses an electron to the group 7 element, so both end up with full outer shells, a stable electronic structure that a noble gas naturally has.
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C2.1.2 Ionic Bonding
- Ionic bonding can hold oppositely charged ions together in giant structures or a giant lattice, structures of which are very regular, due to the ions all being packed neatly, like marbles in a box.
- The force 'emitted' by an ion on other ions in the lattice acts equally in all directions, which is why ions in a giant lattice are held so strongly together.
- Atoms and ions involved in forming ionic bonds can be represented by dot and cross diagrams-only the outer shell of each atom or ion is drawn out, making them quicker to draw than drawing all the electronic layers.
- An example of an ionic compound is calcium chloride- each calcium atom has to lose 2 electrons and each chlorine atom one one, which means that two chlorine atoms must react with every one calcium atom
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C2.1.3 Formulae of Ionic Compounds
- The overall charge in every ionic compound is zero-neutral. Therefore, if we know the charge on each ion in a compound, we can work out its formula by balancing the charges.
- For example, using the method we used in the previous revision card to work out that the formula for calcium chloride is CaCl2- the ratio is 1:2 here.
- The charge on simple ions formed by elements in the groups in the periodic table can be worked out by the number in the group. For transition metals, the charge on the ion is shown by the Roman numeral shown in name of the compound, such as iron (II) sulphate, which has Fe2+.
- In the exam there'll be a data sheet provided, which will also have a periodic table in, so you don't need to learn this by heart.
- Some ions can be made up of more than one element, for example, a carbonate ion is CO32+. The rule about cancelling out charges still applies. For example, calcium carbonate is CaCO3, as the 2+ and 2- ions cancel out in the ratio 1:1.
- Ca(OH)2 has brackets in, in order to help show the ratios between the ions. This shows that the ratio of Ca to O to H is 1:2:2.
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C2.1.4 Covelent Bonding
- This is where non-metallic atoms 'share' electrons to have a full outer shell. This 'sharing' creates a very strong attraction between the two atoms, therefore a covalent bond. Compounds which are made of covalent bonds are called molecules.
- Atoms in group 7 have 7 electrons on the outer shell, and therefore need to gain one for a full outer shell, and therefore form a single bond. Group 6 atoms need two electrons for a full outer shell, and therefore needs 2 bonds, and the same applies for Group 5 and 4.
- A covalent bond can only act between the two atoms it bonds to each other, and many covalently bonded substances have small molecules in, such as water- H2O.
- Some atoms able to form several bonds, such as carbon, can join in giant covalent structures-macromolecules.
- For example, in a diamond, each carbon atom forms four covalent bonds with neighbours, which gives a giant covalent lattice.
- Silicon dioxide-silica-is another substance made of a giant covalent structure.
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- Atoms in metals are all the same size, so build up on each other layer by layer in a regular pattern, forming giant structures, just like the previous marble comparison.
- This means that they can form crystals, which are hard to see with the naked eye. Sometimes we can see them, such as seeing zinc crystals on the surface of steel-steel can be dipped into molten zinc to help prevent rusting.
- In metallic bonding, the electrons in the outer shell delocalise and then move freely between atoms, which makes a 'sea' of electrons surrounding positively charged ions. They act like a bit of a glue, because their negative charge strongly attracts the metal ions, bonding the metal ions very well due to that strong electrostatic attraction.
- The 'sea' of delocalised electrons are no longer linked to any ion in the metallic structure, and help explain properties of metals.
- The reason why metals are such good conductors is because the energy causes the delocalised electrons to move around faster and transfer energy as they bump into other electrons.
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