Chemistry - Additional

• The nucleus of an atom is made up of protons are neutrons.
• Protons have a positive charge, electrons have negative charge and neutrons have no charge.
• The atomic number (proton number) of an element = the number of protons in the nucleus of its atoms.
• Elements are arranged in order of their atomic numbers in the periodic table.

• Electrons in atoms can be represented by shells as energy levels.
• Electrons in the lowest energy level are in the shell closest to the nucleus.
• All elements in a group of the periodic table have the same number of electrons in their outer shell.
• Electrons occupy energy levels from the lowest first eg: neon with 10 electrons is 2.8.

• Noble gases are unreactive becasue they have stable arrangements of their electrons.
• Atoms of other electrons can achieve stable electronic structures by gaining or losing electrons to form ions, or by sharing electrons to form colvalent bonds.
• Group 1 lose their single outer shell electron eg: Sodium Na (2.8.1) forms sodium ions, Na+ (2.8) ---> group 7 gains one electron to form ions eg: Chlorine Cl (2.8.7) forms chloride ions Cl- (2.8.8)
• Positive and Negative Ions attract and form ionic bonds.

• Ionic Bonding holds oppositely charged ions together in giant structures.
• Strong electrostatic forces of attraction act in all directions.
• Each ion in the lattice is held together firmly by surrounding ions with the opposite charge.
• The Sodium Chloride structure contains equal numbers of sodium ions and chloride ions as shown by its formula NaCl.
• Both ions alternate to form a cubic lattice.
• The ratio of ions in the formula and structure nof an ionic compund depensd on the charge on the ions eg: Magnesium ions are Mg²+ and Chloride ions are Cl¯ so chloride needs 2 to make the formula balanced.---> It would make MgCl².
• The ratio would then be 1:2

• A covalent bond is a strong bond between two non metal atoms by sharing a pair of electrons from the outer shell.

• Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds.

• Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.

• Atoms can form more than one covalent bond.

• There is a quick way to figure out how many covalent bonds an atom has.

Group 4Group 5Group 6Group 7 example carbon nitrogen oxygen chlorine number of bonds 8 - 4 = 4 8 - 5 = 3 8 - 6 = 2 8 - 7 = 1

Hydrogen forms one covalent bond. The noble gases in Group 0 do not form any!

Double and triple Bonds:

• A molecule of oxygen (O₂) consists of two oxygen atoms held together by a double bond, like this: O=O. A molecule of nitrogen (N₂) has two nitrogen atoms held together by a triple bond, like this:

Dot and Cross Diagrams:

Ionic bonds form when a metal reacts with a non-metal. Metals form positive ions; non-metals form negative ions. Ionic bonds are the electrostatic forces of attraction between oppositely charged ions.

• The oppositely charged ions are arranged in a regular pattern to form giant ionic lattices. Ionic Compounds often form crystals as a result.

Properties of Ionic Compounds:

• High Melting and Boiling Points- Ionic bonds are very strong - a lot of energy is needed to break them. So ionic compounds have high melting and boiling points.
• Conductive when liquid - Ions are charged particles, but ionic compounds can only conduct electricity if their ions are free to move. Ionic compounds do not conduct electricity when they are solid - only when dissolved in water or melted.

Covalent bonding occurs between non metal atoms. Each bonds consists of a shared pair of electrons, and is very strong. There are two main types of covalently bonded substances: 1.) Simple Molecules and 2.) Giant Covalent Structures.

Simple Molecules:

• Contain only a few atoms held together by strong covalent bonds.
• An example of this could be Carbon Dioxide, where one atom of Carbon is bonded with two atoms of Oxygen.

Properties of simple molecular substances:

• Low Melting and Boiling Points - This is becasue the weak intermolecular forces break down easily.
• Non - Conductive - Substances with a simple molecular structure do not conduct electricity. This is because they do not have any free electrons or an overall electric charge.

• Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds.
• They all have very strong bonds between the atoms, but much weaker forces holding the molecules together.
• When one of these subtances melts or boils, it is these 'weak intermolecular forces' that break, not the strong covalent bonds.
• Simple molecular substances are gases, liquids or solids with low melting and boiling points.