C2.1 Structure and bonding

When two elements react together they make a NEW substance called a COMPOUND. It is DIFFICULT to separate the elements after the reaction. Some atoms react by sharing electrons. We call this CO-VALENT bonding. Other atoms react by GAINING or LOSING electrons. We call this IONIC bonding. When atoms react in this way they get the electronic structure of a NOBLE gas. 

• There are 7 rough 'groups' for the periodic table
• Each group represents the NUMBER of ELECTRONS in the OUTER SHELL. Which are LOST/ GAINED in ionic bonding. 
• GROUP 1 reacts with GROUP 7- 8 electrons in the outer shell- NOBLE GAS. 

Ionic bonding:

• Arranged in LATTICE
• Held in place by very strong forces- ELECTROSTATIC FORCES, acting in all directions. 
• Represent the iosn involved in ionic bonding by using DOT AND CROSS diagrams
• GROUP 1 + GROUP 7
• GROUP 2 + GROUP 6

Examples: 

• NaCl
• MgO
• CaCl
• METALS: form Positive ions
• NON METALS: form Negative ions

When non metal atoms react toogether they make COVALENT bonds. The atoms in these bonds are held together by SHARED electrons. Most substances held together by covalent bonds consist of MOLECULES, but some haev GIANT covlaent structures, sometimes called MACROMOLECULES. 

• Non metal + non metal
• 'share' electrons
• INTERMOLECULAR forces present
• Two types: SIMPLE MOLECULES/ GIANT COVALENT STRUCTURES

SIMPLE MOLECULES:

• Low boiling and melting points= as there are WEAK intermolecular forces, so LESS ebergy is required to break down these bonds
• STRONG COVALENT BONDS- that form small molecules of several atoms
• Do NOT conduct electricity
• Examples: CH4, CO2

• Arranged in layers of the same elelment
• Pure METALS can bend: because layers of atoms can SLIDE over each other. 
• Also each atom has AT LEAST 1 DELOCALISED ELECTRON- forming a sea of delocalised electrons, whcih move through the metal to CONDUCT CHARGE. 
• So the atoms are actually IONS
• The attraction between the DELOCALISED ELECTRONS and the IONS is very strong- keeps lattice in shape. 

Metals have GIANT structures. They are made up of metal ATOMS whicoh are closely packed and arranged in REGULAR patterns. There is evidence of this in CRYSTALS we can sometimes see at the surface of the metal. In metallic bonding, the metal ions are held together by ELECTRONS from the OUTERMOST shell( highest energy level) of the metal atoms. The POSITIVE iosn that thsi produces are held together by strong ELECTROSTATIC forces. The eletcrons in metals are FREE to move throughout the structure. 

DELOCALISED ELECTRONS: act as Glue by- because the attarction between the delocalised electrons and the positive ions. Also acts as REPELLANT: as the electrons repel each other as same charged particles REPEL. 

• Ionic compounds are NEUTRAL- because the ions balance out the charges with each other. 
• Usually have high MELTING POINT- because of the strong electrostatic forces
• They CONDUCT electricity when in SOLUTION- as ions can move about freely in the solution- SOLUBLE
• Formula reflects: RATIO of ions present in the compound
• usually made of CRYSTALS

GIANT COVALENT STRUCTURES: 

• NOT soluble in water
• Particles are NOT CHARGED
• Do not conduct electricity 
• High melting and boiling points- LOTS of energy required to break down. 

DIAMOND: 

• Hard because: 4 STRONG COVALENT bonds for each carbon atom. 
• Strong LATTICE/ HEXAGONAL STRUCTURE
• strong INTERMOLECULAR forces

SILICON DIOXIDE: 

• Can withstand high temperatures because: Strong intermolecular forces- lots of ENERGY required to break them. 

GRAPHITE: 

• Soft because: LAYERS slide over each other easily unless aloyed. 
• CONDUCTS ELECTRICITY: because each CARBON atom has 1 DELOCALISED ELECTRON- which forms a sea of delocalised electrons. They move around Graphite to CONDUCT electricity. 
• ALSO: graphite SUBLIMES ( changes from solid to gas, without melting),a nd alot of energy is required for this: 3000 degrees celsius