covalent substances and covalent bonding

The covalent bond

• bonding between non-metals only

• Electronegativity of non-metals is high - they all want to gain electrons but not donate

• Share pairs of valence electrons to produce full outer shells

Covalent bonding

• when the Electronegativity is almost the same then nobody gets to win the tug of wat

• No transfer of electrons

• Sharing of electrons

Properties of hydrogen

→ Low melting point

• gas at room temp

• MP: -259 degrees Celsius

→ doesnt conduct electricity

→ low density

Properties of covalent molecules

• very low melting points and boiling points → some of the bonding in non-metal compounds must be very weak

• Absence of electrical conductivity in any phrase → non-metal compounds do not contain ions or delocalised electrons

Features of covalent bonds

1. Occurs between non-metallic elements

2. Form molecules rather than lattices

Diatomic molecules: hydrogen

• has 1 electron

• It needs another 1 electron for a full outer shell

• By sharing electrons btwn 2 H atoms → both will have full shell

• As 2 electrons are shared this = single bond

Diatomic molecules - oxygen

• oxygen has 6 electrons in outer shell

• Needs another 2 electrons for full outer shell

• By sharing 2 electrons form each atom btwn 2 oxygen atoms → both have full shell

• 4 electrons shared = double bond

Lewis structures - electron dot diagrams - for atoms

• the atoms nucleus and all inner shell electrons are replaced by elements symbol

• Outershell electrons - represented by dots or small crosses around symbol in square arrangement

→ dots arranged in pairs if more than 4 outer electrons

Molecular compunds - more than 2 atoms - drawing Lewis structures

1. Calculate total no. Of valence electrons of all atoms in compound

2. Identify central atom - atom with least complete valence shell = central atom

3. Draw the outer atoms around central atom & place one pair of electrons btwn each joining atom

4. Arrange the other electrons around outer atoms first & then central atom until all electrons = distributed

5. Count electrons around each atom

6. If any atom doesnt have a full shell shift pairs of electrons from non-bonding space to bonding space → or place remaining electron pairs onto central atom

7. If you want → change bonding pairs of electrons to lines

Polyatomic molecules - CH4

• carbons has 4 electrons in outer shell

• It needs around 4 electrons for a full outer shell

• Hydrogen has 1 electron

• Needs another electron for full outer shell

• By sharing one electron w/t each hydrogen atom → all will have full shell

• As 2 electrons are shared → these - single bonds

Polyatomic molecules: CH2O

• put carbon in the middle with one pair of electrons between the atoms

• Add the electrons around the outside atoms (O)

• Count the electrons around each atom:

H: 2

O: 8

C: 6

Polyatomic molecules: CH2O - continuation

• move one pair to share between C and O

• Redraw with lines for bonds

Representing covalent molecules - representing covalent molecules - electron dot diagrams

• two dimensional

• Does not indicate shape

• Tedious and slow to produce

• Not useful for large molecules

Representing covalent molecules - representing covalent molecules - Valence structure

• two dimensional

• Does not necessarily indicate shape

• Can be tedious and slow to write for larger molecules

• Important lone pairs may be difficult to distinguish from non-important ones

Representing covalent molecules - representing covalent molecules - structural formula

• two dimensional

• Doesn’t necessarily indicate shape

• CNA be tedious and sow to write for larger molecules

Representing covalent molecules - representing covalent molecules - Space-filling

• doesn not show the boding between atoms

• Relative sizes of atoms involved are only approximate

Representing covalent molecules - representing covalent molecules - Ball and stick

• can be physically difficult to produce for larger molecules

• Kits to produce can be expensive

Shapes of molecules

• techniques such as x-ray crystallography give us images that can be put together with complex mathematical equations

• We can also use modelling based on what we know about the octet rule & Lewis dot diagrams

What determines the shape of molecules

• the hate of molecules are determined by the repulsion between electron pairs in the outer shell of the central atom

• Lone pairs repel more than bonded pairs

Valence shell electron pair repulsion (VSEPR) model

• bonding and non-bonding electrons will repel each other due to their like negative charges

• Molecules will arrange themselves in space that requires least energy possible

• Therefore arrange as far away from each other as possible

VSEPR theory

States the electron pairs in the molecule repel each other and take up positions as far from one another as possible

Shape of covalent molecules

• VSEPR theory states that since electron pairs repel, shape of molecule depends on no. Of electron pairs around each atom

• These electron domains are arranged to minimise electrostatic repulsion

Getting the shape steps -

1. Count the no. Of valence electrons for all atoms present

2. Put the atom which can take the most bonds in the middle

3. Place the other atoms around main element

4. Draw one bond to each outer atom from the centre atom

5. Add lone to each outer atom until it has correct number

6. Put any remaining electrons on one central atom

7. Find the parent geometry

8. Find molecular geometry

Structure name - parent geometry tetrahedral (4 electron groups)

• tetrahedral - 4 bonding groups, 0 lone pairs

• Trigonal primarid - 3 BG, 1 LP

• bent - 2 BG, 2 LP

• Linear - 1 BG, 3 LP

Structure name - parent geometry trigonal planar (3 electron groups)

• trigonal planar - 3 BG, 1 LP

• Bent - 2 BG, 1 LP

• Linear - 1 BG, 2 LP

Structure name - parent geometry - linear - (2 electron groups)

• linear - 2 BG, 0 LP