IB Chemistry Topic 4/14

What are polyatomic ions

Ions made up of more than one atom which together have experiences a loss/gain of electrons so carry a charge

What are the charges of polyatomic ions

Nitrate = 1-

Hydroxide = 1-

Hydrogencarbonate = 1-

Carbonate = 2-

Sulfate = 2-

Phosphate = 3-

Ammonium = 1+

What is the co-ordination number in a lattice structure

expresses the number of ions that surround a given ion in a lattice structure

What are the properties of ionic lattice structures

High melting and boiling points due to the strong electrostatic forces.

Solids at room temperature

Non volatile (does not tend to vapourize)

Soluble in polar solvents

Conducts electricity when molten or dissolved as the ions are free to move

Brittle due to the movements in the ions places similar charges next to each other, causing repulsion

How does the electronegativity determine if a substance will bond ionically

if the electronegativity difference between the 2 atoms is greater than 1.8

What makes up a double bond

one sigma bond and one pi bond

What makes up a triple bond

one sigma bond and two pi bonds

How does bond length affect bond enthalpy

A longer bond is weaker due to the electron pair being further from the pull of the nuclei in larger molecules. A larger atomic radius means a longer bond.

How does the bond length and enthalpy change between single, double and triple bonds.

The longest bond is single, and the shortest is triple

Meaning the triple bond is the strongest

What is a coordinate (dative) bond

Both electrons in the shared pair are from one atom. This bond is shown by an arrow

What are the exceptions to the octet rule.

Boron and beryllium

They form stable molecules where the central atom has fewer than 8 electrons in the valence shell. These are said to be electron deficient and have the tendency to form a coordinate bond to complete the octet.

How do London Dispersion forces occur

When at a random moment, the electron cloud can be stronger in a certain area causing an instantaneous dipole. This can induce a dipole of another molecule causing attraction called London dispersion forces.

What is a sigma bond

When 2 s, 2 p or and s and a p orbitals overlap

This occurs along the bond axis

This always forms in a single covalent bond

What is a pi bond

When 2 p orbitals overlap sideways

They are weaker than sigma bonds.

They occur in double and triple bonds

What occurs in sp2 hybridisation

One s orbital and 2 p orbitals hybridise to produce 3 equal sp2 orbitals.

How does the number of electron domains show us the hybridisation

2 electron domains = sp

3 electron domains = sp2

4 electron domains = sp3

What are the properties of London dispersion forces

The weakest type of intermolecular bonds

Increases with a greater number of electrons as this increases the change of an instantaneous dipole

Only forces which exist between non polar molecules

What are dipole-dipole attractive forces

Occurs when there is a difference in electronegativities causing a permanent dipole. These dipoles will attract neighbouring molecules

What are the properties of dipole-dipole forces

Stronger than London Dispersion

Occurs between polar molecules

What are Van der Waals forces

An umbrella term which includes

Dipole-dipole

Induced dipole-dipole

Induced dipole-Induced dipole

What is hydrogen bonding

When the hydrogen electron pair is pulled away by nitrogen, oxygen or flourine due to high electronegativity differences

What are the properties of hydrogen bonds

The strongest type of intermolecular forces

Most prominent in water, causes many of waters distinguishable features

What is the strength of a metallic bond determined by

The number of delocalised electrons (more = stronger)

The charge of the cation (more = stronger)

The radius of the cation (smaller = stronger)

When does a net dipole occur

When there are different elements (with different electronegativities) bonded to the same central ion, and it is non symmetrical

The presence of lone pairs, and it is non symmetrical

What is the equation for formal charge

FC = number of valence electrons - (1/2 electrons in bonded pairs + number of electrons in lone pairs)

What is formal charge used for

to determine the most stable structure from a variety of possible structures

The formal charge closest to zero means a more stable structure