2.2 electrons ,bonding And structure

Question 1

What is electron pair repulsion theory?

Answer 1

The shape of an molecule or ion is determined by the number of electron pairs in the outer shell of the central atom(these pairs can be bonding pairs or lone pairs):

-this is as electrons all have a negative charge, so each electron pair repels other electron pairs
-the shape adopted will be the shape that allows all the pairs of electrons to be as far apart as possible

Question 2

If the central atom has a lone pair what do you do?

Answer 2

Take away 2.5 degrees from the bond angle
As a lone pair of electrons is slightly more electron dense so repels more

Question 3

How do you draw the shape of an ion?

Answer 3

-draw the shape of the ion
-after add square brackets and put the ions charge outside of them

Question 4

Define electronegativity

Answer 4

The ability of an atom to attract the bonding electrons in a covalent bond

Question 5

What is the electronegativity trend on the periodic table?

Answer 5

Electronegativity increases towards the the top right of the periodic table , with fluorine being the most electronegative

Question 6

What is the Pauling scale?

Answer 6

-Invented by Linus Pauling
-Used to measure the electronegativity of an atom

Question 7

Define ionic bonding

Answer 7

The electrostatic attraction between positive and negative ions

Question 8

Define dative covalent bonding

Answer 8

-also known as coordinate bond
-a bond formed by a shared pair of electrons that has been provided by one of the bonding atoms only

Question 9

Why is a H-H molecule non-polar?

Answer 9

-as the two bonding atoms are identical
-so each hydrogen atom has an equal share of the pair of electrons in the bond
-so the electrons in the bond are evenly distributed

Question 10

Why is a HCl molecule polar?

Answer 10

-the two bonding atoms are different
-the Cl atom is more electronegative than the H atom
-so the Cl atom has a greater attraction for the bonding pair of electrons than the H atom
-So there is a small permanent charge difference across the H-Cl bond the charge difference is always present (it is called a permanent dipole)

Question 11

On what type of molecule do dipoles cancel out?

Answer 11

For molecules that are symmetrical the dipoles may cancel out.
E.g. CO2 and CCl4

Question 12

Why do intermolecular forces occur?

Answer 12

-Due to the constant random movements of the electrons within shells of the atoms in molecules

-They do not involve any sharing or transfer of electrons

Question 13

What does Van der walls forces refer to?

Answer 13

-used to describe several types of intermolecular bonding:
.dipole-dipole interactions(which include permanent dipole-induced dipole interactions and permanent dipole-permanent dipole interactions)
.London (dispersion )forces (also known as induced dipole-induced dipole interactions)

Question 14

Describe permanent dipole-induced dipole interactions

Answer 14

-some molecules have permanent dipoles due to polar bonds being present
-a molecule with a permanent dipole has a slightly negative and slightly positive end
-when near to other molecules that are non -polar it is able to cause electrons in the shells of the nearby molecule to shift slightly (either being repelled by negative end or attracted by positive end)
-this causes the non-polar molecule to become slightly polar and then an attraction occurs
-the molecule with a permanent dipole has induced a dipole in the other molecule

Question 15

Describe permanent dipole-permanent dipole interactions

Answer 15

-polar molecules have permanent dipoles
-molecules with permanent dipoles will be attracted to other molecules with permanent dipoles
-their oppositely charged ends are attracted to each other

Question 16

What is a dipole?

Answer 16

A molecule that has both a positive and negative charge on it

Question 17

Describe London (dispersion) forces

Answer 17

-happens in non-polar molecules
-caused by the constant random movement of electrons in atoms’ shells
-this movement unbalances the distribution of charges within the electron shells (like electron density in shell wobbling from side to side )
-at any moment there will be an INSTANTANEOUS dipole across the molecule
-the instantaneous dipole INDUCES a dipole in neighbouring molecules .which in turn induce dipoles on their neighbouring molecules
-the small induced dipoles attract one another causing intermolecular forces known as London forces
-these bond are very weak they switch on and off

Question 18

How do London forces increase?

Answer 18

They increase with increasing number of electrons
The greater the number of electrons the larger the induced dipoles and the greater the attractive forces between molecules

Question 19

What effect do London forces have on boiling points ?

Answer 19

-when a substance is being boiled heat energy is being used to overcome the intermolecular forces between molecules
-as the number of electrons increases so does the strength of the Van der Waals’ forces (e.g. London forces)
-so as number of electrons increases the boiling point increases

Question 20

Compare the strengths of intermolecular forces with ionic forces
Put in order of strongest to weakest

Answer 20

-ionic and covalent bonds
-hydrogen bonds
-permanent dipole-dipole forces
-London (dispersion forces )

Question 21

What is a hydrogen bond?

Answer 21

-a strong permanent dipole-permanent dipole attraction between an electron deficient hydrogen atom (O-H ,N-H,F-H)on one molecule and a lone pair of electrons on a highly electronegative atom (O,N or F) of a different molecule

Question 22

Why is ice less dense than water ?

Answer 22

-as ice has has an open lattice with hydrogen bonds holding the water molecules apart
-when ice melts these hydrogen bonds collapse meaning the water molecules move closer together
-so ice is less dense than water

Question 23

Why does water have a higher than expected melting and boiling point?

Answer 23

-water has a higher m.p and b.p than other hydrides of group 16 elements as:
.there are relatively strong hydrogen bonds between H2O molecules
->these hydrogen bonds are much stronger than other intermolecular forces
->the extra strength of these forces has to be overcome in order to melt or boil water .This results in water having a higher m.p and b.p than would be expected if hydrogen bonds were not present

Question 24

Why does water have high surface tension and viscosity?

Answer 24

-the extra intermolecular bonding from hydrogen bonding explains these properties
-e.g. when small insects walk on water they are walking on a raft of hydrogen bonds

Question 25

Define an atomic orbital

Answer 25

A region around the nucleus that can hold up to two electrons , with opposite spins

Question 26

What happens in a giant ionic lattice?

Answer 26

Each ion attracts oppositely charged ions from all directions:
-each ion is surrounded by oppositely charged ions
-the ions attract each other forming a giant ionic lattice
-ions have huge amount of electrostatic attraction between them

Question 27

Why do ionic compounds have high melting and boiling points?

Answer 27

-they are solid at room temperature as a large amount of energy is needed to break the strong electrostatic bonds that hold the oppositely charged ions together in the solid lattice

-the greater the charge the stronger the electrostatic forces between the ions so more energy is required to break up the ionic lattice during melting

Question 28

Can ionic compounds conduct electricity when solid?

Answer 28

No as the ions are held in fixed positions in the giant ionic lattice so no ions can move
So they do not conduct electricity

Question 29

When can ionic compounds conduct electricity?

Answer 29

-they can conduct electricity when melted or dissolved in water
.as the solid lattice breaks down and the ions are free to move
.so the ionic compound can now conduct electricity

Question 30

When can an ionic lattice dissolve?

Answer 30

-they can dissolve when in polar solvents e.g. water
-as these polar water molecules break down an ionic lattice by surrounding each ion to form a solution
-the slight charges within the polar substance are able to attract the charged ions in the giant ionic lattice
This means the lattice is disrupted and ions are pulled out of it

Question 31

What happens when NaCl is dissolved in water?

Answer 31

-water molecules attract the Na+ and Cl- ions
-the ionic lattice breaks down as it dissolves.Water molecules surround the ions
-Na+ attracts the delta- charges on the O atoms on the water molecules
-Cl- attracts delta+ charges on the H atoms of the water molecules

Question 32

When are simple molecule structures soluble?

Answer 32

They are generally soluble in non-polar solvents , such as hexane
As weak London forces are able to form between covalent molecules and these solvents
This helps the molecular lattice to break down and the substance dissolves

Question 33

What are the melting and boiling points of giant covalent structures?

Answer 33

-they have high m.p and b.p because high temperatures are needed to break the strong covalent bonds within the lattice

Question 34

Are giant covalent structures soluble?

Answer 34

They are insoluble in both polar and non-polar solvents because the covalent bonds in the lattice are too strong to be broken

Question 35

Explain how the structure and bonding in bromine account for its relatively low m.p.

Answer 35

-Forces between molecules
-Which are London forces
-Are weak so are relatively easily overcome by increased kinetic energy
-so has low m.p

Question 36

How many orbitals are occupied in a silicon atom?

Answer 36

8

Question 37

Explain why a CH2Cl2 molecule is polar

Answer 37

-The dipoles do not cancel out
-because the molecule is non-symmetrical

Question 38

Predict whether or not the Cl20 molecule has an overall dipole

Answer 38

Yes as it is non-symetrical

Question 39

Describe what is meant by the term ionic lattice, in terms of type and arrangement of particles present

(2marks)

Answer 39

-repeating pattern
-of oppositely charged ions