AS/A-Level Chemistry Edexcel- Bonding, Polarisation, Trends, Electronegativity

Question 1

Define Ionic Bonding

Answer 1

The electrostatic force of attraction between oppositely charged ions

Question 2

Explain the reason for Giant Ionic Lattice structures:

Why is it structured this way?

Answer 2

[] They are arranged in this way to maximise the attraction of oppositely charged ions

[] Minimises repulsion of similarly charged ions

[] All forces act equally in all directions

[] Form ionic crystals

Question 3

How do Ionic bonds form?

Answer 3

Electrons are transferred from one atom to another forming charged ions which are held together by electrostatic forces of attraction.

• Metals lose e- to form positive cations
• Non-metals gain e- to form negative anions

Question 4

What is the charge of an Ionic compound?

Answer 4

0 because the positively charged ions cancel out negative

Question 5

What are the physical properties of ionic compounds?

Answer 5

1- High melting point due to strong electrostatic forces of attraction
2- Brittle due to layered structure
3- Doesn’t conduct electricity when solid
4- Insoluble in non-polar solvents but soluble in water

Question 6

Why are ionic compounds brittle?

Answer 6

Any force on the structure causes the layers to move meaning similarly charged ions will end up together resulting in the charges to repel and split up the crystal.

Question 7

Why do ionic compounds have high Mp and Bp?

Answer 7

This is due to the strong electrostatic force of attraction needing large amounts of energy to overcome the forces and break it down

Question 8

Why can they only conduct electricity when molten?

Answer 8

When they are solid the ions are held together strongly (compact) meaning the ions are no free to move whereas when molten or aqueous they are mobile and free to move meaning conduction can occur

Question 9

Trend in periodic table: Atomic Radius

What trends can we see across and down the group?

Answer 9

Across a group (left to right)- Atomic radius/atomic size decreases
Down a group- Atomic radius/ size increases

Question 10

Explain why Atomic radius increases down a group?

Answer 10

A new shell is added

Question 11

Explain why Atomic size decreases across a period for metals:

Answer 11

Atomic size decreases across a period for metals because…

They lose an electron to form a cation

Question 12

Explain why Atomic size increases down a group for metals:

Answer 12

Non-metals, they increase down a group because it forms anions meaning they gain an electron resulting in more shells and more no. of electrons than no. of protons leading to an increase in atomic size.

Question 13

Explain how the ionic radius and charge of an ion can affect the bonding: (4 marks)

Answer 13

1. Size of ions- smaller sizes mean they have a stronger force of attraction resulting in stronger ionic bonding. Larger size leads to less attraction resulting in weaker ionic bonding
2. Higher charge- a stronger force of attraction hence stronger ionic bonding

Question 14

Define Isoelectronic:

Answer 14

Same number of electrons and electronic configuration. e.g Aluminium 3+ ion and Oxygen 2- ion

Question 15

Define Electronegativity:

Answer 15

The measure of the ability of an atom to attract the electrons when the atom is part of a compound

Question 16

High electron density

Answer 16

More electrons in that area

Question 17

Role of electron density maps

Answer 17

Show how electrons are distributed in a chemical substance. X rays passed through ionic crystals onto photographic film

• likelihood of finding electrons in that region
• contour lines of equal electron density, with greater electron densities being on contours closer to the nucleus.

Question 18

Electrolysis- How can we use electrolysis as evidence of the existence of ions/movement of ions? (Copper(vI) Chromate)

Answer 18

the behaviour of ionic substances
1- Aqueous Copper(vI) chromate CuCrO4 is an olive green solution
2- It undergoes electrolysis to form a blue colour at the (negative electrode) cathode and a yellow colour at the anode (positive electrode)

Positive Anode Negative Is Cathode (PANIC)

Question 19

What are the ions that turn colour in electrolysis?

Answer 19

Positive blue Copper ions are attracted to the cathode

Negative yellow Chromate ions are attracted to the anode

Question 20

Electron Density structure of NaCl:

Answer 20

• Ions arranged in regular pattern
• Chloride ions are larger than sodium ions (remember not chlorine atom but ions are larger because it has Cl-)
• Ions are separate because the electron density falls 0 between ions
• Anions are a bit distorted

Question 21

Describe the structure of Diatomic (atoms of the same element) molecules in electron density maps?

Answer 21

1- Ions are not separate because of the sharing of electrons
2-Equal size- identical electronegativity
3-Contour lines are similar because of the same electron density around ions

Question 22

Describe the structure of covalent compounds in electron density maps?

Answer 22

1-Ions are not separate due to the sharing of electrons
2-Size of ions show little difference in electron density
3-Contour lines are not exactly the same

Question 23

What evidence show that ionic compounds exist?

Answer 23

1- High melting points, strong electrostatic attractions between oppositely charged ions

2-Electrical conductivity can only occur in a liquid state or aqueous solution because ions need to move

3-Coloured ions can be observed migrating to electrodes during electrolysis

4-Electron density maps show low electron density between the oppositely charged ions

Question 24

Why does the Ionic radius decrease as the number of protons increases?

Answer 24

As the positive charge of the nucleus increases, the electrons are attracted more strongly and therefore are pulled closer to the nucleus

Question 25

What are the Fajan’s rules?

How can polarisation be increased?

Answer 25

• Cation= small size, high charge -> highly polarising and attracts electrons in the anion
• Anion= large size, high charge -> highly polarising and distorts easily

Question 26

Define polarising power:

Answer 26

the ability for a cation to attract electrons from the anion towards itself

Question 27

Charge density formula

Answer 27

charge/r^2

Question 28

What causes covalent character in Ionic compounds?

Answer 28

• during the formation of an ionic bond 2 oppositely charged ions of unequal size attract each other
• smaller ion (cation) attract the outermost electrons of the other anion and repels the nuclear charge
• Results in distortion or polarisation of larger ion
• Electron cloud is no longer symmetrical and elongated towards the cation

Question 29

Define polarizability:

Answer 29

The ability of the Anion to get polarised

Question 30

What factors increase the covalent character?

Increases polarisation

Answer 30

1- Increase in anion size 
2-Decrease in cation size
3-Increase in Nuclear charge 
4-Transition metals 
5-Highly charged density cations

Question 31

Describe the relationship between melting point and covalent characters:

Answer 31

Bigger Anion, greater covalent character, low melting point (due to the sharing of electrons)
Ionic, higher melting point

Question 32

Electrons in a covalent bond are not evenly shared. It all depends on the…..

Answer 32

….Electronegativity of the atoms in the bond

Question 33

The greater the electronegativity…..

Answer 33

….the greater its tendency to attract the bonding pair of electrons. (creates poles at the end of the molecule, dipole)

Question 34

Which element is the most electronegative?

Answer 34

Fluorine

Question 35

What are the trends in electronegativity in the periodic table?

Answer 35

Increases across a period from group 1-17

Decreases down a group, Increases up a group

Question 36

Explain the trend in electronegativity in the periodic table:
Why does it increase across a period and up a group?

Answer 36

Increases across a period (left to right) because the no. of protons increases, nuclear charge increases, greater attraction to the bond pair of electrons

Increases up a group because the atomic size/ radius decreases up a group, shielding effect is less, less repulsion and greater attraction

Question 37

What do you mean by a bond is pure/non-polar?

Answer 37

diatomic molecules/ when 2 covalent bonds have the same electronegativity (no difference) and electron cloud is evenly distributed between 2 atoms.
-Bonding is shared equally, no charges, perfectly covalent

Question 38

What do you mean by a polar covalent bond?

Answer 38

• Bonding is unequal
• Partial charges on atoms
• Small difference in electronegativity

Question 39

Ionic bond:

Answer 39

• complete transfer of one or more valence electrons. Full charges on resulting ions.
• great difference in electronegativity

Question 40

What are the 3 assumptions of electrostatic theory?

Answer 40

1- All bonds are ionic
2-All ions are perfect spheres
3-Charge is evenly distributed in these spheres/point charges

Question 41

Electronegativity difference:

Answer 41

In pure covalent, there is 0 electronegativity difference

Polar covalent, electrons are unequally shared they have a slight electronegativity difference

Polar Ionic, ions are distorted, big electronegativity difference

Pure Ionic, very large electronegativity difference

Question 42

Define polarisation:

Answer 42

covalent character in ionic substances is due to the polarisation of the anion by the cation

-causes a distortion of the electron density in the anion causing it to move closer to the cation resulting in a degree of covalent character.

Question 43

How can we use the electrolysis of sodium chloride as evidence of the existence of ions?

Answer 43

1- Pass a direct electric current through molten sodium chloride
2-Sodium is formed at the negative electrode
3-Chlorine is formed at the positive electrode

[] Positive sodium ions migrate towards the negative electrode where they gain electrons and become sodium atoms

[]Negative chlorine ions migrate towards the positive electrode where they lose electrons and become chlorine molecules

Question 44

Define covalent bond:

Answer 44

Strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms

Question 45

What do you mean by the term dipole?

Answer 45

The differing attraction for the pair of electrons allows there to be a small charge difference between the atoms.

Question 46

What are the differences between polar and non-polar molecules?

Answer 46

Polar

• non-symmetrical
• difference in charge exists across the molecule
• overall dipole

Non-Polar

• symmetry of molecule means the effect of any permanent dipoles are cancelled out
• No difference in charge across the molecule

Question 47

Exam-style question: In Magnesium Iodide some polarisation of the bond occurs. Explain using this example what the term polarisation means:

Answer 47

In Magnesium Iodide there is an unequal sharing of paired electrons meaning there is a difference in electronegativity resulting in polarisation occurring where the cation will attract electrons in the anion causing it to distort.

-unequal distribution of charge

Question 48

Hydration:

Answer 48

process of water molecules being attracted to ions in solution and surrounding the ions; the oxygen ends of the water molecule will be attracted to the cations and the hydrogen ends will be attracted to the anions.
-Hydration of ions is an exothermic process

Question 49

How are Sigma bonds formed? Where is the region of electron density located?

Answer 49

Formed by the head-on overlap of atomic orbitals

-electron density is concentrated in the nuclei of the bonding atoms

Question 50

What are the types of sigma bonds?

Answer 50

1- 2 s orbitals overlap head-on (s-s)
2- S orbital overlaps with a p orbital (s-p)
3- 2 P orbitals overlap head-on (p-p)

Question 51

How are Pi bonds formed? Where is the region of electron density located?

Answer 51

A pi bond is formed by the sideways overlap of atomic orbitals

-results in electron density above and below the plane of the nuclei of the bonding atoms

Question 52

Describe the electron density of 2 atoms of the same element bonded together:

Answer 52

If 2 atoms of the same element bond together we see an identical electronegativity and an evenly distributed electron density. Electron density between the 2 nuclei is identical

Question 53

Type of Covalent bond/ Sigma or Pi :

Answer 53

single covalent - 1 sigma
double covalent- 1 sigma 1 pi bond
triple covalent-1 sigma 2 pi bonds

Question 54

Which bond is stronger? Sigma or Pi?

Answer 54

Sigma bond is a much stronger covalent bond than Pi bonds due to the head-on overlap of the orbitals which results in a stronger hold of the atoms.

The sideways overlap of Pi bonds are smaller compared to the head-on overlaps hence it has a weaker covalent bond.