Electrons, Bonding and Structure

Question 1

How many electrons can fill the:

• 1st shell?
• 2nd shell?
• 3rd shell?
• 4th shell?

Answer 1

• 2
• 8
• 18
• 32

Question 2

What is an atomic orbital?

Answer 2

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

Question 3

What shape are the:

• s-orbitals?
• p-orbitals?

Answer 3

• spherical
• dumbell

Question 4

How many orbitals are there in s-, p- and d-sub-shells and therefore how many electrons can each sub-shell hold?

Answer 4

• s-sub-shell:
  
  • 1, 2 electrons
  • 3, 6 electrons
  • 5, 10 electrons

Question 5

What is an ionic bond?

Answer 5

An electrostatic attraction between two oppositely charged ions.

Question 6

How are dot-and-cross diagrams drawn if there is not a 1:1 ratio of oppositely charged ions in the compound?

Answer 6

Coefficients used to show the ratio e.g. for MgCl₂ :

[Mg]⁺ & 2[Cl]^-

(obviously with outer-shell electrons drawn on as well)

Question 7

Why do ionic lattices form and why are they described as giant?

Answer 7

• Form because each ion experiences an electrostatic attraction in all directions from oppositely charged ions.
• Giant as they are made up of the same unit cell repeated many times.

Question 8

How does ionic structure explain:

• conductivity of ionic compounds?
• melting/boiling points of ionic compounds?
• solubility of ionic compounds?

Answer 8

• Conduct electricity only when molten or dissolved - not when solid. Ions in liquid/aqueous form are mobile so they are free to flow and carry charge. In a solid the ions are fixed in position by the strong ionic bonds.
• Ionic lattices are held together by strong electrostatic forces, which require lots of energy to overcome so they have high melting and boiling points.
• Ionic compounds tend to be soluble as there is an attraction between the charged ions and the polar water molecules.

Question 9

What is a covalent bond?

Answer 9

A strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.

Question 10

What elements can ‘expand the octet’?

Answer 10

Si, P, S, Cl

Question 11

Why do molecules have definite shapes?

Answer 11

The electron pairs repel each other as far apart as possible, minimising repulsion and therfore holding the molecule in a fixed 3D geometry.

Question 12

How to answer ‘Deduce the shape of this molecule’ questions?

Answer 12

1. There are x bonding pairs and y lone pairs around the central atom.
2. Electron pairs repel each other as far apart as possible to minimise repulsion.
3. If no lone pairs: all electron pairs repel each other equally. If lone pairs: lone pairs repel more strongly than bonding pairs.
4. State actual shape and bond angle(s).

Question 13

What are the shapes and bond angles for molecules with up to 6 regions of electron denisty around them, including lone pairs?

Answer 13

Question 14

What is electronegativity

Answer 14

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

Question 15

What are the general trends in electronegativity:

• across periods?
• down groups?

Answer 15

• increases
• decreases

Question 16

How do permanent dipoles form?

Answer 16

• Two atoms involved in a covalent bond have a significant difference in electronegativity.
• The more electronegative atom attracts the bonding electrons more strongly, causing a shift in electron density.
• This causes a slight difference in charge between the two ends of the bond which is called a dipole, meaning the bond is polar.

Question 17

When is a molecule polar?

Answer 17

When it has permanent dipoles which are not arranged symmetrically to cancel each other out, meaning the molecule has an overall dipole and is therefore polar.

Question 18

How do London dispersion forces arise?

Answer 18

• A temporary dipole forms due to the random movement of electrons within a molecule.
• Temporary dipole causes an oppositely orientated induced dipole in a neighbouring molecule.
• The two molecules with oppositely orientated dipoles are then attracted to each other.

Question 19

How do PD-PD interactions arise?

Answer 19

The permanent dipoles on polar molecules cause weak electrostatic forces of attraction between opositely charged ends of different molecules.

Question 20

How do hydrogen bonds form?

Answer 20

• N, O &F are v. electronegative
• the bond between H and N, O or F is v. polar
• H has partial +ve charge so attracts lone pair on N, O or F on another molecule

Question 21

Why is ice less dense than liquid water?

Answer 21

As a solid, the molecules of water are held together in a tetrahedral lattice by hydrogen bonds. Whn ice melts, H bonds are broken so water has fewer H bonds therefore the molecules are closer together.

Question 22

Why does water have a higher melting and boiling point compared to other group 6 hydrides?

Answer 22

It has H bonds in liquid form, which require more energy to overcome than the weak IM attractions between molecules of the other hydrides.

Question 23

Describe the intramolecular and intermolecular bonding in solid simple molecular lattices.

Answer 23

The atom in each molecule are held together by stong covalent bonds.

The molecules are held in a molecular lattice by weak ID-ID attractions.

Question 24

Why do simple covalent compounds have low melting and boiling points?

Answer 24

The IM forces that hol the molecules together are weak and therefore don’t require much energy to overcome.

Question 25

What types of molecule are soluble in water and why?

Answer 25

• Polar molecules because water is a polar molecule.
• Compounds which can form hydrogen bonds with the water molecules.

Question 26

Why don’t simple covalent compounds conduct electricity?

Answer 26

• All the electrons are held tightly in covalent bonds
• Not mobile
• Can’t flow and carry charge