C5- Electrons And Bonding

Question 1

What are shells and their capacity?

Answer 1

• Shells = energy levels.
• As shell number increases, energy increases.
• Shell 1 = 2 electrons
• Shell 2 = 8 electrons
• Shell 3 = 18 electrons
• shell 4= 32 electrons

Question 2

What is an atomic orbital?

Answer 2

A region around the nucleus that can hold up to two electrons, with opposite spins.
- the opposite spins is what allows the electrons to exist together as they would usually repel one another.
- 4 types of orbitals: s- , p- , d- , f-
Each has a different shape.

Question 3

s-orbitals.

Answer 3

• electron cloud in shape of a sphere.
• each shell from n=1 contains one s-orbital.
• can hold one or two electrons.
• radius of the orbital increases with shell number.

Question 4

p-orbitals.

Answer 4

• electron cloud in shape of an 8.
• one orbital can hold one/two electrons.
• there are 3 separate p-orbitals at right angles to each other:
  Px - horizontal 8 shape.
  Py - diagonal 8 shape.
  Pz - vertical 8 shape.
• each shell from n=2 contains three p-orbitals.
• distance from p-orbital to nucleus increases with shell number.

Question 5

d-orbitals and f-orbitals.

Answer 5

d-orbitals:
each shell from n=3 contains five d-orbitals.

f-orbitals:
each shell from n=4 contains seven f-orbitals.

Question 6

What is principal quantum number?

Answer 6

shell number

Question 7

What is a sub-shell?

Answer 7

A group of orbitals of the same type within a shell.

Question 8

Explain the filling of orbitals in terms of energy.

Answer 8

Orbitals fill in order of increasing energy:
- n=2 shell ; 2s, 2p
- n=3 shell; 3s, 3p, 3d
- n=4 shell; 4s, 4p, 4d, 4f
The new sub-shell added always has higher energy.
The 4s sub-shell is at a lower energy level than the 3d sub-shell, therefore it fills first. (Order = 3p, 4s, 3d)

Question 9

What are the rules of the filling of orbitals?

Answer 9

1. Orbitals fill in order of increasing energy.
2. Electrons pair with opposite spins.
3. Orbitals with the same energy are occupied singly first.

Question 10

Why do electrons pair with opposite spins?

Answer 10

-electrons are negatively charged so repel one another.
- electrons have a property called spin; either up or down.
An electron is shown as an arrow indicating its spin.
- the 2 electrons in an orbital must have opposite spins in order to counteract the repulsion between the negative charges.

Question 11

Why are orbitals with the same energy occupied singly first?

Answer 11

To prevent any repulsions between paired electrons until no further orbital is available at the same energy level.

Question 12

What is electron configuration?

Answer 12

Shows how sub-shells are occupied by electrons in an atom.
Can be expressed simply in terms of the previous noble gas plus the outer electron sub-shells. This is useful for emphasising similarities in the outer shell configuration.

Question 13

What blocks is the periodic table divided into?

Answer 13

S-block:
- highest energy electrons in s-sub-shells. (Left block of 2 groups)

P-block:
-highest energy electrons in p-sub-shells. (right block of 6 groups)

D-block:
- highest energy electrons in d-sub-shell. (centre block of 10 groups)

Question 14

Explain the electron configuration of ions.

Answer 14

When forming ions (s and p-block) the highest energy sub-shells lose or gain electrons.

For ions in the d-block, the 4s sub-shell fills and empties before the 3d sub-shell.

Question 15

What is ionic bonding?

Answer 15

The electrostatic attraction between positive and negative ions. It holds together cations and anions in ionic compounds.

Question 16

What are the common cations and anions?

Answer 16

Cations:

• metal ions; Na+ , Ca^2+, Al^3+
• ammonium ions; NH4^+

Anions:

• non-metal ions; Cl- , O^2-
• polyatomic ions; NO3^- , SO4^2-

Question 17

What is a dot and cross diagram?

Answer 17

In dot and cross diagrams, outer shell electrons from a metal atom and transferred to the outer shell of a non-metal atom.

Question 18

Explain the structure of ionic compounds.

Answer 18

• each ion attracts oppositely charged ions in all directions.
  This results in a giant ionic lattice.

Question 19

Explain the melting and boiling point of ionic compounds.

Answer 19

• almost all ionic compounds are solid at room temp.
• most have a high mp and bp.
  High temperatures provide the large quantity of energy needed to overcome the strong electrostatic attraction between the ions.
• melting points are higher for lattices containing ions with greater charges as there is stronger attraction.
• ionic attraction also depends on the size of ions.

Question 20

Explain the solubility of ionic lattices.

Answer 20

• many ionic compounds dissolve in polar solvents (eg. Water)
  Polar water molecules break down the lattice and surround each ion in solution.
  -compounds with ions of large charges may not be very soluble, as ionic attraction is too strong to break.
• depends on the relative strengths of the attractions in the lattice and attractions between ions and water molecules.

Question 21

Explain the electrical conductivity of ionic lattices.

Answer 21

• in solid state = ionic compound does not conduct electricity as there are no delocalised electrons.
• can conduct when melted or dissolved in water as solid lattice breaks down and ions are now free to move.

Question 22

Explain covalent bonding.

Answer 22

It is the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atom.
Occurs between:
- non-metallic elements.
- compounds of non-metallic elements.
- polyatomic ions.
The atoms are bonded together in a single small unit- a small molecule, a giant covalent structure or a charged polyatomic ion.

Question 23

Explain a covalent bond in terms of orbitals.

Answer 23

A covalent bond is the overlap of atomic orbitals (each with 1 electron) to give a shared pair of electrons.

• shared pair is attracted to the nuclei of both atoms.
• bonded atoms often have the same outer electron structure as the nearest noble gas.

Question 24

What is meant by: a covalent bond is localised?

Answer 24

The attraction is localised, acting solely between the shared pair of electrons and the nuclei of the two bonded atoms.
The result can be a small unit called a molecule (consisting of two or more atoms)
A molecule is the smallest part of a covalent compound that can exist whilst retaining the chemical properties of the compound.

Question 25

How can covalent bonds be shown?

Answer 25

Dot and cross diagrams:
-in covalent bonding electrons are shared unlike ionic bonding where they are transferred.
- atoms are drawn as overlapping circles.
- can also be shown as displayed formula which shows the relative positioning of atoms and the bonds between them (as lines).
Both ways also show any lone pairs.

Question 26

What is meant by expansion of the octet?

Answer 26

When the outer shell contains far more than the nearest noble gas, it is called expansion of the octet.
This is only possible from the n=3 shell, when a d-sub-shell becomes available for expansion.

Question 27

What is a multiple covalent bond?

Answer 27

Exists when two atoms share more than one pair of electrons.

Question 28

Double covalent bond?

Answer 28

• the electrostatic attraction between 2 shared pair of electrons and nuclei of the bonded atoms.
• all atoms have 8 electrons in their outer shell and structure is of the nearest noble gas.

Question 29

Triple covalent bond?

Answer 29

• electrostatic attraction between 3 shared pairs of electrons and nuclei of the bonded atoms.
• all atoms have electron structure of the nearest noble gas.

Question 30

What are dative covalent bonds?

Answer 30

A dative bond (aka coordinate bond) is a covalent bond in which the shared pair of electrons has been supplied by one of the bonded atoms only.
- shared pair of electrons was originally a lone pair of electrons on one of the atoms.

Question 31

What is average bond enthalpy?

Answer 31

Serves as a measurement of covalent bond strength.

The larger the value of average bond enthalpy, the stronger the covalent bond.