Periodicity - Period 3

Question 1

When discussing period 3 elements, you can compare atomic properties and physical properties.
What are the 4 topics you can talk about relating to atomic properties?
What are the 3 topics you can talk about relating to physical properties?

Answer 1

Atomic properties:
a) electronic structures
b) first ionisation energy 
c) atomic radius 
d) electronegativity 
Physical properties: 
a) structures of the elements 
b) electrical conductivity 
c) melting and boiling points

Question 2

List which blocks the period 3 elements are in, and why.

Answer 2

sodium and magnesium are in the s-block because their outermost electron is held in an s orbital
the remaining elements (al, si, p, s, cl, ar) are in the p-block because their outermost electrons are held in p orbitals

Question 3

What is first ionisation energy?

Answer 3

first ionisation energy is the energy that is required to remove `1 electron from every atom in a mole of gaseous atoms to produce 1 mole of electrons and 1 mole of ions each with a single positive charge

Question 4

What is the general trend of ionisation energy across period three?

Answer 4

first ionisation energy increases across the period

Question 5

Explain the trend of ionisation energy across a period, with a few words for each point

Answer 5

• increased nuclear charge
• same distance
• same screening

Question 6

Explain in detail, why the trend for ionisation energy generally decreases or increases

Answer 6

Increases because

• across a period the atomic radii is not hugely different so this factor does not influence the trend
• all electrons are in the same energy level (energy level 3 for period 3) so the amount of screening across a period does not change
• but the amount of protons increases as you go along so there is a greater attraction between nucleus and electrons

Question 7

Where is the general trend broken in period 3, and why?

Answer 7

• dips at group 3, aluminium
  because: previous elements are in s block, aluminium however holds outermost electron in p orbital. p orbitals are more remote, the electron is held at greater distance from nucleus and there is additional shielding
• dips at group 6, sulphur
  because: previous element, phosphorus, has unpaired electrons in p orbital. In sulphur, the extra electron means an orbital is occupied by a pair, this causes extra repulsion so it is easier to remove the electron

Question 8

Which period 3 element has the lowest IE?

Answer 8

Sodium

Question 9

Which period 3 element has the largest IE? Mention why.

Answer 9

Argon (it is a noble gas with a full outer shell of electrons)

Question 10

Summarise the trends of IE across period 3

Answer 10

• general increase across period 3

- dips at aluminium and sulphur

Question 11

Across period 3, what happens to the atomic radii of the atoms?

Answer 11

• it decreases between sodium through to chlorine

- argon is much bigger however

Question 12

What causes atomic radii to decrease between Na and Cl?

Answer 12

• there is no difference in shielding
• but nuclear charge is increasing so the attraction between the nucleus and outermost electrons is greater which pulls them closer together

Question 13

Why does increasing proton number effect the atomic radii in the elements of period three (excluding argon)?

Answer 13

• Na, Mg, Al are metallic structures
• Si, P, S and Cl are molecular substances with covalent bonds
• metallic and covalent radii are the measure of distance from the nucleus to the bonding pair of electrons
• increasing proton number pulls the bonding electrons more tightly to it hence decreasing the atomic radius

Question 14

Why is argon’s atomic radius much larger than chlorine, when you might expect it to be smaller due to the increased proton?

Answer 14

Argon only has van der Waals attractions, there are no covalent bonds and bonding pairs of electrons to pull closer.

Question 15

What is electronegativity?

Answer 15

Electronegativity is the measure of the tendency of an atom to attract a bonding pair of electrons towards it.

Question 16

What scale is electronegativity measured on?

Answer 16

The pauling scale that runs 0-4.0

Question 17

Which element in period 3 is the most electronegative?

Answer 17

Chlorine (think: FONCl)

Question 18

What is the trend in electronegativity across a period and why?

Answer 18

There is a steady increase as each element has an extra proton

Question 19

Which element is not assigned an electronegativity value at all and why?

Answer 19

Argon because it does not form compounds

Question 20

What are the three types of structure you see in period 3? Name the elements that belong to each group.

Answer 20

Metallic - Na, Mg, Al
Giant covalent - Si
Simple Molecular - P4, S8, Cl2, Ar

Question 21

Give a brief description of the general structures of Na, Mg and Al

Answer 21

• metallic structures
• closely packed metal ions surrounded by a sea of delocalised electrons that have been delocalised from the atoms outer shell of electrons
  Na: ions have 1+ charge, 1 electron per atom in the sea of electrons, 8-co-ordinated structure
  Mg: ions have 2+ charge, 2 electrons per atom in the sea of electrons, 12-co-ordinated structure
  Al: ions have 3+ charge, 3 electrons per atom in the sea of electrons, 12-co-ordinated structure

Question 22

What is meant by 8-co-oridinated and 12-co-ordinated structure, and which is more effective and why

Answer 22

8-co-ordinated structure means that each atom is touched by 8 other atoms. 12-co-ordinated structure means that each atom is touched by 12 other atoms.
12-co-ordinated is more effective because there is less space wasted

Question 23

Which period 3 element has a giant covalent structure? Give a description of this element.

Answer 23

• it is silicon
• 3 dimensional covalent bonds like diamond
• silicon is a metalloid (properties lie intermediate between metal and non metal)

Question 24

In the simple molecular structures, how are the atoms held together?

Answer 24

• by covalent bonds (in P4, S8 and Cl2)
• argon is monatomic so there are no covalent bonds
• in the liquid or solid state molecules are held together by van der waals attractions

Question 25

Which period 3 elements have good electrical conductivity and why?

Answer 25

• Sodium, Magnesium and Aluminium

- metallic structures that have a sea of delocalised electrons that can move throughout the structure

Question 26

Put the following elements in order of increasing electrical conductivity. Explain your answer.
Magnesium, Sodium, Aluminium

Answer 26

1. Na 2. Mg 3. Al

Because of the increased charge density, more electrons in the sea

Question 27

Describe silicons electrical conductivity?

Answer 27

Silicon is a semi conductor

Question 28

Which period 3 elements do not conduct electricity and why?

Answer 28

Phosphorus, Sulphur, Chlorine and Argon

They are simple molecular structures and have no electrons that are free to move

Question 29

What happens when a substance melts?

Answer 29

Some of the bonds between the particles are broken which lets the particles move freely around each other whilst still being close together

Question 30

What happens when a substance boils?

Answer 30

All of the bonds between molecules are broken and the particles can move freely in all directions

Question 31

What do the melting and boiling points of a substance depend on?

Answer 31

The substances structure and strength of the bonding involved

Question 32

Describe the trends of the boiling points of the period 3 elements, explaining the trend

Answer 32

Increase Na- Al (increasing strength of metallic bonds due to charge on the ions and charge density)
Slight dip at Si (giant covalent structure that requires lots of energy to break all of the bonds)
Decreases to P (P4, only van der waals forces)
Increases at S slightly (S8 rings make Sulphur the largest simple molecular molecule)
Decreases at Cl and Ar (Cl is diatomic, Ar is monatomic, so van der waals forces are pretty weak)

Question 33

How does the trend of boiling points differ from the trend of melting points of the elements of period 3?

Answer 33

It is the same except Silicon’s melting point increases past aluminium’s