1. HC
CHEMISTRY HC
CHEMISTRY
(B) Periodicity

2. Covalent radius and IonisationHC
CHEMISTRY HC
CHEMISTRY
(B) Periodicity
Covalent radius and Ionisation
After completing this topic you should be able to :
The covalent radius is a measure of the size of an atom.
The trends in covalent radius across periods and down groups can be explained in terms of the number of occupied shells, and the nuclear charge.
The trends in ionisation energies across periods and down groups can be explained in terms of the atomic size, nuclear charge and the screening effect due to inner shell electrons.

3. Covalent radius

4. Atomic Size There is no definite edge to an atom.
However, bond lengths
can be worked out.
Atomic radius,
½ the distance between nuclei.
To find the bond length, add 2 covalent radii together.
pm = picometre X 10 – 12 m

5. Variation of covalent radius with atomic number
Adapted from New Higher Chemistry E Allan J Harris Rb K Na Li I Br Cl
The covalent radii of the elements in
any period decrease with increasing atomic number. F

6. Variation of covalent radius with atomic number
Adapted from New Higher Chemistry E Allan J Harris Cs Rb K Na Li I Br Cl
The covalent radii of the elements in
any group increase with increasing atomic number. F

7. Variation of covalent radius with atomic number
Adapted from New Higher Chemistry E Allan J Harris
No values are given for the Nobel gases Why?
Unreactive so do not form bonds

8. Covalent radius

9. COVALENT RADIUS
Explain the change in covalent radius as you go along a period.
The covalent radius of an element is half the distance between the nuclei of two of its covalently bonded atoms.
The covalent radius decreases as you go along a period
As you go along a period there is a greater positive charge on the nucleus The shells or energy levels of electrons are more strongly attracted to the nucleus and therefore the size of the atoms decreases.

10. COVALENT RADIUS
Explain the change in covalent radius as you go down a group.
The covalent radius of an element is half the distance between the nuclei of two of its covalently bonded atoms.
The covalent radius increases as you go down a group.
As you go down a group there are more energy levels of electrons. The outer electrons are further away from the nucleus so the atoms are larger.

11. Ionisation energy

12. Ionisation energies
This is defined as "the amount of energy required to remove one mole of electrons from one mole of atoms in the gaseous state”
Energy
M (g)  M+(g) + e 1st ionisation e e + +
M (g)
The outermost electron will be the most weakly held and is removed first

13. Ionisation energies
This is defined as "the amount of energy required to remove one mole of electrons from one mole of atoms in the gaseous state”
Energy e
M (g)  M+(g) + e 1st ionisation e +
M +(g) 2
M(g)+  M(g)2+ + e 2nd ionisation

14. ionization energy
The ionization energy of a chemical element, expressed in joules (or electron volts), is usually measured in an electric discharge tube in which a fast-moving electron generated by an electric current collides with a gaseous atom of the element, causing it to eject one of its electrons.

15. The ionisation energy is an enthalpy change and therefore is measured per mole.
Units kJmol-1 (kilojoules per mole).

16. Ionisation energies kJ mol-1
Overall increase along period Li
526 Be
905 B
807 C
1090 N
1410 O
1320 F
1690 Ne
2090 Na
502 Mg
744 Al
584 Si
792 P
1020 S
1010 Cl
1260 Ar
1530 K
425 Ca
596 Ga
577 Ge
762 As
953 Se
941 Br
1150 Kr
1350 Rb
409 Sr
556 In Sn
715 Sb
816 Te
870 I Xe
1170
Decrease down group

17. First and Second ionisation energies of the first 20 elements
Adapted from New Higher Chemistry E Allan J Harris
In each period there is an overall increase peaking at the noble gas He Ne Ar H Li Na

18. FIRST IONISATION ENERGY
Explain the change in first ionisation energy as you go along a period.
The first ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of atoms in the gaseous state.
The first ionisation energy increases as you go along a period
As you go along a period there is a greater positive charge on the nucleus. There is a greater attraction between the outer electron and the nucleus. More energy needs to be supplied to remove the electron.

19. First and Second ionisation energies of the first 20 elements
Adapted from New Higher Chemistry E Allan J Harris
Down a group first ionisation energy decreases He Ne Ar Li Na K

20. FIRST IONISATION ENERGY
Explain the change in first ionisation energy as you go down a group.
The first ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of atoms in the gaseous state.
The first ionisation energy decreases as you go down a group.
As you go down a group there are more energy levels of electrons. The outer electron is further from the nucleus. The inner electrons shield the outer electron from the effect of the nucleus. Less energy is needed to remove the outer electron.

21. First and Second ionisation energies of the first 20 elements
Adapted from New Higher Chemistry E Allan J Harris
For each element the second ionisation energy is higher than the first ionisation energy.

22. First ionisation energy – first mole of electrons removed
Explain why the second ionisation energy of an element is always greater than the first ionisation energy:
First ionisation energy – first mole of electrons removed
M(g)  M+(g) + e
Second ionisation energy – second mole of electrons removed
M+(g)  M2+(g) + e
In the second ionisation energy negative electrons are being removed from positive ions rather than neutral atoms.
In the positive ion there is a greater attraction for the electrons so more energy is needed to remove the second mole of electrons.

23. This requires a lot of energy
Explain why the second ionisation energy of K is much greater than the second ionisation energy of Mg:
K (g)  K+ (g) + e
Mg (g)  Mg+ (g) + e
2,8,8,1
2,8,8
2,8,2
2,8,1
K+ (g)  K2+ (g) + e
Mg+ (g)  Mg2+ (g) + e
2,8,8
2,8,7
2,8,1
2,8
The second ionisation of K involves removing an electron from a stable electron arrangement.
This requires a lot of energy
The second ionisation of Mg involves removing an electron to form a stable electron arrangement.
This requires less energy

24. Successive ionisation Energies
first ionisation energy E(g) E+(g) e-
second ionisation energy E+(g) E 2+ (g) + e-
third ionisation energy E 2+(g) E 3+ (g) e-
fourth ionisation energy E 3+(g) E 4+ (g) e-
ionisation energies increase as successive electrons are removed
- removing an electron from a filled inner shell requires a large increase in energy

25. The first four ionisation energies of aluminium, for example, are given by
Al(g) Al+(g) e st I.E. = 577 kJ mol-1
Al+(g) Al2+ (g) + e nd I.E. = 1820 kJ mol-1
Al2+(g) Al3+ (g) e rd I.E. = 2740 kJ mol-1
Al3+(g) Al4+ (g) e th I.E. = kJ mol-1
In order to form an Al3+(g) ion from Al(g) you would have to supply:
= 5137 kJ mol-1

26. First and Second ionisation energies of the first 20 elements
Adapted from New Higher Chemistry E Allan J Harris
It is worth noting the Nobel gases have the highest value for each period. This goes some way to explaining the great stability of filled orbital's and the resistance of the Nobel gases to form compounds. He Ne Ar

27. Nobel gas compounds
If some other change can compensate for the energy required then ionic compounds of Nobel gases can be made.
Can you suggest why the first Nobel gas compound prepared contained Xe?

28. Questions for you to try:
Explain why the third ionisation energy of Magnesium is so much greater than its second.
2. Calculate the energy change for the following changes.
Ca (g) → Ca2+ (g) + 2e-
B2+(g) → B4+ (g) + 2e-
Removing the third mole of electrons involves breaking into a stable complete energy level of electrons.
= 1756 kJmol-1
= kJmol-1

29. 3. Which of the following equations represents the first ionisation energy of fluorine?
A F–(g) → F(g) + e–
B F–(g) → F2(g) + e–
C F(g) → F+(g) + e–
D F2(g) → F+(g) + e– C

30. 4. Which line in the table is likely to be correct for the element francium?
State at 30 °C
First ionisation
energy/kJ mol–1 A
solid
less than 382 B
liquid C
greater than 382 D B

31. 5. As the atomic number of the alkali metals increases
A the first ionisation energy decreases
B the atomic size decreases
C the density decreases
D the melting point increases. A

32. 6. Explain why
A potassium atom is larger than a sodium atom
The Chlorine atom is smaller than a sodium atom
Potassium has an extra energy level (shell) of electrons.
Both atoms have the same number of energy levels, but the chlorine has a greater nuclear charge than sodium. This attracts the outer electrons more strongly and causes the decrease in atomic radius.

33. 7. Atoms of different elements are different sizes.
What is the trend in atomic size across the period from sodium to argon?
Decreases or gets smaller

34. 8. Which of the following reactions refers to the third ionisation energy of aluminium?
A Al(s) → Al3+(g) + 3e–
B Al(g) → Al3+(g) + 3e–
C Al2+(g) → Al3+(g) + e–
D Al3+(g) → Al4+(g) + e– C

35. 9. Atoms of different elements have different ionisation energies.
Explain clearly why the first ionisation energy of potassium is less than the first ionisation energy of sodium.
Potassium has more electron shells (or outer electron is further from the nucleus)
The inner electrons (electron shells) shield (screen) the outer electron from the attraction of the nucleus.
Therefore the outer electron is held less tightly in potassium

36. Periodic Table of Data Visual database of the physical and thermochemical properties of the chemical elements which allows the user to plot graphs and tables, play games and view diagrams. Periodic Landscapes The Periodic Landscape images are computer-generated landscape views and models based on patterns and relationships within the periodic table. The models are sculpted to achieve a sense of general trends or patterns.