Periodicity

Periodicity Objectives• understand periodicity in terms of a repeating pattern

across different periods

• understand reasons for the trends in the following properties of the elements from periods 2 and 3 of the Periodic Table:i) the melting and boiling temperatures of the elements, based on given data, in terms of structure and bondingii) ionisation energy based on given data or recall of the plots of ionisation energy versus atomic number

• be able to illustrate periodicity using data, including electronic configurations, atomic radii, melting and boiling temperatures and first ionisation energies

PeriodicityDefinition:

Periodicity is a regular periodic variation of properties of elements with atomic number and position in

the Periodic Table.

Order of the Periodic Table 1. The first attempts to arrange the

periodic table arranged the elements in order of atomic mass.

2. The modern periodic table organises them in a closely related way –by atomic number the number of protons.

3. Identify the elementswhere these two orders are different.

1234567

Transition metals

Periods (rows)

The Periodic TableGroups (columns)1234567

1 2 3 4 5 6 7 0

• Elements are arranged in order of their atomic number.

• The elements in each group have similar properties.

• Elements show changing trends across a period.

Metals, left of red lineNon metals, right of red line

Periodicity• Across a period elements change

from metallic to non-metallic.• What do you notice as you descend

the table?• The dividing line moves to the right.

Your task.....• Look at the information you have

been given• Describe all the trends you can see

for each one and explain them

• First look at 1 – 4 first

Group 4:

What do you notice about this group?

1

Periodic table and electron structure

• In terms of electron structure:• What do elements of a group have in

common?– Same number of electrons in outer shell– Same type of orbitals– Hence similar chemical behaviours

First ionisation energies of the first 20 elements

© Pearson Education Ltd 2008This document may have been altered from the original

Describe all the trends you can see in the graph.Can you explain this trend in terms of the atomic

structure?

2

Ionisation EnergyThe nuclear attraction is affected by

three things:• Atomic radius– Greater radius = less attraction.

• Nuclear charge– Larger charge = larger attraction.

• Electron shielding– Inner shells repeal electrons outside

them, this is called shielding or screening.–More inner shells = lower attraction

Across a Period

• There is a general increase across the period• The number of protons increases, so there is more

attraction acting on the electrons • Electrons are added to the same shell, so the outer

shell is drawn inwards slightly. Same no. of shells = same level of shielding, but more nuclear charge.

Period 2Period 3

P1

Across a Period

There is a sharp decrease in 1st IE between the end of one period and the start of the next.

This reflects the addition of a new shell. •Increased distance from the outer shell to the nucleus•Increased electron shielding of the outer electrons by the inner shells.

Atomic radius gets smaller

© Pearson Education Ltd 2008This document may have been altered from the original

Trend in first ionisation energy and atomic radius across a period

Describe all the trends you can see in the diagramCan you explain these in terms of atomic structure?

3

Across the period electrons are being added to the same principal energy level.•The nuclear charge increases.•The degree of shielding is the same for each successive element.•As a result, the electrons are pulled nearer the nucleus, reducing the atomic radius.

The first ionization energies for the elements in period 3 show a general increase.

All principal energy levels contain one or more sub-levels, with different but exact energy values.

However, aluminium’s value is below that of magnesium. This suggests that the third principal energy level is not one single energy level.

Aluminium’s electron is in a different sub-shell which is slightly further from the nucleus than magnesium's.

element

first

ioni

zati

on e

nerg

y(k

J mol

-1)

600

800

1000

1200

1400

Na Mg Al P Ar400

Si S Cl

1600

Remember the general trend is that first ionisation energies increase

Evidence for Hund’s ruleThe first ionization energies for the elements in period 3 show a general increase.

element

first

ioni

zati

on e

nerg

y(K

J mol

-1)

600

800

1000

1200

1400

Na Mg Al P Ar400

Si S Cl

1600 However, sulfur’s value is below that of phosphorus.

In phosphorous the electrons in the 3 p sub level are unpaired (3p3).

Sulfur has an extra electron so this is paired (3p4 ). This electron is easier to remove because of repulsion of the other electrons.

© Pearson Education Ltd 2008This document may have been altered from the original

Trend in first ionisation energy and atomic radius down a group

Describe all the trends you can see in the diagram

Can you explain these in terms of atomic structure?

4

• 1st IE’s decrease

• The number of shells increases, so the distance between the nucleus and the outer electrons – weaker attractive force.

• More inner shells, so the shielding increases – less attraction.

© Pearson Education Ltd 2008This document may have been altered from the original

Trend in first ionisation energy and atomic radius down a group

Describe all the trends you can see in the diagram

Can you explain these in terms of atomic structure?

4

• Ionic radius increases

• More shells of electrons

• Increased electron shielding

• Also less attraction means electrons are pulled in less by the nucleus

Periodicity Objectives• understand periodicity in terms of a repeating pattern

across different periods

• understand reasons for the trends in the following properties of the elements from periods 2 and 3 of the Periodic Table:i) the melting and boiling temperatures of the elements, based on given data, in terms of structure and bondingii) ionisation energy based on given data or recall of the plots of ionisation energy versus atomic number

• be able to illustrate periodicity using data, including electronic configurations, atomic radii, melting and boiling temperatures and first ionisation energies

• Now look at numbers 5 – 6

• Describe all the trends that you can see

• Explain them

Boiling points of Period 2 and Period 3 elements

Describe all the trends you can see in the graph.Can you explain this trend in terms of the bonding

and structure of the elements?Look at the elements.What happens to the elements as you move across

a period?

5

Trends in Boiling Points• Boiling points are based on the type

of bonding present.• And there are trends within each

type of bonding….

Boiling PointsTrend in BP across each period:

Group 1 – 4 Group 4 – 5 Group 5 – 0 Li C C N N NeNa Si Si P P ArGeneral increase in Boiling point

Sharp decrease in boiling point

Comparatively low boiling points

Distinct change between group 4 and 5 in structure and forces

• From giant structures to simple molecular• From strong to weak forces. Trend in melting point is similar – sharp decrease

between group 4 and 5 marking the change from giant to simple molecular.

Increase in boiling point from Na to Al

Describe all the trends you can see in the diagramCan you explain these in terms of atomic structure?

The ions are drawn to reflect their relative size.

6

From sodium to aluminium:•The positive charge increases and the atomic radii decreases•The number of delocalised electrons per atom increases.•This results in a higher charge density•This increases the strength of the metallic bond which in turn increases the melting and boiling points.

Complete the worksheets.

Atomic Radius Across Period 31. State and explain the general trend in atomic

radius across Period 3 (excluding Argon). (4 marks)

2. Atomic radius is a general term. Measurements are taken of metallic radii for metals and covalent radii for molecules. Draw a diagram to show how you could calculate the atomic radius of 2 covalently bonded atoms. (3 marks)

3. Why does Argon not follow the trend? (2 marks)

4. State the effect of atomic radius on the first ionisation energy of an element. (1 mark)

Atomic Radius Across Period 31. State and explain the general trend in atomic

radius across Period 3 (excluding Argon). (4 marks)

Atomic Radius Across Period 3

2. Atomic radius is a general term. Measurements are taken of metallic radii for metals and covalent radii for molecules. Draw a diagram to show how you could calculate the atomic radius of 2 covalently bonded atoms. (3 marks)

Atomic Radius Across Period 3

3. Why does Argon not follow the trend? (2 marks)

4. State the effect of atomic radius on the first ionisation energy of an element. (1 mark)

6 marksJune 2012 Q2

Exam Questions

Periodicity Objectives• understand periodicity in terms of a repeating pattern

across different periods

• understand reasons for the trends in the following properties of the elements from periods 2 and 3 of the Periodic Table:i) the melting and boiling temperatures of the elements, based on given data, in terms of structure and bondingii) ionisation energy based on given data or recall of the plots of ionisation energy versus atomic number

• be able to illustrate periodicity using data, including electronic configurations, atomic radii, melting and boiling temperatures and first ionisation energies