AQA Science © Nelson Thornes Ltd 2006 1 C3 1.2 The modern periodic table How are the electrons arranged in an atom?

AQA Science © Nelson Thornes Ltd 2006 1

C3 1.2 The modern periodic table

How are the electrons arranged in an atom?



ATOMSAll atoms of the same

element have the same proton number.

1H

2He

3Li

5B

4Be 6C

7 N

9F10Ne

11Na12Mg

16S

17Cl

18Ar

19K

20Ca8O

13Al

14Si15P

Elements 1–20

Number of electrons = number of protons (unless the atom is charged).



The size of the nucleus compared with the size of the atom is like ‘the size

of a man’s fist compared with the dome of St Paul’s Cathedral’ (Rutherford).

INSIDE AN ATOM

The nucleus contains almost the entire mass of the atom.

Tiny electrons whizz around the nucleus.

Almost all of the atom is empty space.



ELECTRONSElectrons can’t just go

anywhere. They move in spaces called orbitals.

nucleuselectron

Diagram not to scale.

Orbitals have complicated shapes

which are represented by the circles.

The further away they are from the nucleus, the higher the energy

of the electrons.



ENERGY LEVELS ON A HILLSIDE

Part of a hill

These stones have least energy because they are

nearest the bottom.

This stone has most energy because it has furthest to fall.

Each different height from the ground is an ENERGY LEVEL. Stones must be at one level or

another, not in between.

Increasing energy



Increasing energy

Level 1

Level 2

Level 3

Level 4

2

Nucleus

8

18

32

The further away you move from the nucleus, the more orbitals available, but they are of higher energy.

ENERGY LEVELS AROUND THE NUCLEUS OF AN ATOM

Figures in red show maximum number of electrons in each energy level.



Energy

Level 1

Level 2

Level 3

Level 4

2

Nucleus

8

18

32

ARRANGEMENT OF ELECTRONS IN A HYDROGEN ATOM: 1H

Electronic configuration: 1.

The electron occupies the lowest available

energy level.



Energy

Level 1

Level 2

Level 3

Level 4

2

Nucleus

8

18

32

ARRANGEMENT OF ELECTRONS IN A HELIUM ATOM: 2He

Electronic configuration: 2.

The electron occupies the lowest available

energy level.



Energy

Level 1

Level 2

Level 3

Level 4

2

Nucleus

8

18

32

ARRANGEMENT OF ELECTRONS IN A LITHIUM ATOM: 3Li

Electronic configuration: 2, 1.

The new electron occupies the lowest

available energy level.

filled orbital

most recently filled orbital



Energy

Level 1

Level 2

Level 3

Level 4

2

Nucleus

8

18

32

ARRANGEMENT OF ELECTRONS IN A NEON ATOM: 10Ne

Electronic configuration: 2, 8.



filled orbital




Energy

Level 1

Level 2

Level 3

Level 4

2

Nucleus

8

18

32

ARRANGEMENT OF ELECTRONS IN A POTASSIUM ATOM: 19K

Electronic configuration: 2, 8, 8, 1.



filled orbital


not available



Energy

Level 1

Level 2

Level 3

Level 4

2

Nucleus

8

18

32

ARRANGEMENT OF ELECTRONS IN A CALCIUM ATOM: 20Ca




filled orbital


not available



Energy

Level 1

Level 2

Level 3

Level 4

2

Nucleus

8

18

32

ARRANGEMENT OF ELECTRONS IN A SCANDIUM ATOM: 21Sc




filled orbital




ELECTRON ARRANGEMENTS AND THE PERIODIC TABLE

This periodic table shows all the elements in proton number order. The number of elements in each period is shown in red.

2

8

8

18

18

32

30



FAMILIES OF METALS

Group 1

3Li 2, 1

11Na 2, 8, 1

19K 2, 8, 8, 1

37Rb 2, 8, 18, 8, 1

55Cs 2, 8, 18, 18, 8, 1

Group 2

4Be 2, 2

12Mg 2, 8, 2

20Ca 2, 8, 8, 2

38Sr 2, 8, 18, 8, 2

56Ba 2, 8, 18, 18, 8, 2

Increasing reactivity




FAMILIES OF NON-METALS

Group 6

8O 2, 6

16S 2, 8, 6

Group 7

9F 2, 7

17Cl 2, 8, 7

35Br 2, 8, 18, 7

53I 2, 8, 18, 18, 7





THE NOBLE GASES

Group 0

2He 2

10Ne 2, 8

18Ar 2, 8, 8

36Kr 2, 8, 18, 8

54Xe 2, 8, 18, 18, 8Almost completely

unreactive



FORMING A SODIUM ION

Sodium atom, Na, 2, 8, 1. Sodium ion, Na+, 2, 8.

Na(g) Na+(g) + e–

The sodium ion is much smaller than the sodium atom.



Energy

Level 1

Level 2

Level 3

Level 4

Nucleus

FORMING POSITIVE IONS

The more energy the electron already has, the less extra energy is needed to remove it.

Complete orbitals shield the outer electrons from the pull of the nucleus.

This means it takes less energy to remove an electron from an outer orbital than from an inner one.

Li Li+ + e– requires 520 kJ/mol.

Cs Cs+ + e– requires 376 kJ/mol.

Given enough extra energy, an electron can escape from the pull

of the nucleus.

The higher its energy level, the less extra energy it will

need.

Na Na+ + e– requires 496 kJ/mol.

Estimate the values for K and Rb.



FORMING A CHLORIDE ION

Cl(g) + e– Cl–(g)

The chloride ion is largerthan the chlorine atom.

Chlorine atom, Cl, 2, 8, 7. Chloride ion, Cl–, 2, 8, 8.



Energy

Level 1

Level 2

Level 3

Level 4

Nucleus

FORMING NEGATIVE IONS

The stronger the attraction of the nucleus for an extra electron, the more energy will be given out when a negative ion is formed.

Complete orbitals shield the outer electrons from the pull of the nucleus. So the larger the atom, the less its attraction for an extra electron.

Cl + e– Cl– gives out 349 kJ/mol. Estimate the value

for iodine.

The nucleus can attract

electrons into the outer shell of the atom.

The smaller the atom, the

stronger the pull of the nucleus.

Br + e– Br –

gives out 325 kJ/mol.

Documents

AQA Science © Nelson Thornes Ltd 2006 1 C3 1.2 The modern periodic table How are the electrons arranged in an atom?