1.3 Atomic Structure - Chemistry Teaching Resources

Chemical Changes & Structure 16/06/2013 

1.3 Atomic Structure 

This lesson topic revises and extends your understanding of Atomic Structure. 

Atomic Models 

Dalton Model 

Thompson Model 

Rutherford Model 

Bohr Model 

Cloud Model 

Early models of the atom imagined hard 

indestructible spheres similar to "Snooker 

Balls" colliding and bouncing off each other. 

This Model remains effective as part of our 

Particle Model of Matter. 

Scientists such as JJ Thompson were able to 

show, firstly, that atoms contained very small 

negatively charged particles (electrons) and 

later that they also contained positive particles 

(protons). The "Plum Pudding" model. 

Rutherford then showed that all the protons 

were concentrated in a tiny nucleus in the 

centre of the atom. and that over 99% of an 

atom was empty space. Finally the presence 

of neutral particles (neutrons) was proven. 

Bohr put forward the theory that electrons 

orbited the nucleus in shells rather like 

planets around the sun. This is the model 

most often used, though we now know that 

electrons do not move like this. 

We can also imagine electrons occupying 

cloud-like regions in space called "orbitals". 

This model is particularly useful when trying 

to visualise the shape of molecules and when 

dealing with multiple bonds. 

SUMMARY 3 types of particles; protons (+ve), neutrons and electrons (–ve). 

The protons and neutrons are squashed together in the nucleus. The 

nucleus is extremely small, heavy and positively charged. 

The electrons 'move' around the nucleus in a complex pattern 

KHS May 2013 page 13 

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Important Numbers 

Atomic Number - is 

the number of protons 

in the nucleus of all 

atoms in an element 

Mass Number - is 

the total number of 

protons and neutrons 

in the nucleus of an 

atom 

Electrons - In neutral atoms the number of electrons is equal to the 

number of protons so we can usually use the Atomic Number 

to tell us the number of electrons as well. 

Neutrons - The number of neutrons is simply the number of protons 

(Atomic Number) subtracted from the Mass Number. 


Each element has a different atomic number and 

they are listed in order of this number. Elements with 

similar properties are found in the same group. 

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Element Symbol Atomic Mass number of number of number of 

Number Number protons electrons neutrons 

Nitrogen 

Oxygen 

Neon 

Sodium 

Magnesium 

Silicon 

Phosphorus 

Sulphur 

Potassium 

Nickel 

Zinc 

Silver 

Tin 

Platinum 

Mercury 

N 7 14 7 7 14 - 7 = 7 

O 

Ne 

Na 

Mg 

Si 

P 

S 

K 

Ni 

Zn 

Ag 

Sn 

Pt 

Hg 


8 

10 

11 

12 

14 

15 

16 

19 

28 

30 

47 

50 

84 

80 

16 

20 

23 

24 

28 

31 

32 

39 

59 

66 

108 

119 

195 

201 

Number of protons = Atomic Number 

8 

10 

11 

12 

14 

15 

16 

19 

28 

30 

47 

50 

84 

80 

8 

10 

11 

12 

14 

15 

16 

19 

28 

30 

47 

50 

84 

80 

66 - 30 = 36 

Number of electrons = Number of protons = Atomic Number 

Number of neutrons = Total in nucleus - Number of protons 

= Mass Number - Atomic Number 

The Mass Number can only ever refer to one particular atom. However, when 

we want to talk generally about the mass of the atoms of an element, we can 

usually safely assume that the average mass (RAM) rounded to the nearest 

whole number can safely be used as the 'most likely' Mass Number for an atom 

of this element - but be careful, Hg has RAM 79.9 so we would assume 'most 

likely' Mass Number = 80, but only 79 Hg and 81 Hg exist naturally. 

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Nuclide Notation Nuclide Notation is the system which adds information 

about an atom to its Symbol. 

proton in the nucleus 1 

Isotopes 

2 

1 H 

Heavy hydrogen 

(deuterium) 

1 

1 H 

Ordinary hydrogen 

3 

1 H 

Very heavy hydrogen 

(tritium) 

Relative Atomic Mass (RAM) 


⎯⎯→ 

⎯⎯→ 

1 0 

orbiting around the nucleus - 1 

mass number 

atomic number 

7 

3 Li 

Isotopes are atoms of the same element which 

have the same number of protons but have 

different numbers of neutrons. 

This means that atoms of the same element 

can have different masses . 

Isotopes are atoms of the same atomic number but 

different mass numbers . 

Since atoms of the same element can 

have different masses, it is necessary to 

know the average mass - the 

relative atomic mass of an element. 

Information provided by a machine 

called a mass spectrometer can be used 

to calculate the RAM of an element. 

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① Each atom has an electron knocked 

off which leaves the atom as a 

positively charged ion . 

① 

➁ The ions are accelerated by an electric field; repelled 

by a positive plate, attracted towards a negative. 

➂ The strength of the magnetic field is gradually increased. 

④ Any ions that are of the correct mass will be deflected ‘round the corner’. 

⑤ Any ions which are still too heavy for the magnetic field will crash into 

the wall of the chamber. They will be detected later when the field is 

stronger. 

⑥➆ Any ions which are too light will be deflected too far. They would have 

been detected earlier when the field was weaker. 

⑦ Any ions arriving here are detected and counted. 

The mass spectrometer is able to tell us 3 things about an element: 

1. the number of isotopes that element has, 

2. the mass number of each isotope, and 

3. the relative amounts of each isotope. 

The information is printed out in the form of a mass spectrum. 


➁ 

④ 

➂ 

⑥ 

⑦ 

⑤ 

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Atomic 

No. (Z) 

3 Lithium 

5 Boron 

12 Magnesium 

14 Silicon 

24 Chromium 

* Some values within this table have been rounded / modified for simplicity 


From this information it is now possible to 

calculate the average mass of all the atoms 

in the element. 

RAM = ( mass 1 x % 1 ) + ( mass 2 x % 2 ) 

___________________________ 

100 

= ( 35 x 75 ) + ( 37 x 25 ) 

___________________________ 

100 

= ( 3575 ) + ( 37 ) 

___________________________ 

100 

= 

Name Symbol % Abundance RAM (Relative Atomic Mass) 

6 Li 

7 Li 

10 B 

11 B 

24 Mg 

25 Mg 

26 Mg 

28 Si 

29 Si 

30 Si 

50 Cr 

52 Cr 

53 Cr 

54 Cr 

7.59 

92.41 

19.90 

80.10 

78.99 

10.00 

11.01 

92.23 

4.68 

3.09 

4.35 

83.79 

9.50 

2.36 

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Isotopic Ions 

It is not just the number of neutrons that can be different 

in atoms of the same element. Atoms can also change their 

number of electrons. 

7 

3 Li 6 

3 Li 7 

3 Li+ 6 

3 Li+ 

protons = 3 

neutrons = 4 

electrons = 3 

protons = 3 

neutrons = 3 

electrons = 3 


protons = 3 

neutrons = 4 

electrons = 2 

protons = 3 

neutrons = 3 

electrons = 2 

The number of protons never changes. This is why the Atomic Number for an 

element is defined as the number of protons. 

Element Symbol Atomic Mass number of number of number of 

Number Number protons neutrons electrons 

Lithium 

Oxygen 

Chlorine 

Sodium 

Phosphorus 

Iron (II) 

Iron (III) 

Tin (II) 

Tin (IV) 

7 + Li 3 

16 

8 

O 2- 

37 

- Cl 17 

23 

+ Na 11 

31 

3- P 15 

56 

2+ Fe 26 

58 

3+ Fe 26 

2 + H 1 

116 

2+ Sn 50 

119 

4+ 

Sn 50 

3 

8 

17 

11 

15 31 15 15 16 

26 

26 

1 

50 

50 

7 

16 

37 

23 

56 

58 

2 

116 

119 

3 

8 

17 

11 

26 

26 

1 

50 

50 

4 

8 

20 

12 

30 

32 

1 

66 

69 

2 

10 

18 

10 

18 

24 

23 

0 

48 

46 

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Q1. SC Q5. Int2 

The grid shows information about some particles. 

Number of 

Particle protons neutrons electrons 

A 11 12 11 

B 9 10 9 

C 11 13 11 

D 19 20 18 

E 9 10 10 

a) Identify the particle which is a negative ion. 

_______ 

a) Identify the two particles which are isotopes. 

_______ and _______ 

Q2. Int2 

An atom has 26 protons, 26 electrons and 30 neutrons. The 

atom has. 

A atomic number 26, mass number 56 

B atomic number 26, mass number 52 

C atomic number 30, mass number 56 

D atomic number 30, mass number 82 

Q3. Int2 

Which line in the table describes a neutron? 

Mass Charge 

A 1 - 1 

B negligible 0 

C 1 + 1 

D 1 0 

Q4. Int2 

The isotopes of carbon and oxygen are given in the table. 

12 

Isotopes of carbon C 6 

16 

Isotopes of oxygen O 8 

13 

C 6 

17 

O 8 

14 

C 6 

18 

O 8 

A molecule of carbon dioxide with mass 46 could contain 

A one 12 C atom and two 16 O atoms 

B one 14 C atom and two 18 O atoms 

C one 12 C atom, one 16 O atoms and one 18 O atom 

D one 14 C atom, one 16 O atoms and one 18 O atom 


In the manufacture of glass, other chemicals can be added 

to alter the properties of the glass. The element boron can 

be added to glass to make oven proof dishes. 

Information about an atom of boron is given below. 

Particle Number 

proton 5 

electron 5 

neutron 6 

Use this information to complete the nuclide notation for 

this atom of boron. 

B 

Atoms of boron exist which have the same number of protons 

but a different number of neutrons from that shown 

in the table. 

What name can be used to describe the different atoms of 

boron? 

______________________________________ 

Q6. Int2 

Elements are made up of atoms. 

An atom of an element is represented by the diagram 

below. 

= protons 

= neutrons 

= electrons 

What name is given to the part of the atom which contains 

protons and neutrons? 

______________________________________________ 

Using the information in the diagram: 

a) state the mass of this atom; 

______________________ 

b) explain why this atom is electrically neutral; 

_______________________________________ 

_______________________________________ 

c) name the family of elements to which this atom 

belongs. 

_______________________________________ 

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1.3 Atomic Structure - Chemistry Teaching Resources

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