1. 1.1.2 Atomic Structure
• Describe protons, neutrons and electrons.
• Describe the distribution of mass and charge in an atom.
• Describe the contribution of protons and neutrons to the nucleus in terms of atomic number and mass number.
• Explain the term isotopes.
• Deduce the atomic structure in atoms and ions.

2. THE STRUCTURE OF ATOMS
Atoms consist of a number of fundamental particles,
the most important are ...
Mass / kg
Charge / C
Relative mass
Relative charge
PROTON
NEUTRON
ELECTRON

3. 1 +1 1 -1 1 1836 THE STRUCTURE OF ATOMS
Atoms consist of a number of fundamental particles,
the most important are ...
Mass / kg
Charge / C
Relative mass
Relative charge
PROTON
NEUTRON
ELECTRON 1 +1
1.672 x 10-27
1.602 x 10-19 1
1.675 x 10-27 1
1836 -1
9.109 x 10-31
1.602 x 10-19

4. 1 +1 1 -1 1 1836 THE STRUCTURE OF ATOMS
Atoms consist of a number of fundamental particles,
the most important are ...
Mass / kg
Charge / C
Relative mass
Relative charge
PROTON
NEUTRON
ELECTRON 1 +1
1.672 x 10-27
1.602 x 10-19 1
1.675 x 10-27 1
1836 -1
9.109 x 10-31
1.602 x 10-19
Calculate the mass of a carbon-12 atom; it has 6 protons, 6 neutrons and 6 electrons

5. 1 +1 1 -1 1 1836 THE STRUCTURE OF ATOMS
Atoms consist of a number of fundamental particles,
the most important are ...
Mass / kg
Charge / C
Relative mass
Relative charge
PROTON
NEUTRON
ELECTRON 1 +1
1.672 x 10-27
1.602 x 10-19 1
1.675 x 10-27 1
1836 -1
9.109 x 10-31
1.602 x 10-19
Calculate the mass of a carbon-12 atom; it has 6 protons, 6 neutrons and 6 electrons
6 x x x x x x =

6. 1 +1 1 -1 1 1836 THE STRUCTURE OF ATOMS
Atoms consist of a number of fundamental particles,
the most important are ...
Mass / kg
Charge / C
Relative mass
Relative charge
PROTON
NEUTRON
ELECTRON 1 +1
1.672 x 10-27
1.602 x 10-19 1
1.675 x 10-27 1
1836 -1
9.109 x 10-31
1.602 x 10-19
Calculate the mass of a carbon-12 atom; it has 6 protons, 6 neutrons and 6 electrons
6 x x x x x x = x kg

7. MASS NUMBER AND ATOMIC NUMBER
Atomic Number (Z) Number of protons in the nucleus of an atom
Mass Number (A) Sum of the protons and neutrons in the nucleus

8. MASS NUMBER AND ATOMIC NUMBER
Atomic Number (Z) Number of protons in the nucleus of an atom
Mass Number (A) Sum of the protons and neutrons in the nucleus
Mass Number (A)
PROTONS + NEUTRONS Na 23 11
Atomic Number (Z)
PROTONS

9. Na 23 11 MASS NUMBER AND ATOMIC NUMBER
Atomic Number (Z) Number of protons in the nucleus of an atom
Mass Number (A) Sum of the protons and neutrons in the nucleus
Mass Number (A)
PROTONS + NEUTRONS Na 23 11
THESE ALWAYS GO TOGETHER – ANYTHING WITH 11 PROTONS MUST BE SODIUM
Atomic Number (Z)
PROTONS

10. MASS NUMBER AND ATOMIC NUMBER
Atomic Number (Z) Number of protons in the nucleus of an atom
Mass Number (A) Sum of the protons and neutrons in the nucleus
Mass Number (A)
PROTONS + NEUTRONS Na 23 11
Atomic Number (Z)
PROTONS

11. MASS NUMBER AND ATOMIC NUMBER THERE WILL BE 12 NEUTRONS IN THE NUCLEUS
Atomic Number (Z) Number of protons in the nucleus of an atom
Mass Number (A) Sum of the protons and neutrons in the nucleus
Mass Number (A)
PROTONS + NEUTRONS
THERE WILL BE 12 NEUTRONS IN THE NUCLEUS
23 – 11 = 12 Na 23 11
Atomic Number (Z)
PROTONS

12. MASS NUMBER AND ATOMIC NUMBER
Atomic Number (Z) Number of protons in the nucleus of an atom
Mass Number (A) Sum of the protons and neutrons in the nucleus
Mass Number (A)
PROTONS + NEUTRONS Na 23 11
Atomic Number (Z)
PROTONS

13. MASS NUMBER AND ATOMIC NUMBER
Protons
Neutrons
Electrons
Charge
Atomic Number
Mass Number
Symbol A 19 21 B 20 40 C + 11 23 D 6 E 92
235 F 13 G 16 2- H
27Al3+

14. MASS NUMBER AND ATOMIC NUMBER
Protons
Neutrons
Electrons
Charge
Atomic Number
Mass Number
Symbol A 19 21 40
40K B 20
40Ca C 11 12 10 + 23
23Na+ D 6
12C E 92
143
235
235U F 7 13
13C G 16 18 2- 32
32S2- H 14 3+ 27
27Al3+

15. Subatomic particles in ions
In reduction and oxidation reactions, electrons are transferred.
When elements react to form ions, electrons are either lost or gained.
This results in a stable electron arrangement.
What changes to electrons occur during the formation of positive ions?
What changes to electrons occur during the formation of negative ions?

16. Name Symbol Protons neutrons electrons
Hydrogen - 1 H
Hydrogen - 2
Hydrogen - 3
Lithium - 6
Lithium - 7
Chlorine - 35
Chlorine - 37
Copper – 63
Copper – 65
Zirconium – 90
Zirconium - 94 1 2 1
Worksheet – isotope table

17. atom - ion
symbol
protons
neutrons
electrons
charge
hydrogen H
hydrogen ion
+ 1
sodium Na
sodium ion
magnesium
magnesium ion
+ 2
aluminum
aluminum ion
+ 3
nitrogen
nitride ion
- 3
oxygen
oxide
- 2
fluorine
fluoride -1
Worksheet – Atoms & ions

18. The arrangement of the electrons
The electrons are found at considerable distances from the nucleus in a series of levels called energy levels.
Each energy level can only hold a certain number of electrons.
The first level (nearest the nucleus) will only hold 2 electrons,
the second holds 8,
and the third also seems to be full when it has 8 electrons.
These levels can be thought of as getting progressively further from the nucleus.
Electrons will always go into the lowest possible energy level (nearest the nucleus) - provided there is space.

19. What is the electronic arrangement in chlorine ?
The Periodic Table gives you the atomic number of 17.
Therefore there are 17 protons and 17 electrons.
The arrangement of the electrons will be 2, 8, 7
(i.e. 2 in the first level, 8 in the second, and 7 in the third).
The electronic arrangements of the first 20 elements

20. Two important generalisations
If you look at the patterns in this table:
The number of electrons in the outer level is the same as the group number. (Except with helium which has only 2 electrons.
T he noble gases are also usually called group 0 - not group 8.) This pattern extends throughout the Periodic Table for the main groups (i.e. not including the transition elements).
So if you know that barium is in group 2, it has 2 electrons in its outer level; iodine (group 7) has 7 electrons in its outer level; lead (group 4) has 4 electrons in its outer level.
Noble gases have full outer levels.

21. 23Na+ 19F- 35Cl- 37Cl- 40Ca2+ 39K+ 32S2- 16O2- 14N3- 24Mg2+ 7Li+
Name
Symbol
Number of protons
Number of neutrons
Number of electrons
Electron configuration
23Na+
19F-
35Cl-
37Cl-
40Ca2+
39K+
32S2-
16O2-
14N3-
24Mg2+
7Li+
27Al3+
Exercise
Worksheet - Atomic Structure of ions

22. Tasks
Write out these Key definitions
Isotopes . .
Atomic ( proton ) number . .
Mass ( nucleon ) number . . .
An ion
Questions 1, 2
Examiners tips are . .

24. 1.1.3 Atomic Masses
• Explain why 12C is used as the standard measurement of relative mass.
• Define the terms relative isotopic mass and relative atomic mass.
• Calculate relative atomic masses.
• Work out relative molecular masses and relative formula masses.

25. Relative Atomic Mass (Ar)
RELATIVE MASSES
Relative Atomic Mass (Ar)
The mass of an atom relative to the 12C isotope having a value of
Ar = average mass per atom of an element x 12
mass of one atom of carbon-12
Relative Isotopic Mass
Similar, but uses the mass of an isotope 238U
Relative Molecular Mass (Mr)
Similar, but uses the mass of a molecule CO2, N2
Relative Formula Mass
Used for any formula of a species or ion NaCl, OH¯

26. ISOTOPES Definition Atoms with…
the same atomic number but different mass number or
the same number of protons but different numbers of neutrons.

27. ISOTOPES Definition Atoms with…
the same atomic number but different mass number or
the same number of protons but different numbers of neutrons.
Properties Chemical properties of isotopes are identical
Physical properties (such as density) can differ

28. ISOTOPES Definition Atoms with…
the same atomic number but different mass number or
the same number of protons but different numbers of neutrons.
Properties Chemical properties of isotopes are identical
Physical properties (such as density) can differ
Theory Relative atomic masses measured
by chemical methods rarely produce
whole numbers but they should do
(allowing for the low relative mass of
the electron). This was explained
when the mass spectrograph revealed
that atoms of the same element could
have different masses due to the
variation in the number of neutrons in
the nucleus. The observed mass was
a consequence of the abundance of
each type of isotope.
ISOTOPES OF HYDROGEN
Protons
Neutrons 1 H 2 3

29. ISOTOPES - CALCULATIONS
There are two common isotopes of chlorine. Calculate the average relative atomic mass of chlorine atoms
Protons
Neutrons % 35 Cl 17 18 75 37 20 25

30. ISOTOPES - CALCULATIONS
There are two common isotopes of chlorine. Calculate the average relative atomic mass of chlorine atoms
Method 1 Three out of every four atoms will be chlorine-35
Average = = 35.5 4
Protons
Neutrons % 35 Cl 17 18 75 37 20 25

31. ISOTOPES - CALCULATIONS
There are two common isotopes of chlorine. Calculate the average relative atomic mass of chlorine atoms
Method 1 Three out of every four atoms will be chlorine-35
Average = = 35.5 4
Method 2 Out of every 100 atoms 75 are 35Cl and are 37Cl
Average = (75 x 35) + (25 x 37) = 35.5
100
Protons
Neutrons % 35 Cl 17 18 75 37 20 25

32. MASS SPECTRA
An early application was the demonstration by Aston, (Nobel Prize, 1922), that naturally occurring neon consisted of 3 isotopes Ne 21Ne Ne.
• positions of peaks gives atomic mass
• peak intensity gives relative abundance
• highest abundance is scaled up to 100%
- other values are adjusted accordingly.
m/z
90.92
0.26
8.82
Abundance / %
Calculate the average relative atomic mass of neon using the above information.
Out of every 100 atoms are 20Ne , are 21Ne and are 22Ne
Average = (90.92 x 20) + (0.26 x 21) + (8.82 x 22) =
100
Relative atomic mass =

33. Can we calculate the % composition if we know the isotopes ?
Naturally occurring potassium consists of potassium-39 and potassium-41.
Calculate the percentage of each isotope present if the average is 39.1.
Assume there are x nuclei of 39K in every 100; so there will be (100-x) of 41K
so x (100-x) =
100
therefore x x = 3910
thus x =
and x = 95
ANSWER There will be 95% 39K and
5% 41K

34. Tasks
Write out these Key definitions
Relative isotopic mass
Relative atomic mass
Relative molecular mass
Relative formula mass

35. Calculate the relative atomic mass of the following
– give your answers to 3 significant figures
Bromine : 79 Br 50% , 81 Br 50%
Copper : Cu 69% , 65 Cu 31%
Zirconium : Zr 51.5% , Zr 11.2%, Zr 17.1%,
94 Zr 17.4%, Zr 2.8%
Lead : 204 Pb 1.5% , Pb 23.6%, Pb 22.6%, Pb 52.3%
Neon : Ne 90.9% , Ne 0.2%, Ne 8.9%
63.6
91.3
207.
20.2

36. Tasks
Worksheet Exercise Atomic Masses
Questions : 1, 2, 3