2008 SPECIFICATIONS
ATOMIC
STRUCTURE

A guide for A level students
KNOCKHARDY PUBLISHING
ATOMIC STRUCTURE
INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm

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THE STRUCTURE OF ATOMS
Atoms consist of a number of fundamental particles,
the most important are ...
THE STRUCTURE OF ATOMS
Atoms consist of a number of fundamental particles,
the most important are ...
0
-1
+1
1
1
1836
1
9.109 x 10-31
1.602 x 10-19
1.672 x 10-27
1.602 x 10-19
1.675 x 10-27
0
THE STRUCTURE OF ATOMS
Atoms consist of a number of fundamental particles,
the most important are ...
0
-1
+1
1
1
1836
1
Calculate the mass of a carbon-12 atom; it has 6 protons, 6 neutrons and 6 electrons
9.109 x 10-31
1.602 x 10-19
1.672 x 10-27
1.602 x 10-19
1.675 x 10-27
0
THE STRUCTURE OF ATOMS
Atoms consist of a number of fundamental particles,
the most important are ...
0
-1
+1
1
1
1836
1
Calculate the mass of a carbon-12 atom; it has 6 protons, 6 neutrons and 6 electrons
9.109 x 10-31
1.602 x 10-19
1.672 x 10-27
1.602 x 10-19
1.675 x 10-27
0
6 x 1.672 x 10-27 + 6 x 1.675 x 10-27 + 6 x 9.109 x 10-31 =
THE STRUCTURE OF ATOMS
Atoms consist of a number of fundamental particles,
the most important are ...
0
-1
+1
1
1
1836
1
Calculate the mass of a carbon-12 atom; it has 6 protons, 6 neutrons and 6 electrons
9.109 x 10-31
1.602 x 10-19
1.672 x 10-27
1.602 x 10-19
1.675 x 10-27
0
6 x 1.672 x 10-27 + 6 x 1.675 x 10-27 + 6 x 9.109 x 10-31 = 2.0089 x 10-26 kg
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 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
Atomic Number (Z)
PROTONS
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
Atomic Number (Z)
PROTONS
THESE ALWAYS GO TOGETHER – ANYTHING WITH 11 PROTONS MUST BE SODIUM
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
Atomic Number (Z)
PROTONS
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
Atomic Number (Z)
PROTONS
THERE WILL BE 12 NEUTRONS IN THE NUCLEUS
23 – 11 = 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
Atomic Number (Z)
PROTONS
MASS NUMBER AND ATOMIC NUMBER
MASS NUMBER AND ATOMIC NUMBER
RELATIVE MASSES
Relative Atomic Mass (Ar)
The mass of an atom relative to the 12C isotope having a value of 12.000

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¯
ISOTOPES
Definition Atoms with…

the same atomic number but different mass number or
the same number of protons but different numbers of neutrons.
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
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
ISOTOPES - CALCULATIONS
There are two common isotopes of chlorine. Calculate the average relative atomic mass of chlorine atoms







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 + 35 + 35 + 37 = 35.5
4
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 + 35 + 35 + 37 = 35.5
4

Method 2 Out of every 100 atoms 75 are 35Cl and 25 are 37Cl

Average = (75 x 35) + (25 x 37) = 35.5
100
MASS SPECTRA
An early application was the demonstration by Aston, (Nobel Prize, 1922), that naturally occurring neon consisted of 3 isotopes... 20Ne 21Ne 22Ne.


• positions of peaks gives atomic mass

• peak intensity gives relative abundance

• highest abundance is scaled up to 100%
- other values are adjusted accordingly.
Calculate the average relative atomic mass of neon using the above information.

Out of every 100 atoms 90.92 are 20Ne , 0.26 are 21Ne and 8.82 are 22Ne

Average = (90.92 x 20) + (0.26 x 21) + (8.82 x 22) = 20.179
100

Relative atomic mass = 20.18
MASS SPECTRA
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 39x + 41 (100-x) = 39.1
100

therefore 39 x + 4100 - 41x = 3910

thus - 2x = - 190

and x = 95

ANSWER There will be 95% 39K and
5% 41K
ATOMIC
STRUCTURE

THE END
© 2008 JONATHAN HOPTON & KNOCKHARDY PUBLISHING