Chapter 4
Periodic Table
An element is any substance which cannot be split into similar substances by
chemical means.
History of the Periodic Table.
Dobereiner’s Triads.—Group of 3 elements with similar chemical properties in
which the atomic weight of the middle element is approximately equal to the average
of the other two.
Cl
Ca Li
Br St
Na
I
Ba
K
Earliest attempt to trace a relationship between properties of elements and their
atomic weights.
Newland’s Octaves.-Group of elements arranged in order of increasing atomic
weight in which the 1st and 8th element of each group have similar chemical
properties.
Discrepancies: O and S in same group as Fe.
N and P in same group as Mg
Newland’s did not realise that some elements had still to be discovered.
Mendeleef’s Periodic table
Arranged elements in order of increasing atomic weight-each 8th element had similar
chemical properties.
Periodic Law-when elements are arranged in order of increasing atomic weight the
properties of elements vary periodically.
1. Left gaps for undiscovered elements.
2. In a few cases, he had to reverse the order to make them fall into groups of
elements of similar chemical properties. e.g. I and Te
Mosley and the modern Periodic Table
The modern periodic table is an arrangement of elements of increasing atomic
number.
MENDELEEF’S PERIODIC TABLE
In order of Atomic weight
Gaps
No Nobel gases
No block for Transition Metals
MODERN PERIODIC TABLE
In order of Atomic number
No gaps
Nobel Gases
Transition metals-separate block
Isotopes- Atoms of the same atomic number but different mass number.
Eg.
6
C
12
6
C
14
10
Ne
20
10
Ne
22
1
H
1
1
H
2
1
H
3
Due to different numbers of neutrons in the nucleus.(See Page 31 BOOK)
Mass Spectrometer-may be used to identify the presence of isotopes, measure their
relative atomic masses and measure the relative abundance of elements.
It works on the principal that ions can be separated by mass while moving in a
magnetic field.
(See BOOK Page 32)
1. Vaporisation
2. Ionisation
3 Acceleration
4. Deflection
5. Detection and analysis
Relative atomic mass- mass of the atom compared to 1/12th of the mass of the carbon
12 isotope.
Writing electronic configuration of atoms and ions
(See book pages 35-38 ).
Note: Chromium and Copper are exceptions
Aufbau principle- Electrons occupy the lowest available energy level.
Hund’s Rule of Max multiplicity-When two or more orbitals of equal energy are
available to the electrons, they occupy them singly before occupying them in pairs.
Pauli’s exclusion principal- No more than two electrons can occupy an orbital and
they must have the opposite spin
s-orbital-------2 electrons
p-orbital-------6 electrons
d-orbital-------10 electrons
Examples Pages 39 and 40 BOOK