C2.1 STRUCTURE AND BONDING
C2.1.1
The electronic configuration of atoms in terms of main energy levels, and
s, p and d atomic orbitals.
Classification of elements into s, p and d blocks.
Up to the atomic number = Z = 36.
Electrons as particles. Atomic
orbitals and electron configurations (in terms of s, p and d notation).
QU
1
In module 1 you considered the arrangement of electrons in shells.
Within the shells the electrons are split up into subshells.
(a) How has the emission spectrum of hydrogen helped chemists in their understanding of subshells?
Ans
As the hydrogen atom has
only one electron its spectrum is relatively easy to interpret.
Elements other than hydrogen have much more complex spectra indicating
that there are also sub-shells and not just the electron shells.
(b) How are the subshells labelled.
Ans
The sub-shells are labelled as follows:
first shell
1s
second shell
2s 2p
third shell
3s 3p 4s 3d
fourth shell 4p
(c)
In what ways do subshells vary from one another.
Ans
Sub-shells vary in :-
1) Their distance
from the nucleus
2) The amounts of energy they
have
3) The trajectories of the
electrons within the sub-shells
4) The number of electrons
each sub-shell can hold
(d)
Show how the energies of the subshells of different shells overlap for
shells three and above.
Ans
See
part (h) for the energy levels of a potassium atom.
(e) What are the possible consequences of (d)
Ans
This means that, once the 4s level is filled in calcium for example, the
next element, scandium, has the electronic structure 1s2 2s2
2p6 3s2 3p6 3d1 4s2.
The 3d sub-shell continues to be filled
across the period in the elements Sc to Zn.
Zinc has the electronic configuration 1s2 2s2 2p6
3s2 3p6 3d10 4s2.
Since electronic structure is the basis of an element's chemistry, the
periodic repetition of configuration accounts for the periodicity of the
chemistry of the elements.
QU
2
The subshells are themselves divided further into ATOMIC ORBITALS.
(A)
How many atomic orbitals are found in each type of sub-shell?
Ans
An electron in a given orbital can be found in a particular region of
space around the nucleus.
·
an s sub-shell
always contains one s atomic orbital
·
a p sub-shell
always contains three p atomic orbitals
·
a d sub-shell
always contains five d atomic orbitals
·
an f sub-shell
always contains seven f atomic orbitals
(B) These orbitals occupy a particular region of space around the nucleus. Show diagramatically the region of space occupied by s and p atomic orbitals.
Ans
(C) Do you think an atomic orbital as represented by your lines in part (b) map the exact path taken by an electron in that orbital?
Ans The atomic orbital is not in a fixed electron orbital. The position of an electron cannot be mapped exactly. For an electron in a given atomic orbital, we only know the point/orbit that an electron is statistically most likely to be found at/in.
(D)
Do the orbitals in the same subshell have the same energy in an isolated
atom?
Ans
Yes they do. The orbitals
are filled in order of increasing energy. Where
there is more than 1 orbital with the same energy, these orbitals are first
occupied singly by electrons. This
keeps the electrons in an atom as far apart as possible.
Only when every orbital is singly occupied do the electrons pair up in
orbitals. For the lowest energy
arrangement, electrons in singly occupied orbitals have parallel spins.
(E)
How many electrons can each atomic orbital hold?
Ans
Each atomic orbital can hold 2 electrons s, p, or d.
(F)
Complete the following:
Every electron spins at the same rate but one spins in a clockwise
direction and the other in an anti-clockwise
direction. Electrons
can only occupy the same orbital if they have opposite
or paired spins
which we can write as E
where the box represents the atomic orbital and the arrows the spinning
electrons.
(G)
What four pieces of information need to be supplied in order to describe
accurately the state of an electron?
Ans
1) The electron shell it is in
2) Its sub-shell
3) Its orbital within the
shell
4) Its direction of spin
(H)
Show diagrammatically how
the electrons are arranged in the subshells of the atoms of Mg, P, Ar, K, Ti, Fe
& Cu.
Ans This diagram shows the position of energy sub levels in a potassium atom. Others can be represented in a similar manner.
C2.1.2
QU 3 Describe the organisation of elements in the Periodic Table in relation to their proton numbers and electronic structures.
Ans
Hydrogen is the simplest element, with atomic number Z = 1. It has one electron which will occupy the s orbital of the n = 1
shell.
#
1s H 1s1
The next element, helium (Z = 2) has 2 electrons which both occupy the 1s
orbital with paired spins.
1s E
He 1s2
Lithium (Z = 3) has 3 electrons. The
third electron cannot fit in the n = 1
shell and so occupies the next lowest orbital, the 2s orbital,
1s E
2s #
Li 1s2
2s1
And so on across the first short period.
Nitrogen (Z = 7) has 7 electrons
1s E
2s E
2p #
#
#
N 1s2
2s2 2p4
The three electrons occupying the 2p
sub-shell must occupy the three
separate p orbitals singly and their
spins must be parallel.
1s E
2s E
2p E
#
#
O 1s2
2s2 2p4
Oxygen (Z = 8) has 8 electrons
C2.1.3
QU 4 Discuss the relationships which exist between atomic structure, ionisation energies and the Periodic Table. Include -
·
Plots of log
(successive ionisation energy) against number of electrons removed for an
element to introduce energy levels.
·
Plots of first
ionisation energies against atomic number to introduce sub-divisions of energy
levels.
Ans
Ref: Chemistry
in Context Pgs74-8
QU
5
Explain what is meant by the terms –
(a)
Unit cells
-
simple cubic
Ans The
description of crystals is simplified by deciding on the size and shape of the
unit cells. These are the
fundamental blocks which can be stacked together to construct the entire
crystal. E.g.
(b)
Empirical Formula
Ans
This shows only the simplest whole number atomic ratio of the various
elements present in a compound. E.g.
Glucose = C6H12O6
= CH2O
(c)
Co-ordination numbers.
Ans
Which type of ionic structure is described by the co-ordination numbers
of ions present. The co-ordination
number tells us how many oppositely charged ions surround a particular ion.
E.g. In sodium chloride the co-ordination number of sodium ions is 6 and
is also 6 for the chloride ions.
(d)
Formula unit
Ans
This is the simplest representation of an ionic compound. E.g. NaCl, CaCl2
QU
6
X-ray diffraction is a technique used to determine crystal structures.
Discuss.
Ans
The wavelengths of x-rays vary from about 10nm to 0.001nm, and are about
10,000 times smaller than those of light. X-ray
wavelengths are diffracted by particles (atoms or ions) of a solid crystal if
they are arranged in a regular pattern.
Diffracted x-rays from a regular pattern of spots when they fall on a
photographic plate. A single
crystal can be replaced by a powder, and concentric circles are then formed on
the photographic plate. Distances
between layers of molecules, atoms or ions in a crystal or powder can be
determined from the photographs. Since
the diffraction of x-rays is produced by the electrons around an atom or ion, a
hydrogen atom produces practically no diffraction because it has only one
electron. It is possible to
calculate bond lengths and bond angles from the photographs.
Computers are used to deduce
the detailed structure.
Ionic
lattices.
QU 7 Compare the structure of NaCl and CsCl.
Ans
Sodium chloride and caesium chloride
In sodium chloride, each positive sodium ion is surrounded by six Cl-
ions and each negative chloride is surrounded by six Na+ ions.
The structure of sodium chloride is said to have 6:6 co-ordination.
In CsClthe co-ordination number of Cs+ ions in the structure
is 8 in the same way the co-ordination number of Cl- ions is also 8.
The structure of caesium chloride is therefore said to have 8:8
co-ordination.
Sodium chloride has a simple cubic structure thus
Caesium chloride has a body centred cubic structure thus
QU 8 Explain what is meant by the term ‘co-ordination numbers’ as applied to ionic lattices and give some different examples.
Ans Each type of ionic structure is described by the co-ordination numbers of ions present. It tells us how many oppositely charged ions surround a particular ion. E.g. Close packed - each atom is in contact with six others. The central atom has six other atoms in the same layer in contact with it. In the second layer, atoms pack as closely as possible to those in the first layer by ‘sitting’ in the depressions between atoms in the first layer. Around each first layer atom there are six depressions. This means that any one atom touches 12 others in these close packed arrangements – 6 in its own layer. Three in the layer above and three in the layer below. This is summarised by saying that its co-ordination number is 12, e.g. magnesium .
Body centred cubic shows that the structure is basically cubic with an atom at the centre of each cube. In this case each atom is surrounded by eight others. Thus….
QU
9
Discuss the importance of the cation: anion radius ratio in
ionic structures.
Ans
Metal atoms have a tendency to lose one or more electrons when they form
compounds with non metal atoms. The
non-metal atoms in these combinations show a tendency to gain one or more
electrons. Since the loss of
electrons produces a positive ion (cation) and the gain of electrons produces a
negative ion (anion), the combination of a metal and non-metal produces an ionic
compound. By finding the ionic
radius the type of ionic lattice can be predicted.
To find this it is half the internuclear distance between the two
neighbouring monotomic ions in a crystalline salt. To make the prediction of the ionic lattice you must find the
radius ration.
Radius ratio = radius of cation
radius of anion
Therefore the importance of this ratio is to enable chemists to predict the type of lattice structure that will be present. The bigger the ratio, the bigger the cation is compared to the anion.
QU
10
Define the term ‘Lattice enthalpy’.
Ans
Refer to- pages 177 – 179 in
‘Chemistry in Context
pages 87 – 92 in
‘Understanding Chemistry’