C4.1
Predicting physical properties of the Halogens
Summary
Models of
how substances react and the different types of chemical reactions that can
occur enable us to predict the likelihood and outcome of a chemical reaction.
The current periodic table was developed based on observations of the
similarities and differences in the properties of elements. The way that the
periodic table is arranged into groups and periods reveals the trends and
patterns in the behavior of the elements. The model of atomic structure
provides an explanation for trends and patterns in the properties of elements.
The arrangement of elements in groups and periods reveals the relationship
between observable properties and how electrons are arranged in the atoms of
each element.
Common
misconceptions
Learners
consider the properties of particles of elements to be the same as
the bulk
properties of that element. They tend to rely on the continuous matter model
rather than the particle model. Learners confuse state changes and dissolving
with chemical changes. Also, since the atmosphere is invisible to the eye and
learners rely on concrete, visible information, this means they therefore often
avoid the role of oxygen in their explanations for open system reactions. Even
if the role of oxygen is appreciated, learners do not realize that solid
products of an oxidation reaction have more mass than the starting solid.
Underlying
knowledge and understanding
Learners
should be familiar with the principles underpinning the Mendeleev periodic
table; the periodic table: periods and groups; metals and non-metals; the
varying physical and chemical properties of different elements; the chemical
properties of metals and non-metals; the chemical properties of metal and
non-metal oxides with respect to acidity and how patterns in reactions can be
predicted with reference to the periodic table.
C4.1a-b
To be able to recall the simple properties of Group 7 both physical and
chemical properties.
To be able to explain how observed simple properties of Group 7 depend on
the outer shell of electrons of the atoms and predict properties from given
trends down the groups including the ease of electron gain or loss.
The Halogens form a group of non- metals on the right hand side of the
Periodic Table Group 7 next to the noble gases. Molecules of the halogens are
diatomic: each molecule has two atoms:
F-F, Cl-Cl, Br-Br and I-I as in the diagram opposite:
What does this tell you about their physical properties?
Halogens are
non-polar molecules
All Halogens are non-polar molecules.
The molecules do not carry permanent charges but because of the movement of
electrons in these molecules instantaneous dipoles exist.
The instantaneous dipoles can set up or induce similar dipoles in adjacent
molecules.
In this way, forces of attraction do exist between halogens but they are
very weak forces.
The forces between halogen molecules are called London or induced dipole
forces.
The induced dipole forces become stronger down the group since there are
more electron shells and electrons down the group.
So that means more energy is required to separate a molecule of iodine from
another molecule of iodine than a molecule of chlorine from another molecule of
chlorine.
This difference is seen in the way the melting and boiling points increase
down the group.
Halogen
states at rtp and colours in different solvents
This means that the states of the halogens at room temperature change going
down the group.
The table below shows these changes in state and colour and also colour of
their solution in hexane and water.
Halogen
|
Colour
|
State at room temperature and pressure
|
Colour of aqueous solution
|
Colour of halogen in organic solvent
|
Fluorine
|
Pale yellow
|
Gas
|
||
Chlorine
|
Green
|
Gas
|
Pale green
|
Pale green
|
Bromine
|
Dark red
|
Liquid
|
Orange
|
Orange
|
Iodine
|
Grey black
|
Solid
|
Brown
|
Purple
|
Astatine
|
Black
|
Solid
|
In my next post I’ll discuss the chemical properties of the halogens their
reaction with metals and their reactions with each other.
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