Monday 6 February 2017

GCSE OCR Gateway C4:1 a-b Halogen physical properties

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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