The periodic table is a method of classifying elements
into groups and periods and therefore predicting their properties.
groups = vertical column of elements ; within a
group we see distinct chemical similarities
periods = horizontal column of elements
Periodic Trends
All elements in the periodic table are either metals or non-metals. The metals are found towards the left of the table, while the non-metals towards the right.
There is a change from metallic to non-metallic
character across a period. The non-metals tend to share electrons unlike metals
which tend to donate them.
Relationship Between Electronic Configuration and Position
The group number of an element is equal to its number of valence electrons (i.e. the number of electrons in the outermost shell)
The period number of an element is equal to its
number of shells that have electrons in them
Chemical reactions all have to do with electrons.
Therefore those elements with similar chemical properties are found in the same
groups as they have the same number of valence electrons.
There are 8 groups of elements from 1-8 all though group 8 is often called group 0. However, elements along a period show a gradual trend in properties:
There are 8 groups of elements from 1-8 all though group 8 is often called group 0. However, elements along a period show a gradual trend in properties:
- change from metal to non-metal
- increase in number of valence electrons
Group Properties
Here we study the properties of certain groups:
- Group 1 – The Alkali Metals
- Group 7 – The Halogens
- Group 8 or 0 – The Noble Gases
Group 1 - The Alkali Metals
Group 1 consists of 6 of the most reactive metals – lithium, sodium, potassium, rubidium, caesium and francium. We only really consider lithium, sodium and potassium as they are the only alkali metals most see in a lifetime.
The three metals mentioned above have the following
properties:
- low densities
- good conductors of heat and electricity
- soft metals (easily cut with a knife)
- react very violently with halogens
- they float
- rapid effervescence (bubbling)
- potassium and sodium catch fire but lithium doesn’t
- when universal indicator is added to the water after the reaction is completed it turns blue indicating that the solution is alkaline.
2Li + 2H2O = 2LiOH + H2
2Na + 2H2O = 2NaOH + H2
2K + 2H2O = 2KOH + H2
Trends
Trends are gradual changes. They are useful as they
allow predictions about elements not seen in reaction.
From above we know all the following properties
that group 1 elements share, however as you go down the group you
notice:
- melting and boiling point decrease (lithium has the lowest m.p. and b.p.)
- densities increase (francium has highest density)
- reactivity with water (and other substances) increases
We don’t actually know the density of francium but
since we know the properties of the other metals we can assume it follows a
trend. This is a skill that is required.
Why does the reactivity of the element with
substances increase with movement down the group?
When group 1 metals react they lose their single
valence electron, i.e. they oxidise to form +1 ions. Normally the negative
valence electron is attracted by the positive nucleus by electrostatic forces.
As you go down the group 1, the atoms increase in
size. This means that the valence electron of francium is the furthest away
from the nucleus. It therefore experiences the least attractive force. This is
why potassium loses it valence electron more easily than lithium and therefore
why potassium is more reactive than lithium.
The more reactive a metal = the higher ability to
form ions.
Group 7 – The Halogens
Group 7 consists of the 5 elements fluorine,
chlorine, bromine, iodine and astatine. Halogens have the following properties:
- they are all coloured non-metals
- all are diatomic molecules (exist as molecules with two atoms)
- have 7 valence electrons
- all react with metals to form ionic compounds
- similarly to metals, a more reactive halogen can displace a less reactive halogen from a solution of its salt.
