7 Science
Éanna OBoyle
The
Periodic Table
What is the whole idea of the
periodic table?The idea of the Periodic Table is to arrange the elements in
a way that enables chemists to understand patterns in the properties of the
elements. The elements are laid out in order of Atomic (proton) Number (at.
no.). Many of the similarities and differences in the properties of elements
can be explained by the electronic structure of the atoms (electron configuration,
arrangement in shells or energy levels).
Periods and Groups
The
periodic table is organized like a big grid. The ELEMENTS are placed
in specific places because of the way they look and act. There are ROWS (left
to right) and COLUMNS (up and down) and they each mean something different.
Periods (rows)
When you look at figure 1 you can see that the
rows are different shades. Even though the rows skip some squares in between,
all of the rows go from left to right. When you look at a periodic table, each
of the rows are considered to be different PERIODS (Like PERIODic table!).
All
of the elements in a period have the same number of occupied atomic SHELLS.
Every
element in the top row (the first period) has one shell occupied with its electrons.
All of the elements in the second row (the second period) have two shells occupied
with electrons. It goes down the periodic table like that. The maximum number
of shells so far found is seven.
Groups (columns)
See the picture The periodic table has a special
name for its columns too. When a column goes from top to bottom, it's called
a GROUP.
The elements in a group have the same number of electrons
in their outer shell.
Elements of the
one group(e.g. lithium, sodium, potassium, rubidium, caesium, and francium)
are usually similar chemically and physically but there are often important
trends in physical properties and chemical reactivity (for example goind
down group 1, the elements get more reactive!). Some of
the groups have special names, others do not! Complete the table (this symbol
“-”: leave!) below:
|
Group |
Name |
Number
of outer electrons |
What
do elements in this group do when bonding?
(How do you make them happy?) |
| 1 |
Alkali
metals |
1 |
Gives
away electron |
| 2 |
Alkaline-earth
metals |
2 |
Gives
away electrons |
| 3-12 |
Transition
metals |
- |
Give
away 1, 2, or 3 electrons |
| 13 |
Boron
group |
3 |
- |
| 14 |
Carbon
group |
4 |
- |
| 15 |
Nitrogen
group |
5 |
- |
| 16 |
Oxygen
group |
6 |
Takes
electron(s) |
|
17 |
Halogens |
7 |
Takes
electron(s) |
|
18 |
Noble
gases |
8 |
|
Period 1: Two
exceptional elements
You'll may notice that HYDROGEN is special. Hydrogen
can have the talents and electrons of two groups, one and seventeen. To scientists,
Hydrogen is sometimes missing an electron, and sometimes it has an extra.
HELIUM is another exception. Helium is different
than all of the other elements. It can only have two electrons in its outer
shell. Even though it only has 2 it is still grouped with elements that have
eight. Why? Because it has a fully occupied shell!
The elements from group 3 to group 12 are called TRANSITION
elements. They have special electron rules, where electrons squeeze in between
the last two occupied electron shell.
What information is in each square in the periodic table?
That
carbon square above tells you the mass of one atom of carbon (12 atomic
mass units or amu), how many protons are inside (6, which is also the number
of electrons in a neutral atom of carbon) and the name and/or symbol for carbon.
With 6 electrons, carbon’s electron arrangement (or configuration)
is 2,4. In the same group is silicon which has the electron arrangement 2,8,4.
Some basic facts about the periodic table ...
- The elements in a
group tend to have similar physical and chemical properties because of their
similar outer shell electron structure.
- A Period is a horizontal
row of elements with a variety of properties, changing from very metallic
elements on the left to non-metallic elements on the right. A period
starts when the next electron goes into the next available main energy level
or shell (Group 1 alkali Metals). The period ends when the main energy level
is full (Group 18 - the Noble Gases).
- The last element in
a period is when the outer shell is full (Group 18 Noble Gases eg argon 2,
8, 8). The next electron for the next element goes into the next highest level
(shell) available, and so starts the next period.
- So in terms of electrons
....
- Period 1 is elements
1-2, H (1) to He (2)
- Period 2 is elements
3-8, Li (2, 1) to Ne (2, 8)
- Period 3 is elements
11-18, Na (2, 8, 1) to Ar (2, 8, 8)
- Period 4 is elements
19-36, starts with K (2, 8, 8, 1) and Ca (2, 8, 8, 2) and finishes with Kr
(2, 8, 18, 8).
- Note that the number
of shells containing electrons is equal to the period number.
- From Period 4 onwards
the length of a period significantly increases because it includes horizontal
series of similar metals with their own characteristic physical and chemical
properties eg The 1st Transition Metals Series.
- More than three-quarters
of all of the known elements are metals (elements naturally occur up to uranium
92, 93 and above are 'man-made' elements from the experiments of nuclear physicists).
- This work will continues
as heavier and heavier elements are likely to be made in nuclear reactions.
They will be all metals and radioactive. But one theory suggests that
'super-heavy' elements of about atomic number 150? may be in a nuclear stability
region and would prove most interesting to study. Chemists are trying to predict
their properties now!
- Only about 19 elements
in the periodic table are definitely are non-metal but about 7 more are semi-metals
(or sometimes called METALLOIDS) of mixed physical and chemical character.
- The metals in the
periodic table are mainly found in the left hand columns (Groups 1 and 2)
and in the central blocks of the transition elements.
- There is a 'rough'
diagonal division between the two principal types of element zig-zagging from
a line under boron (B) to a line under astatine (At and the rarest element
found on earth!) in Group 17.
- The elements in this
'band' are sometimes referred to as 'semi-metals' or 'metalloids' because
of their 'mixture' of metallic and non-metallic character eg the semi-conductor
silicon in group 14.
- There tends to be
gradual changes in physical and chemical properties down a group eg
- Down Group 1 (Alkali
Metals) and Group 2 the metals get more reactive.
- Down Group 17 (Halogens)
the non-metals get less reactive, their colour gets darker, their melting/boiling
points increase.
- Down Group 14 you
start with a definite non-metal carbon, and end up at the bottom with a the
definite metal lead, so there are quite significant changes in both physical
and chemical character.
- There tends to be
major changes in physical and chemical properties across a period eg
- Period 2 starts with a solid
low melting reactive metal lithium, in the middle there are the high melting
and rather unreactive non-metals boron and carbon, next to the end is
the very highly reactive non-metal gas fluorine, and the period finishes
with the very unreactive gas neon. Very complicated pattern!
- Period 4 starts with a solid
low melting very reactive metal potassium (K). After calcium (Ca) there
are ten transition metals with a wide variety of chemistry, followed by
the metallic gallium, semi-metal germanium and more non-metallic arsenic/selenium.
Next to the end is the very reactive non-metal liquid bromine, and the
period finishes with the very unreactive gas krypton. Even more complicated
pattern!
- The Noble Gas elements have full,
very stable, outer valency shells.
Metals and
Non-Metals
Typical Properties of Metallic
Elements
Physical properties of metals
- high melting points
and boiling points so all solid bar one (exceptions like mercury the only
liquid metal at room temperature and the Alkali Metals have untypical
low melting points)
- good conductors of
heat and electricity
- high density
(exceptions like the Alkali Metals have untypical low densities, the first
three Li, Na and K float on water before the 'fizzing'!)
- appearance - shiny
(usually silvery grey except for copper and gold)
- usually quite strong
materials (exceptions like the Alkali Metals which are untypically soft)
- easily beaten into
shape (malleable) or drawn into wire (ductile)
- solids sonorous
Chemical Properties of metals
- Metals tend to form
basic oxides that react with acids to form salts (if the oxide is soluble
in water it forms an alkali of pH > 7, universal indicator blue or violet)
- most react with acids
to form a salt and hydrogen
Typical Properties
of Non-metallic Elements
Physical properties of non-metals
- usually low melting
points and boiling points and so can be gases, liquids or solids (exceptions
like silicon, and carbon as diamond or graphite)
- poor conductors of
heat and electricity (exceptions like carbon in the form of graphite)
- generally low density
- appearance - dull
if solid
- usually weak materials
eg soft or brittle solids (exceptions like silicon, and carbon as diamond,
which are very hard and strong)
- if solid, not easily
beaten into shape or drawn into wire, tend to be too brittle
- solids not usually
sonorous
Chemical properties of non-metals
- form acidic oxides
when burned in air or oxygen, these react with alkalis to form salts, if soluble
in water they form acid solutions of pH <7, universal indicator yellow-orange-red
- they do not usually
react with acids
A Look at Some
Groups!
Group 1 The Alkali Metals
- Alkali metal atoms
have one outer electron, which is readily lost to form a stable single positive
ion M+.
- Because of above,
they are very reactive.
- Thay have low density
(some float on water).
- They readily react
with non-metals to form ionic compounds eg NaCl or Na+Cl-,
Li2O or (Li+)2O2-.
- These are colourless
crystals or white solids, soluble in water to give colourless solutions
- The metals react rapidly,
maybe violently, with water to form alkaline hydroxides and hydrogen gas.
- Down the group, the
metals get more reactive, and the melting points and boiling points decrease.
Group 2 The Alkaline-Earth
Metals
- They are not quite
so reactive as the Alkali Metals for the same period.
- They have two outer
electrons and readily lose them to form the M2+ ion.
- This ion occurs in
the ionic compounds they readily form with non-metals like the Group 17 Halogens
or oxygen and sulphur from Group 16 eg MgCl2 or CaO.
Group 17 Halogens
- The are coloured non-metals
with low melting points and boiling points.
- They are brittle when
solid eg iodine and poor conductors of heat and electricity when liquid or
solid.
- Halogens exist as
molecules of pairs of atoms, X2 (diatomic molecules), form ionic salts with
metals eg KBr or MgCl2, but form covalent molecular compounds with
other non-metallic elements eg HCl, CBr4.
- Down the group the
melting points and boiling points increase and the reactivity decreases.
Group 18 Noble Gases
- These are all unreactive.
As a result of this:
- they exist as
individual atoms.
- they are all
gases.
- When electricity
passes through them they emit colourful light such as red (neon).
- In recent years some
noble gases have been "forced" to react with other elements (for
example xenon hexafluoride, XeF6)
Groups 3 - 12 The Transition Metals
- The ten horizontal
elements Sc to Zn are called the 1st series of Transition Metal Elements
eg iron and copper.
- These elements in
the central blocks of the periodic table are typical metals - good conductors
of heat and electricity and can be bent or hammered into shape (malleable)
and they can be drawn into wire (ductile).
- However, compared
to the group 1 Alkali Metals, they have higher melting points (except mercury
- a liquid at room temperature); they are harder, tougher and stronger; they
are much less reactive and so do not react (corrode) as quickly with oxygen
or water.
- These properties make
them useful structural materials (eg steel) and were things need to be good
conductors eg copper electrical wiring or steel radiators.
- Most transition metals
form coloured compounds (eg blue copper salt solutions) and are used in pottery
glazes, stained glass and weathered copper roofs turn green!
- an alloy means a
metal mixed with at least one other element.