HISTORY OF THE PERIODIC TABLE
One of the first suggestions about
the organization of the elements
came from Dalton (1808) who
arranged the 20 known elements in
order of atomic mass. He was also
the first to propose symbols for
each element though they are not
the symbols we use today.
Dobereiner (1829) found that with some groups of three elements with
similar properties, for example lithium, sodium and potassium, the atomic
mass (mass number) for the second element was the average of the first
and third element. So, lithium has a mass number of 7 and potassium has a
mass number of 39. The average is (7 + 39) ÷ 2 = 23. Sodium has a mass
number of 23. The same was found for some other groups of three
elements for example chlorine (35), bromine (80) and iodine (127).
These groups became known as Dobereiner's Triads.
John Newlands (1863) noticed that by arranging the elements in order of
increasing atomic mass every eighth element seemed to have similar
properties. He proposed a similarity with music, where the eighth note is
an octave above the first note. This idea became known as Newlands
Octaves.
Newlands assumed that all the elements had been discovered even though
new ones were still being discovered. His attempt to arrange the
elements saw him try to squeeze two elements into the same position. His
table was also missing the noble gases which were yet to be discovered.
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Dmitri Mendeleev is credited as being the Father of the modern periodic
table. In 1869 he arranged the 50 or so known elements in order of
atomic mass, putting elements with similar properties in the same vertical
group, and leaving gaps for unknown elements, yet to be discovered. When
the elements were later discovered, they were found to have the
properties predicted by Mendeleev's table. Knowing nothing of protons,
nuclei or atomic number, Dmitri Mendeleev's periodic table was broadly
correct.
The modern periodic table
The modern periodic table is very useful for giving a summary of the
atomic structure of all the elements. It is arranged in order of atomic
number (proton number) rather than mass number, which sorts out 1 or 2
anomalies such as argon and potassium being the wrong way around.
Groups
Elements in the same group of the periodic table have similar chemical
properties since they have the same number of electrons in their outer
shell and it is this which determines their reactivity. The group number is
equal to the number of electrons in the outer shell.
As each group is descended the atoms get bigger since there are more
shells of electrons. This means that:
Metal atoms (which are looking to lose electrons) can do so more easily
 Non-metal atoms (which are looking to gain electrons) can do so less
easily
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This happens because the outer electrons get further from the positively
charged nucleus and so are attracted to it less and less. Not only that,
but the inner shells of electrons ‘shield’ the outer electrons from the pull
of the nucleus. This results in an increase in reactivity as group I or II is
descended since the outer electron is lost more easily the bigger the
atom. For group VII (which are looking to gain electrons) the reactivity
decreases as the group in descended since electrons are attracted less
strongly into the outer shell as the atom gets bigger.
Periods
The horizontal divisions of the periodic table are known as periods. The
period number is equal to the number of electron shells present in an
atom.
Group I
Lithium, sodium and potassium are all very reactive metals and have to be
stored in oil. They are soft and can be cut with a knife an, have a shiny
silvery surface when first cut. They all react with cold water, forming a
soluble alkaline hydroxide and hydrogen gas. Group I compounds are all
white solids which dissolve to form colourless solutions.
lithium + water
2Li(s) + 2H2O(l)
lithium hydroxide + hydrogen.
2LiOH(aq) +
H2(g)
sodium + water
2Na(s) + 2H2O(l)
sodium hydroxide + hydrogen.
2NaOH(aq) +
H2(g)
potassium + water
2K(s) + 2H2O(l)
potassium hydroxide + hydrogen.
2KOH(aq) +
H2(g)
Group I metals have a low density and all three metals above float on
water while reacting. Lithium, sodium and potassium will all react
vigorously with halogens to form the corresponding halide salt. The
halogens are fluorine, chlorine, bromine and iodine.
For example,
sodium + chlorine
2Na(s) + Cl2(g)
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sodium chloride.
2NaCl(s)
3
In all of these reactions the group I metal has lost one electron and
formed an ionic compound in which it is present as an M+ ion.
Transition metals
Transition metals have many desirable properties:




They are tough enough to be used as structural materials (e.g. iron
in the form of steel)
They are good electrical conductors, especially copper, which is
used for electrical cables
Many are good catalysts e.g. iron (Haber Process) and platinum
They have high melting points (except mercury)
The transition metals are much less reactive that group I or II metals.
They do react with water or oxygen in the air but much more slowly. Eg
iron takes days or weeks to rust. Other metals corrode much more slowly
than this.



Most transition metals form coloured compounds. The colours of
many minerals and gemstones is due to the presence of transition
metal ions.
Transition metals can exist as ions with different charges. Eg Fe2+
and Fe3+, Sn2+ and Sn 4+.
The ability of these metals to have ions with different charges
makes them useful as catalysts in industry.
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METALS IN THE PERIODIC TABLE
The chemical elements can be arranged in order of their relative atomic masses. This list can then be arranged in rows so
that elements with similar properties are in the same column (Group). The resulting table is known as the Periodic Table.
Although this arrangement puts most elements in their appropriate Group, a few are clearly misplaced. For example, argon
(Ar) atoms have a greater relative atomic mass than potassium (K) atoms, but argon is better placed before potassium in the
Periodic Table, so that it falls in Group 0 (noble gases) and potassium falls in Group I (alkali metals). Other anomalies are
tellurium (Te) / iodine (I) and cobalt (Co) / nickel (Ni).
In the modern Periodic Table, where the elements are arranged in order of their atomic numbers (no. of protons), these
anomalies are resolved.
Group
I
II
Li
non-metals
Na
Ar
K
Fe
Rb
Co
Ni
Cu
transition metals
Te
Cs
Hg
Fr
METALS
alkali metals
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I
Comparing Alkali Metals & Transition Metals
Alkali Metals
Transition Metals
●low melting points
high melting points (except for mercury (Hg))
●soft and easily cut
hard, tough and strong
●very reactive with water much less reactive with water
●white compounds
coloured compounds
●not catalysts
good catalysts
Metalloids or semi-metals
The boundary between metals (on the left of the periodic table) and nonmetals (on the right) is not clear. Elements such as boron, silicon,
germanium, arsenic, antimony, and tellurium demonstrate some properties
considered to be metallic whilst also demonstrating other non-metallic
properties. They all lie along the ‘zig-zag’ line on the periodic table on
page 5 of these notes.
Group VII
The halogens are all in group 7 on the right of the periodic table. They
are Fluorine, Chlorine, Bromine and Iodine. They are all diatomic
covalently bonded molecules. Diatomic means that each molecule contains
two atoms.
The formulae are F2, Cl2, Br2, I2, (see structure of chlorine).
All of the halogens will either
1) gain one electron from a metal to form an X- ion
or
2) share one electron with a non-metal to form a covalent bond
(see structure of chlorine or hydrogen chloride).
Fluorine is an extremely reactive pale yellow gas. Chlorine is a reactive
yellow-green gas. Bromine is a less reactive red-brown liquid (and gas).
Iodine is a still less reactive dark grey (or black) solid.
As you go down group 7 from fluorine to astatine, the atoms
 get bigger
Each successive element has an extra electron shell. It is the electron
shells which take up nearly all the space of an atom.
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 Are less reactive. Fluorine is the most reactive, astatine the least.
As the size of the atoms increase, the extra outer electron
(which it wants to gain or share to become a stable ion or molecule) is
further and further from the positive nucleus. It is the attraction
between the negative incoming electron and the positive nucleus which
determines how easily the electron will be gained. The smaller the atom,
the closer the incoming outer electron is, the more easily it is gained, and
the more reactive the halogen is.
Also, the smaller the atom, the less shielding there is.
 Have higher melting points and boiling points because the atoms are
bigger.
The relative reactivity of the halogens, as described above, can be shown
by displacement reactions. For example, Bromine gas bubbled through a
solution of potassium iodide in water will displace (take the place of) the
less reactive iodine, forming iodine and potassium bromide.
bromine + potassium iodide
Br2(g) + 2KI(aq)
potassium bromide + iodine.
2KBr(aq) +
I2(s)
Similarly, chlorine will displace less reactive halogens. Chlorine will
displace both bromine and iodine from the appropriate salt.
chlorine + potassium iodide
Cl2(g) + 2KI(aq)
potassium chloride + iodine.
2KCl(aq) +
I2(s)
The equations can be written in terms of ions (called ionic equations). For
example, the last equation can be written as
Cl2(g) +
2I-(aq)
2Cl-(aq) +
I2(s)
Potassium iodide, on the left, exists as potassium ions (K+) and iodide ions
(I-)
and potassium chloride, on the right, exists as potassium ions (K+) and
chloride ions (Cl-). Potassium ions (or other metal ions) can be left out of
the ionic equation because they do not take part in the reaction. They are
called 'spectator ions', as though they just sit back and watch!
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Noble gases
The elements in Group 8 or 0 are known as the noble gases as they are
unreactive or inert. This is due to them already having full outer shells of
electrons so they do not try to bond with other elements to fill them.
They are all colourless gases that exist as monatomic forms (ie. as one
atom, not grouped in a molecule like the halogens F2, hydrogen H2 or
oxygen O2). They all give off light when a current is passed through them.
As you go down group 0 from Helium to Xenon, the atoms get bigger.
Each successive element has an extra electron shell. It is the electron
shells which take up nearly all the space of an atom. As the group is
descended, the gases also become more dense. Helium is used in balloons
as it is lighter than air, whilst a balloon full of Xenon would drop like a
stone.
Argon is used in light bulbs to prevent the white hot filament would
react with oxygen and quickly burn away. Neon is used in advertising signs
since it glows red when electricity is passed through it. Krypton is used in
the lasers which are used for corrective eye surgery, whereas Xenon in
lighting.
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Topic 12: The Periodic Table
Summary questions
1
2
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3
4
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5
6
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7
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8
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