The Periodic Table.pdf - TLS

SLU CALM Resources
Chemistry
The Periodic Table
In the periodic table, elements have been placed in order of increasing atomic
number and similar properties. The first element is hydrogen (H) with an atomic
number of 1, indicating that each atom has one proton with one electron orbiting
around the nucleus. The second element is helium (He) with an atomic number
of 2, indicating two protons with two electrons orbiting around the nucleus. The
next element is lithium (Li) with an atomic number of 3, and so on.
Although not necessarily a requirement, it is useful to learn the order of the first
20 elements of the periodic table as they contain many of the important elements
for life. An easy way to tackle such a task is to use a mnemonic or memory aid,
usually involving learning an easier thing that enables you to work out a more
complicated set of things. For instance,
Hey Hello Little Beryl Brown Counting Nuts On Friday Near Nana’s Mg,
Always Singing Pretty Songs Clearly Around Kings Castle
This mnemonic represents the first 20 elements, and includes symbols (bold),
referring to H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca.
Of course, you need to know what elements the symbols represent - another
important step in the learning process - but this learning aid makes the task a
little easier. Read more about learning strategies.
Properties of elements include metallic or non-metallic, reactive or non-reactive,
solid, liquid (rare) or gaseous. Although distinguishable from each other,
elements can show some remarkable regularities in behaviour and properties.
For example, the metals sodium and potassium are both silvery white, both react
violently with water, both form white oxides, both form white crystalline
compounds with fluorine, chlorine, bromine and iodine. The point is that there is a
similarity between sodium and potassium and, what's more, indications of a
similarity between fluorine, chlorine, bromine, and iodine.
Although the periodic table is based around the number of protons, it is the
number of electrons or, to be more exact, the number of electrons that are readily
available for reactions, that affects the way elements behave. Chemical reactions
involve redistribution (loss or gain) of electrons, therefore similarity of elements is
1

eflected by the number of electrons which each element either gains or loses.
The number of electrons is not constant because they may be redistributed from
atom to atom during chemical reactions. This is why atomic number refers to the
number of protons, not electrons.
Thus, elements with similar properties have atoms that lose or gain the same
number of electrons during chemical reactions. Elements are classified according
to vertical groups (Roman numerals). There are 8 main groups, namely groups I,
II, III, IV, V, VI, VII, and 0 (sometimes called VIII). These group numbers coincide
with the number of electrons in the outer shell (energy level) of the atoms.
Patterns also occur periodically, thus the table is arranged horizontally in periods.
The Periodic Table - first 20 elements
Groups
Period I II III IV V VI VII 0
1 H
1
He 2
2 Li 3 Be 4 B 5 C 6 N 7 O 8 F 9 Ne 10
3 Na 11 Mg 12 Al 13 Si 14 P 15 S 16 Cl 17 Ar 18
4 K 19 Ca 20
More information on the Periodic Table.
Electron Shells (energy levels)
The atomic theory suggests that electrons in atoms orbit around the nucleus at
discrete set distances called electron shells. Each electron shell contains
electrons with the same energy level. There is a maximum number of electrons in
each shell. The nearest shell to the nucleus can have up to 2 electrons, the
second shell can have up to 8 electrons, the third shell up to 18 electrons, etc.
The maximum number of electrons in the outer shell, however, is 8 which seems
to be a very stable number chemically (as is 2).
The elements of Group 0, the inert gases, appear to have such a stable outer
layer of electrons that they do not easily react, if at all. These elements all have 8
electrons in the outer shell (except Helium, which has 2).
2

Elements in other groups do not have the stable number of 8 elements in the
outer shell. These elements tend to be chemically reactive as electrons move to
or from atoms. Chemical reactions result from the tendency of electrons to fill the
outer shell to the most stable number (usually 8).
There is a strong chemical imperative towards a full outer shell, particularly in
elements containing atoms that have close to a full shell of electrons. Groups I
and VII contain elements that are amongst the most reactive in the periodic table.
Group VII elements tend to gain one electron, resulting in a full outer shell. Group
I elements tend to lose one electron, resulting in a full outer shell (but one less
shell).
3