The Periodic Table.pdf - TLS

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