As we have seen in the previous section, in the
periodic table the elements are organised according to their properties. These
properties and the position in the periodic table are related to the
characteristics of the atom’s shell. The atom’s shell splits into different
levels with different number of electrons in each level. The outer electron
shell (the farthest from the nucleus) is called the valence shell. All the elements in the same group (column) have
an equal number of electrons in their valence shell and, for that reason, they
have the same chemical properties.
In nature, we find all the substances made up of
groups of atoms, except noble gases. Atoms bond to become more stable, that
means they have less energy as a group than as separated atoms. The force of
attraction that keeps two or more atoms together is called a chemical bond.
But, why and how atoms create a chemical bond? Are all
chemical bonds equal? And, why do the atoms in noble gases not bond? The reason
is related to their valence shell; noble gases have 8 electrons in their
valence shell so they are stable despite of the fact of being isolated.
Chemist observed that when atoms bound acquire the
electronic configuration of the noble gas nearest to them, that is, they obtain
8 electrons in their valence shell. This fact is explained by the octet rule, which means that a
neutral atom can lose or gain the specific number of electrons needed to have 8
electrons in their valence shell. Some atoms are far from a noble gas and would
have to lose or gain an excessive number of electrons, in this case, atoms can
share electrons with one another to comply with the octet rule. There are
exceptions to the octet rule because there are some atoms with less than 8
electrons, such as Hydrogen, Lithium or Beryllium. Those elements need to
acquire the electronic configuration of Helium, which means having 2 electrons
in their valence shell.
The elements are classified into metals, non-metals,
semimetals and noble gases depending on their properties, that is, on their
valence shell. In general terms, metals need to lose electrons, non-metals want
to gain electrons and semimetals can do both things. When some atom gains, or
loses electrons it creates an ion, negative or positive respectively, so metals
create positive ions, called cations, and non-metals create negative ions,
called anions.
We define three types of chemical bond depending on
which elements form it and how is the octet rule complied. We summarize the three types of chemical bond and the properties of the compounds formed in each case in the next mindmap.
I encourage you to do another mind map but from macro
structures to micro (atoms), I mean, you must separate the matter into the
substances we find in nature (gases, liquids and solids) and you have to split
each state into the different compounds depending on their chemical bond. You do not need to write all the properties but you must use them to analyse the
type of substance in each case.
Sources of information:
- Chemical bonds: www.khanacademy.org
- Covalent bonding-single bonds: www.chemguide.co.uk
- Física y química. 4ºESO. Grupo Anaya,
2015.
