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What is Valency? - Atomic Valency explained
Following the success of What Is Density it seemed a good idea to explain what valency is, as well.
Atoms have a tendency to connect up to other atoms, to form molecules which are compounds, or to form structures. Each type of atom has a preference for connecting to a particular number of atoms. A hydrogen atom likes to join up with one other atom, whereas an oxygen has two vacancies for other atoms to join on to it. This, the "number of other atoms it likes to connect with" is the valency.
Carbon has a valency of 4, so in compounds with carbon there are four bonds to other atoms. Nitrogen has a valency of 3, niobium 5, iridium 3, etc.
The inert gases such as helium, neon, krypton, etc all have a valency of zero, ie the atoms don't really connect to any other atoms, and so there aren't a lot of chemical compounds for inert gas elements.
Valency in practice:
H20: (water) H - O - H
CO2: (carbon dioxide) O = C = O (double bonds)
CH4: (methane gas) a carbon atom with four hydrogen atoms connected to it.
In a structure such as a piece of metal or a crystal, the valency results in the atoms aligning in a regular way so each atom attaches to the appropriate number of other atoms.
In semiconductor material with a regular array of valency 4 atoms, small impurities of valency 3 or valency 5 atoms can cause the material to have spare valencies or spare absences of valency, giving it a particular electrical persuasion (n-type and p-type). By combining these materials, junctions and transistors can be made.
Inspired by the idea of Valency, it could be said that a household tends to maintain an inhouse-quota of cats, the cat valency, which is the number of cats that is instinctively felt appropriate to that household!