Introduction to Chemistry - Part II

2. Chemical Symbols           
All Elements have been named by the scientists who had discovered them and do not have any particular nomenclature. Quite a few have Latin names. Some have English names also. Although the names are long, generally abbreviation of either first or first two letters represents the element. This is done for convenience only.  Some elements are known commonly by their English names but the abbreviations followed are of their Latin names!! The abbreviations are called symbols and all the elements are represented by different symbols. 

Table below gives some of the names of the elements and their symbols. 

It has to be borne in mind that for an element represented by two lettered symbol, the first one is in capitals and second one is written as a lower case letter. This again is an adopted convention by scientists and is completely accepted by all.  

3. Chemical Formulae       
Substances (either compounds or mixtures) can be written in combination of the symbols, but have to be written in correct proportion of the elements that are the constituents. The written representation of a molecule of a substance using symbols of constituent element is called the molecular or chemical formula. For example, a molecule of potassium permanganate is written as KMnO₄; this means that one molecule of potassium permanganate contains one atom of potassium (K), one atom of manganese (Mn) and four atoms of Oxygen (O). A molecule of water is written as H₂O; which means that there are 2 atoms of Hydrogen (H) and one atom of Oxygen (O). In an ordinary table salt molecule, there is one atom of Sodium (Na) and one atom of Chlorine (Cl) and hence its chemical symbol is NaCl. To be able to write a chemical formula, we must first know the chemical reaction that precedes the formation of the compound. 

4. Valency 
Our knowledge about the structure of atoms depends on the mathematical formulations predicted by Neils Bohr. He suggested that electrons are distributed in orbits and the number of electrons held in the orbit depends on the number of the orbit. The orbits are counted outwards from the nucleus. Higher the orbit number, farther are the electrons in that orbit from the nucleus. If the orbit number is “n”, then the maximum electrons held in the orbit is given as 2n². The first orbit has n=1, and will hold maximum of 2 electrons, the second orbit has n=2 and is capable of holding a total of 8 electrons; similarly the third orbit will be able to contain 18 electrons and so on. 

Electrons within an atom have definite energies[1]. The electrons closest to the nucleus (n=1) are most tightly bound; the reason is because of stronger electrostatic attraction with the nucleus. Electrons in the highest orbit are least tightly bound. Electrons in the same orbit have same energies. The electron orbits are also called as electron energy levels or shells. Electronic shells are known as K shell, L shell, M shell, N shell corresponding to orbit number n=1,2,3 and 4 respectively. Higher number orbits are assigned shell names in alphabetical order after N.

Graphic Showing Shells

The distribution of electrons in various orbits in an atom is known as the electronic configuration of that atom. The electronic configuration gives each atom, and hence each element in nature, its unique physical and chemical characteristics.  

Table below gives electronic configuration of some elements. 

The number of electrons in the outermost orbit decides the chemical nature of the atom. An atom tries to have its outermost orbit complete with generally 8 electrons. Helium is the only stable element whose (first and the last) outermost orbit has 2 electrons[2]. If this situation is not met, then the atom is unstable and reacts with other elements to either give or borrow electrons. The tendency of atoms to donate or accept electrons to stabilize their outermost orbits is known as the valency of the element. Valency is the measure of reactivity of the element. For example, Sodium (Na) has Z=11, no of electrons =11. Its electronic configuration will be K=2, L=8, M=1. The last M shell has vacancy for accommodating 7 electrons. But instead it is far easier for an Na atom to give up 1 electron from the M shell so that its outermost orbit has L=8. Chlorine (Cl) has Z=17, no. of electrons = 17. Its electronic configuration will be K=2, L=8, M=7. The last M shell needs to borrow 1 electron or give up 7 electrons. The chemical compound Sodium Chloride (NaCl) is very easily formed as both Na and Cl complement each other’s need for stabilizing their outermost orbits. Both Na as well as Cl are said to have valency 1. (Na valency is said to be +1, Cl valency is said to be –1)

We will discuss more about valency of atoms when we discuss how compounds are formed. Also it has to be kept in mind that several elements show variable valency, that is their combining capacity with other elements change with the condition of reaction.  

[1] Other than n=1 that is K shell, all other shells have sub-shells. The sub-shells very slightly differ in energy within each shell.

[2] In reality, the outermost orbit of an atom is stable when the electrons are paired.

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