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. Altho Althoug ugh h the names names are long, long, gener generall ally y abbre abbrevia viatio tion n of eithe eitherr first first or first first two letter letters s represent represents s the element. element. This is done for convenie convenience nce only. Some elements 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. Common name of element Hydrogen Carbon Sodium Chlorine Iron Silver Gold
Latin Name Natrium Ferrum Argentum Aurum
Symbol H C Na Cl Fe Ag Au
It has to be borne in mind that for an element represented 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 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. 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 H 2O; 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 knowledg knowledge e about about the structure structure of atoms atoms depends depends on the mathema mathematical tical formulation formulations s 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] 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 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
The distribu distributio tion n of electr electrons ons in vario various us orbit orbits s in an atom atom is known known as the the electr electroni onic c 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. Name of the Atomic number (Z) element (symbol) Hydrogen (H) Carbon (C) Oxygen (O) Chlorine (Cl)
1 6 8 17
Total number of electrons 1 6 8 17
Electronic Configuration K 1 2 2 2
L
M 4 6 8
N
7
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] [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 elem elemen ent. t. For For exam exampl ple, e, Sodi Sodium um (Na) (Na) has has Z=11 Z=11,, no of elec electr tron ons s =11. =11. Its Its elec electr tron onic ic 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 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. Variable Valency Quite a lot of transition metals and non-metals show variable valency. As mentioned before, valency is a measure of reactivity of an element. Physically, valency means how many electrons an element can donate or accept to make its outermost orbital stable. A stable orbit may mean mean that there are no unpaired unpaired electrons left in the outermost orbit. Generally it is observed that an atom tries to acquire electronic configuration of its nearest noble gas atom. Amongst transition metals, Iron, Nickel show variable valency. Amongst non-metals, Oxygen and Nitrogen are good examples. These metals and non-metals combine to give various compounds compounds having different properties. For example Fe 2O3 is different from FeO. In Fe2O3, Fe shows valency +3 and O shows valency –2. Fe2O3 is a magnetic compound. In FeO, Fe shows valency +2. FeO is non magnetic.
Water (H2O) and hydrogen peroxide (H 2O2) are compounds of hydrogen and oxygen. In H2O, H shows a valence of +1 and O shows a valence of –1. In H 2O2, H has a valency of +2 but O has a valency of –1. Water is a neutral compound but H2O2 is a highly acidic compound.