substances made up of same type of atoms, having same atomic number and cannot be decomposed into simple substances by ordinary chemical means are called elements. Or substance in which all the atoms are identical and valenticle is called element.
Total elements in the world: 118 elements have been discovered, out of which 92 are naturally occurring elements and 18 elements are artificially synthesized in the labortary and are called synthetic elements..
Explanation: Each element is made up of unique type of atoms that have very specific properties.
Elements occur in nature in free or combined form. All the naturally occurring elements found in the world have different percentages in the earth’s crust, oceans and atmosphere. Natural occurrence by weight percent of some naturally occurring elements.
Classification of elements:
Elements may be solids, liquids or gases. Majority of the elements exist as solids e.g. sodium, copper, zinc, gold, etc. There are very few elements which occur in liquid state e.g. mercury and bromine. A few elements exist as gases e.g. nitrogen, oxygen, chlorine and hydrogen.
On the basis of their properties, elements are divided into metals (sodium) , non-metals (Chlorine) and metalloids (Boron). About 80 percent of the elements are metals.
Chemical composition of living body: Major part of a living body is made up of water i.e. 65% to 80% by mass.
Six elements constitute about 99% of our body mass; namely: Oxygen 65 %, Carbon 18%, Hydrogen 10 %, Nitrogen 3%, Calcium 1.5% and Phosphorus 1.5%. Potassium, Sulphur, Magnesium and Sodium constitute 0.8% of our body mass. Whereas Copper, Zinc, Fluorine, Chlorine, Iron, Cobalt and Manganese constitute only 0.2% of our body mass.
Symbol of the elements: The abbreviated name of an element is called symbol. Elememns are represented by these symbols.
Roles for assigning symbols to an element: A symbol is taken from the name of that element in English, Latin, Greek or German. If it is one letter, it will be capital as H for Hydrogen, N for Nitrogen and C for Carbon etc. In case of two letters symbol, only first letter is capital e.g. Ca for Calcium, Na for Sodium and Cl for Chlorine.
Valency: The combining capacity of an element with other elements is called valency. It is the unique property of an element. It depends upon the number of electrons in the outermost shell.
In simple covalent compounds, valency is the number of hydrogen atoms which combine with one atom of that element or the number of bonds formed by one atom of that element e.g. in the following compounds.
HCl, H2O, NH3 and CH4.
The valency of chlorine, oxygen, nitrogen and carbon is 1, 2, 3 and 4, respectively.
In simple ionic compounds valency is the number of electrons gained or lost by an atom of an element to complete its octet. Elements having less than four electrons in their valence shell; prefer to lose the electrons to complete their octet. For example, atoms of Na, Mg and Al have 1, 2 and 3 electrons in their valence shells respectively.
They lose these electrons to have valency of 1, 2 and 3, respectively. On the other hand, elements having five or more than five electrons in their valence shells, gain electrons to complete their octet. For example, N, O and Cl have 5, 6 and 7 electrons in their valence shells respectively. They gain 3, 2 and 1 electrons respectively to complete their octet. Hence, they show valency of 3, 2 and 1, respectively. A radical is a group of atoms that have some charge. Valencies of some common elements and radicals are shown in Table.
Some elements show more than one valency, i.e. they have variable valency. For example, in ferrous sulphate (FeSO4 ) the valency of iron is 2. In ferric sulphate (Fe2(SO4)3), the valency of iron is 3. Generally, the Latin or Greek name for the element (e.g., Ferrum) is modified to end in ‘ous’ for the lower valency (e.g. Ferrous) and to end in ‘ic’ for the higher valency (e.g. Ferric).