If we group the elements having similar properties, study of large numbers of elements becomes easier. Example: Be, Mg, Ca, Sr, Ba, Ra all have the same properties. So, instead of studying them separately, they could be grouped together to study their properties. Thus, classification also helps us to understand the patterns of behaviour of elements in a more simple and systematic way.
The modern classification of elements is based on the atomic number of the elements rather than the atomic mass. The principle involved in this classification was stated by Mosley in the form of a law now known as the modern periodic law. He has been given the credit for the modern periodic table. Now it is known that properties of elements depend on the electronic configuration, which depends on the atomic number. The atomic number, instead of atomic weight, is chosen for the following reasons:
1. Atomic number increases regularly by one from element to the next, but atomic mass does not vary regularly from one element to next. 2. No two elements can have the same atomic number though the same element can have atoms with different masses. 3. The property of an element depends on the electronic configuration, which depends on the atomic number and not the atomic mass.
PERIODS AND GROUPS:
The basic structure of the periodic table is its division into rows and columns, or periods and groups. A period consists of the elements in any one horizontal row of the periodic table. A group consists of elements in any one column of the periodic table.
The first period of elements is short, consisting of only hydrogen (H) and helium (He). The second period has 8 elements, beginning with lithium ( Li) and ending with neon ( Ne). There is then another period of 8 elements followed by a period comprising of 18 elements, beginning with potassium (K) and ending with krypton (Kr). The fifth period also has 18 elements. The sixth period actually consists of 32 elements, but in order for the row to be fit on a page, part of it appears at the bottom of the table. Otherwise the table would have to be expanded, with the additional elements placed after lanthanum (La, atomic number 57). The seventh period, though not complete, also has some of its elements placed as a row at the bottom of the table.
The groups are usually numbered. The numbering frequently seen in North America labels the groups with Roman numerals and A's and B's. In Europe, a similar convention was followed, but some columns have A's and B's interchanged. To eliminate this confusion, the International Union of Pure and Applied Chemistry (IUPAC) suggested a convention in which the columns are numbered 1 to 18. When we refer to an element by its periodic group, we will use the traditional North American convention. The A groups are called main-group elements while the B groups are called transition elements. The two rows at the bottom of the table are called inner-transition elements, with first row being lanthanides, and the second row actinides.
As mentioned earlier, the elements in any one group have similar properties. For example, the element in Group 1A, often known as the alkali metals, are soft metals that react easily with water. (Hydrogen, a gas is an exception and might be put in a group by itself). Next in the order are sodium and potassium. The group VIIA elements, known as halogens, are also reactive elements. For example, chlorine and bromine are halogens, which reacts vigorously with sodium.
METALS, NONMETALS AND METALLIODS:
The elements of the periodic table are divided by a heavy " staircase" line into metals on the left and nonmetals on the right. A metal is a substance or mixture that has a characteristic luster, or shine, and is generally a good conductor of heat and electricity. Except for mercury, the metallic elements are solids at room temperature. They are malleable and ductile.
A nonmetal is an element that does not exhibit the characteristics of a metal. Most of the nonmetals are gases (chlorine, oxygen, etc.) or solids (phosphorus, sulfur, etc.). The solid nonmetals are usually hard and brittle substances. Bromine is the only liquid nonmetal.
Most of the elements bordering the staircase line in the periodic table are metalloids, or semimetals. A metalloid is an element having both metallic and nonmetallic properties. Elements such as silicon (Si), germanium (Ge) are metalliods which are usually good semiconductors. They are poor conductors of electricity, in pure form, at room temperature, but become moderately good conductors at higher temperatures.