10th Class Science Periodic Classification of Elements

Periodic Classification of Elements

• Scientists have discovered 118 elements till date. Some of these elements occur in free state and some in combined state.

• Lavoisier classified elements into metals and non-metals.

• In 1817, German chemist Johann Wolfgang Dobereiner proposed Law of Triads.

• In a Dobereiner's triad, the atomic weight of middle element is nearly the arithmetic mean of the first and the third element.

• In 1866, John Newlands, an English chemist showed that, when elements are arranged in the order of their increasing atomic masses, the eighth element, starting from a given element is a kind of repetition of the first one like the eighth one in an octave of music. Newlands' law is known as law of octaves.

• Mendeleev and Lother Meyer used atomic weights to classify elements.

• Mendeleev's periodic law states that "the properties of elements are the periodic functions of their atomic masses Mendeleev's periodic table corrected the wrongly assigned masses of some elements. He predicted the properties of some undiscovered elements such as eka-boron, eka-aluminium, etc. Mendeleev's periodic table contains vertical columns called "groups" and horizontal rows called "periods"

• Modern periodic table is divided into 7 periods and 18 groups. The first six periods contain 2,8,8,18,18 and 32 elements respectively. The 7th period is incomplete. Lanthanoids and actinoids are placed at the bottom of the periodic table in separate blocks.

• Based on the electronic configuration, elements are classified into 4 types: inert gases, representative elements, transition elements and inner transition elements.

• The maximum number of electrons that a shell can accommodate is determined by the formula\[\text{2}\,{{\text{n}}^{\text{2}}}\], where 'n' represents the shell number from the nucleus.

• The valency of an atom can be determined by the number of electrons it loses or gains or shares while combining with other atoms.

• Atomic size is the distance between the centre of the nucleus and outermost shell of an isolated gaseous atom.

• Atomic radius decreases from left to right in a period and in a group it increases from top to bottom.

• An element which shows both metallic and non-metallic characters is called a metalloid or a semi-metal.