History of Periodic Table
* Dobereiner’s law of triads: In a triad, the atomic mass of the middle element is equal to the arithmetic mean of the atomic masses of the other two.
* Newlands law of octaves: When elements are arranged in increasing order of their atomic weights, the properties of the eighth element are a repetition of the properties of the first element.
* Mendeleev’s law: The physical and chemical properties of elements are a periodic function of their atomic weights.
Modern Periodic Table
* Atomic number is the number of protons in an atom.
* The number of electrons in an atom is equal to the number of protons.
* An atom is electrically neutral.
* Valence electrons: Electrons in the shell of an atom.
* Electronic configuration: The arrangement of electrons in an atom around its nucleus.
* Bohr’s theory: Electrons are not located randomly around an atom’s nucleus, but occur in definite electron shells.
* The physical and chemical properties of elements are a periodic function of their atomic numbers.
* Elements are arranged in the increasing order of atomic number.
The modern periodic table consists of:
* 7 horizontal periods
* 18 vertical groups
* The properties of the elements in the periodic table change gradually from left to right.
* Group I elements are called alkali metals.
* Group II elements are called alkaline earth metals.
* Group VII elements are called halogens.
* Zero group elements are referred to as inert gases.
Periodic Properties
* The properties of elements, which recur at regular intervals, are called periodic properties.
* Atomic size increases in a group and decreases in a period.
* Ionization energy increases across a period and decreases down a group.
* Electron affinity increases across a period and decreases down a group.
* Electronegativity increases across a period and decreases down a group.
* Metallic character decreases across a period and increases down a group.
* Non-metallic character increases across a period and decreases down a group.