Periodic Table
■ Lavoisier classified the elements into metals and non-metals.
■ Dobereiner classified the elements as triads, in which the atomic weight of the middle element was the arithmetic mean of the other two.
■ The law of octaves was proposed by Newland.
■ Mendeleev's periodic table is based on atomic weights.
■ Mendeleev's periodic table consists of 7 periods and 9 groups.
■ Modern periodic table contains 7 horizontal rows called periods and 18 vertical columns called groups and 118 elements.
■ Elements of group 1 are called Alkali metals.
■ The periodic table is divided into four main blocks
1. s-block - Elements of group 1 and 2
2. p-block - Elements of group 13 to group 18
3. d-block - Elements of group 3 to group 12
4. f-block - Lanthanides and Actinides
■ Elements of group 2 are called Alkaline earth metals.
■ Elements of group 17 are called Halogens.
■ There are 118 elements known today, out of which 90 are naturally existing.
■ First period contains only 2 elements, this is the shortest period.
■ The sixth period consists of 32 elements, this is the longest period.
■ Elements of group 18 show little reactivity and are termed noble gases.
■ Elements from atomic number 58 to 71 are lanthanides or rare earths.
■ Elements from atomic number 90 to 103 are termed as actinides.
■ The atomic number was first calculated by Moseley.
■ Man made elements are known as transuranics.
■ Elements from atomic number 93 onwards do not occur in nature. So they are called synthetic or transuranic elements.
■ s and p-block elements are called representative elements.
■ d-block elements are called transition elements.
■ f-block elements are called inner transition elements.
■ In a group, from top to bottom atomic size increases.
■ What is electro negativity ?
Ans: The power of atom to attract electron pairs. The order of electro negativity is F (4.00) > O (3.50) > Cl (3.10) > N (3.00). The electro negativity scale was introduced by Linus Pauling.
■ Atomic size decreases from left to right in a period.
■ Ionization Energy is the energy required to remove an electron completely from an isolated gaseous atom or ion of an element.
■ Ionization energy generally increases from left to right in a period with increase in atomic number or decrease in atomic size.
■ In a group, from top to bottom ionization energy decreases.
■ Electron Affinity is the energy change when an electron is gained by a neutral gaseous atom converting it into a negatively charged ion.
■ In a group electron affinity, decreases from top to bottom and in a period it increases as we move from left to right.
Classification of Elements
1. Metals and non metals - Lavoisier
2. Triads – Dobereiner
3. Law of octaves – Newland
4. Based on atomic weight – Mendeleev
5. Based on atomic number – Moseley
