After the discovery of large number of elements it became necessary to classify them and arrange them in a regular manner in order of their periodic properties. In 1817, Johann Wolfgang Dobereiner tried to arrange the elements with similar properties into groups. He identified some groups of three elements having similar physical and chemical properties, known as Dobereiners triads.
In 1865, John Newlands arranged all known elements in the order of increasing atomic masses and found that the properties of every eighth element are similar to the properties of the first element.
(i) If CI, Br, I is a Dobereiners triad and the atomic masses of CI and I are 35.5 and 127 respectively, then the atomic mass of Br is

(ii) Example of Dobereiner's triad is

(iii) A and B are two elements having similar properties which obey Newlands' law of octaves. How many elements are there in between A and B?

(iv) According to the Newlands' law of octaves, the properties of magnesium are similar to those of

(v) On what basis the elements are arranged in Dobereiner's triad?

Mendeleev arranged the 63 elements known at that time in order of their ascending atomic masses and prepared a periodic table. Mendeleev's periodic table contains vertical columns called 'groups' and horizontal rows called 'periods': Elements with similar properties were placed in same groups.
The basis of Mendeleev's classification is his periodic law which states that:
(I) Atomic mass is the fundamental property of elements.
(II) The physical and chemical properties of elements are periodic function of their atomic masses.
(i) Which of the following metals is not placed in eighth group of Mendeleev's periodic table?

(ii) In Mendeleevs periodic table, silver belongs to IB group. The group to which silver belongs in the modern periodic table is

(iii) In Mendeleev's periodic table, gaps were left for the elements to be discovered later. Which of the following elements found a place in the periodic table later?

(iv) The properties of eka-aluminium predicted by Mendeleev were the same as properties of which element that was discovered later?

(v) Which of the following statements is not correct about Mendeleev's periodic table?
(a) In the mendeleev's periodic table, some places were left vacant for new elements which were not discovered at that time.
(b) Group VIII like groups I-VII has been divided into two sub-groups A and B.
(c) The group of an element in the periodic table represents its valency.
(d) Li and C belong to same period in Mendeleevs periodic table.

In 1913, Henry Moseley showed that the atomic number of an element is the more fundamental property than its atomic mass. Accordingly, Mendeleev's periodic law was modified and atomic number was adopted as the basis of modern periodic table.
In this periodic table, the elements are arranged in increasing order of their atomic numbers.
There are 18 vertical columns in the periodic table which constitute 18 groups or families. The groups are numbered as 1, 2, 3, ... up to 18. All the members of a particular group have similar outer shell electronic configuration. There are seven horizontal rows of the periodic table which are known as periods.
(i) According to modern periodic law, the properties of elements are the periodic function of their

(ii) All the elements in a period in the periodic table have the same

(iii) Which of the following combinations of elements belong to the same group?

(iv) The atoms of elements belonging to the same group of periodic table have same number of

(v) In the periodic table, the element with atomic number 16 will be placed in the group

Maximum number of electrons that can be accommodated in a shell is given by the formula: 2n², where n is the number of the outermost from the nucleus.
For example,
K shell - 2 x (1)² => 2, hence, K-shell can accommodate maximum 2 electrons.
L shell - 2 x (2)² => 8, hence, L-shell can accommodate maximum 8 electrons.
In the modern periodic table, elements are placed according to their electronic configuration. The elements present in any group have the same number of valence electrons. The elements present in any period contain the same number of shells. The first period of the modern periodic table corresponds to the filling of electrons in the first energy shell, i.e., K-shell, first period has two elements. The second period of the periodic table corresponds to the filling of electrons in the second energy shell, i.e., L-shell, second period contains eight elements. The third, fourth, fifth, sixth and seventh periods have 8, 18, 18,32 and 32 elements respectively.
(i) Electronic configuration of an element 'X' is 2, 1. The number of elements present in the period to which 'X'belongs is

(ii) Among the given elements A, B, C, D and E with atomic numbers 2, 3, 7, 10 and 30 respectively, which of these belong to the same period?

(iii) The elements A, B, C and D have atomic numbers 4, 12, 17 and 19 respectively. Which pair of elements belong to the same period?

(iv) Which of the following have the same number of electrons in outermost shell?
(a) Elements with atomic numbers 3, 11, 19
(b) Elements with atomic numbers 14, 15, 16
(c) Elements with atomic numbers 12, 20, 28
(d) Elements with atomic numbers 10, 18, 26
(v) Which of the following elements has two shells and both are completely filled?

All the elements on the left side and in the middle of the periodic table (except hydrogen) are metallic elements or metals. Also, majority of elements in periodic table are metals. In the modern periodic table, the metals are separated from non-metals by a zig-zag line. Some non-metals are gases, some are liquids and rest are solids at room temperature. They generally differ from metals in appearance and in other physical properties. Some elements that lie along the zig-zag line that separates metals from non-metals, have properties that fall between those of metals and non-metals. These elements are regarded as semi-metals or metalloids.
(i) From the given set of metals and non-metals, identify the non-metals.
 \(\mathrm{S}, \mathrm{Mg}, \mathrm{Al}, \mathrm{P}, \mathrm{N}, \mathrm{Na}, \mathrm{K}\)

(ii) Which of the following groups contains metals, non-metals and metalloids?

(iii) Which of the following elements is a metalloid?

(iv) Silicon is a metalloid because
(a) its valency is 4
(b) it has three electron shells
(c) it shows properties of both metals and non-metals
(d) it is a liquid metal
(v) The lightest metal is