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

Generally metals possessing 1, 2 or 3 electrons in their respective valence shells have a strong tendency to lose electrons to form positive ions. Non-metals, on the other hand, having 4 to 8 electrons in their respective outermost shells generally have a tendency to gain electrons to form negative ions. Metallic character is called electropositive character and non-metallic character is called electronegative character. The metallic character increases down a group and non-metallic character increases along a period.
(i) Which of the following electronic configurations represent most electropositive element?

(ii) Considering the elements B, C, N, F and Si, the correct order of their non-metallic character is

(iii) Which of the following is least metallic?

(iv) To which of the following categories does the element with atomic number 14 belong?

(v) Non-metals are present in the periodic table at

The distance between the centre of the nucleus and the outermost shell of electrons is known as atomic radius.On moving from left to right along a period, atomic radii decrease because effective nuclear charge increases.
For example, the atomic size decreases regularly from Li to F in the second period and from Na to CI in the third period. It may, however, be noted that in any period, the noble gas has the largest size. On moving down in a group, atomic radii increase.

(i) Which of the following has the maximum atomic radius?

(ii) The element with the smallest size in group l3 is

(iii) The atomic radius decreases as we move across a period because
(a) atomic mass increases
(b) atomic number decreases
(c) effective nuclear charge increases
(d) electrons are removed
(iv) In the third period of the periodic table, the element having smallest size is

(v) Among O, C, F, CI, Br, the correct order of increasing atomic radii is

Study the following table in which positions of six elements A, B, C, D, E and F are shown as they are in the modern periodic table:

(i) Which element in the given table has same number of electrons as in K⁺ and CI^-.

(ii) The formula of the oxide of element D will be

(iii) Which of the following elements has most metallic character?

(iv) Element E forms a chloride with formula

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

The recurrence of properties of the elements after a certain regular intervals, when they are arranged in the increasing order of their atomic numbers, is called periodicity. There are a number of physical properties such as atomic size, metallic and non -metallic character, etc. which show periodic variation. In periodic table, various properties vary differently from moving left to right in a period and going down in a group. In a period, properties vary because from moving left to right in a period, number of shells remain same but valence electron increases by one number hence nuclear charge increases. In a group, on going down, number of valence shells increases while number of valence electrons remains same.
(i) From top to bottom in a group of the periodic table, the electropositive character of the element

(ii) Which element has the largest size in the second period?

(iii) Which of the following elements has three valence electrons?

(iv) In the periodic table, the metallic character of elements
(a) decreases from left to right and decreases down the group
(b) decreases from left to right and increases down the group
(c) increases from left to right and increases down the group
(d) increases from left to right and decreases down the group
(v) Which of the following increases along the period?

"Properties of elements are the periodic function of their atomic numbers".This is known as modern periodic law. It means that the properties of elements depend on their atomic numbers and the elements are given positions in the periodic table on the basis of their increasing atomic number. Atomic number determines the distribution of electrons in the orbit, and electrons of the outermost orbit determine the properties of an element. There are 18 groups (vertical columns) and 7 periods (horizontal lines) in modern form of the periodic table. The number of the period is equal to the number of shells in the atoms of the elements belonging to that period.
(i) What is the atomic number of element of period 3 and group 17?

(ii) Atomic number of an element is 2, 8, 6. Its period number and valency are respectively

(iii) An element has mass number 40 and contains 20 neutrons in its atom. To which period and group of the periodic table does it belong?

(iv) An elements 'X' has an atomic number of 16. With which of the following elements will it show similar chemical properties?

(v) Identify the statement(s) which is(are) true for the modern periodic table.
(a) It reflects trends in physical and chemical properties of the elements.
(b) It helps to reflect the relative atomicity of bonds between any two elements.
(c) It helps to predict the stable valency state of the elements.
(d) All of these

The electron configuration of an element describes how electrons are distributed in its atomic orbitals. Electron configurations of atoms follow a standard notation in which all electron-containing atomic subshells (with the number of electrons they hold written in superscript) are placed in a sequence. The diagram below shows the electron arrangement of three atoms of elements X, Y and Z.

(i) Which of the following is the electronic configuration of X?

(ii) Number of valence electrons in Y is

(iii) Which of the following has the most metallic character?

(iv) Element Y achieve the electron structure of a noble gas by gaining

(v) Which of the following statement is incorrect for 2?

In his periodic table, Mendeleev arranged all the then known 63 elements in the order of increasing atomic masses in horizontal rows but in such a way that elements having similar properties came directly under one another in the same vertical column. In the classification of the then known elements, Mendeleev was guided mainly by two factors. In order to make sure that the elements having similar properties fell in the same vertical column, Mendeleev left some gaps in his periodic table. Though the leaving of gaps in the periodic table was considered to be a big drawback of his classification of elements at that time but Mendeleev stuck to his decision.

(i) What are the horizontal rows of Mendeleev’s periodic table known as ?

(ii) How many horizontal rows of elements were there in Mendeleev’s original periodic table ?

(iii) How many vertical rows were there in Mendeleev’s original periodic table ?

(iv) The three elements having chemical symbols of Si, B and Ge are :

(v) According to Mendeleev’s periodic law, the elements were arranged in the periodic table in the order of :

Study the following table in which positions of six elements A, B, C, D, E and F are shown as they are in the Modern Periodic Table :
On the basis of the table, answer the following questions :

(i) The element which forms only covalent compounds is

(ii) Which of the following elements is a metal with valency three?

(iii) Which of the following elements is a non- metal with valency three?

(iv) Which of the following elements has largest atomic size?

(v) C and F belongs to