Periodic Table

Question 1

Triads

Answer 1

Groups of three chemically analogous elements in order of increasing atomic weight with the central element having an atomic weight equal to the arithmetic mean of the atomic weight of the first and third element were known as triads.

Question 2

Dobreiner’s law of triads

Answer 2

Chemically analogous elements arranged in increasing order of their atomic weights formed well marked groups of three called triads in which the atomic weight of the middle element was found to be generally the average [arithmetic mean] of the atomic weights of the other two elements.

Question 3

Demerits of Dobreiner’s law of triads

Answer 3

1. The method created disordered groups of elements and it didn’t hold true for all elements.
2. The chemical properties of the elements in a triad were always not found to be similar.`

Question 4

Newland’s law of octaves

Answer 4

Elements when arranged in increaisng order of their atomic weights show resemblance in physical and chemical properties between the eighth and first element [ similar to the eighth and first note of a musical scale]

Question 5

Octaves

Answer 5

Groups of 8 elements in order of increasing atomic weight created by Newland where the first and eighth elements show resemblance in their physical and chemical properties.

Question 6

Periodicity of properties

Answer 6

The phenomenon of appearance of similar properties at regular intervals is known as the periodicity of properties.

Question 7

Periodic properties

Answer 7

The properties which repeat themselves at regular intervals are known as periodic properties.

These include atomic radius, ionisation potential, electron affinity, etc.

Question 8

Mendeleef’s periodic law

Answer 8

The physical and chemical properties of elements are periodic functions of their atomic weight.

Question 9

Mendeleef’s contributions(any 3)

Answer 9

1. Elements were arranged in increasing order of atomic weights in horizontal rows called periods and vertical columns called groups.
2. Elements which are similar with respect to their chemical properties are grouped together and have atomic weights of nearly the same value.
3. Elements in the sam egroup had the same ‘valency nad similar chemical properties.
4. Based on the periodicity of properties, a number of gaps were left in the table for undiscovered elements i.e. elements now discovered eg: Scandium Gallium and Germanium originally called eka-boron, eka aluminium and eka-silicon respectively.
5. The properties of undiscovered elements left in the vacant gaps was predicted.
6. Incorrect atomic weights of some of the arranged elements were corrected with the knowledge of the atomic weight of adjacent elements.

Question 10

Demerits of Mendeleef’s periodic table

Answer 10

1. Certain pairs of elements having high atomic weights were placed before elements having low atomic weights. For example, Cobalt, having a lower atomic weight, is placed before Nickel in the table. This could have been solved by ordering based on atomic number and not atomic weight.
2. The position of Lanthacides and rare Earths could only be justified when using atomic numbers instead of atomic weights.
3. Isotopes had to be placed at different positions in the table when using atomic weights instead of atomic number.

Question 11

Number of periods in the periodic table

Answer 11

7

Question 12

Modern periodic law

Answer 12

The physical and chemical properties of elements are periodic functions of their atomic numbers

Question 13

GR: Periodicity in properties occur

Answer 13

1. Due to increasing atomic weight, there will be a repetition of similar electronic configuration at regular intervals.
2. Since electronic configuration and number of valence electron are responsible for the chemical properties of an element, there is a repetition of properties of elements at regular intervals and periodicity in properties occurs.

Question 14

Atomic radius

Answer 14

Distance between centre of nucleus and outermost shell.

Question 15

Ionisation potential

Answer 15

Amount of energy required to remove an electron from the valence shell of an isolated atom.

Question 16

Electron affinity

Answer 16

The amount of energy released when an atom in the gaseous state accepts an electron to form an ion.

Question 17

Periods

Answer 17

Seven horizontal rows of elements arranged in increasing order of atomic number in the periodic table are known as periods.

Question 18

Periods

Answer 18

Seven horizontal rows of elements arranged in increasing order of atomic number in the periodic table are known as periods.

Question 19

Groups

Answer 19

Eighteen vertical columns of elements [with eight main groups] in the periodic table are known as groups.

Question 20

Transition property characteristics down a sub-group

Answer 20

Similarity: the number of valence electron stay the same and chemical properties stay the same or vary very gradually.

Transition: down a group, there is an increase of metallic character

Question 21

Properties of Alkali metals

Answer 21

1. Present in group 1 of the periodic table.
2. Light in character.
   [VALENCY]
3. The elements of group 1 have one valence electron so they are univalent(they have a valency of 1).
4. These elements are strong reducing agents. They are electron donors and have one electron in their outermost shell which can be easily removed.
5. They are highly reactive, electropositive metals which are soft and can be easily cut with a knife. They are good conductors of heat and electricity.
6. They form electrovalent compounds(such as NaCl, KBr, etc.) with non-metals.

Question 22

Properties of Alkali Earth metals

Answer 22

1. They are found in group 2 of the Periodic table.
2. They are light metals.
3. They have 2 valence electron so they are divalent.
4. Their chemical properties are extremely similar to alkali metals but they are slightly less reactive.

Question 23

Properties of Halogens

Answer 23

1. They are found in group 17 of the Modern Periodic Table.
2. They have 7 valence electrons so they are monovalent.
3. They are strong oxidising agents. They are electron acceptors since, having 7 valence electrons, it is easy for them to gain an electron and become stable.
4. They are highly reactive, electronegative non metals.
5. They are poor conductors of heat and electricity.
6. They form covalent bonds with other non-metals.
7. They are called Halogens because of their property of producing salt when reacting with a metal.

Question 24

What is the need for classification of elements?

Answer 24

To analyse the properties of the elements and their compounds in a systematic manner