Periodic Classification of elements

Question 1

Explain Dobreiner’s triads.

Answer 1

He tried to arrange elements with similar properties into groups with increasing atomic mass. Each of the group had 3 elements and he figured out that the sum atomic masses of the 1st and 3rd elements divided by 2 was equal to the atomic mass of the 2nd element.

eg; group of LI, Na and K

Question 2

State the limitations of dobreiner’s triads.

Answer 2

1. He was able to identify only 3 triads from the 33 elements known at that time.

Question 3

Explain newland’s law of Octaves.

Answer 3

• He arranged the known elements in the order of increasing atomic masses.
• He found out that he current element had the similar properties to the eight element from that place.
• He compared this to the octaves found in music.

Question 4

State some limitations of newland’s octates.

Answer 4

1. It was found that the law of octaves was applicable only upto calcium as after calcium every eight element did not possess properties similar to the first element.
2. Only 56 elements were known at that time. But later new elements were discovered to whom this law was not applicable.
3. In order to fit the elements according to the rule, the adjusted two elements in the same slot and some elements with unlike properties were fit under the same group

Question 5

Describe mendeleev’s periodic table

Answer 5

• At that time only 63 elements were known,
• He sorted out the elements with similar properties and pinned them on a wall. Among chemical properties. he concentrated on the compounds formed by elements on reacting with hydrogen and oxygen.
• He grouped the elements with the same properties together and arranged them in increasing atomic mass. He also leaved black spaces where no elements could fit.
• He also said that “The properties of elements are periodic functions of their atomic masses”

Question 6

State the merits of mendeleev’s periodic table

Answer 6

1. In his periodic table, he left some blank spaces, these black spaces were left for the elements that were not discovered by that time.
2. Mendeleev also predicted the properties of these unknown elements on the basis of the properties of elements lying vertically adjacent to these vacant spaces. He predicted the presence of 3 elements - eka-silicon, eka-aluminium, eka-boron. for eg; the eka-aluminium was late discovered to be gallium and the properties of gallium was similar to that of aluminium which was vertically adjacent to it.
3. when late noble gases were discovered, they were accomodated in the table in the form of a separate group without disturbing the positions of the other elements.
4. He was able to correct the values of atomic masses of elements like beryllium from 13.5 to 9 as he place these elements strictly by the similarities in their properties.

Question 7

state the demerits of mendeleev’s periodic table

Answer 7

1. Mendeleev could not assign a correct position to hydrogen in the periodic table as hydrogen resembles alkali metals as well as halogens.
2. Position of the isotopes could not be assigned.
3. It was observed that at certain places, an element of higher atomic mass was place before an element of lower atomic mass. for eg Cobalt (58.93 amu) was placed before Nickel (58.71 amu).

Question 8

The modern periodic table was made by ___________ in _____

Answer 8

henery mosely, 1913

Question 9

__________ elements were present in the d-block and __________ elements were present in the f-block.

Answer 9

transition, inner-transition

Question 10

Vertical columns in the modern periodic table are called _______ and the horizontal rows in the modern periodic table are called _________.

Answer 10

groups, periods

Question 11

Describe the modern periodic table.

Answer 11

According to the modern periodic table, the properties of elements are a function of their atomic numbers. There are 18 vertical columns (groups) and 7 horizontal rows (periods).
Elements are arranged in the order of increasing atomic numbers and elements in the group have the same properties.

Question 12

In the modern periodic table,
1st group elements are called ______________ .
2nd group elements are called _______________ .
17th group elements are called ______________ .
18th group elements are called ____________ .

Answer 12

• alkali metals
• alkaline earth metals
• halogens
• noble gases

Question 13

in modern periodic table,
group number indicates __________
periodic number indicates __________

Answer 13

valence electrons, number of orbits

Question 14

explain the valence electron trend in modern periodic table.

Answer 14

valence electrons increases in atoms of elements as we move from left to right of a period and valence electrons remains same as we move from top to down of a group.

Question 15

explain the valency trend in the modern periodic table.

Answer 15

valency of elements remain the same as we move from top to bottom of group. when we move left to right of a period then the tendency of atoms from loosing electrons shift to the tendency of gaining electrons.

Question 16

explain the number of orbits trend of the modern periodic table.

Answer 16

the number of orbits increases as we move from top to down of a group and remains same as we move from left to right of period.

Question 17

explain the atomic size trend in the modern periodic table.

Answer 17

atomic size increases from top to bottom in a group as the number of orbits increases but decreases from left to right as the effective nuclear charge increases. Effective nuclear charge means the number of protons which controls the electrons, so, as the number of electrons controlling the number of protons increases the overall force increases and the orbits size shrinks.

Question 18

Explain the trend of metallic character in the modern periodic table.

Answer 18

• metallic character increases form top to down the group because as we move down the distance between the positively charged nucleus and the negative valence electrons increases, so, now the valence electrons can move out of the atoms easily and hence can show more metallic character.
• metallic character decreases across the period as now the valence electrons increases and hence the tendency to gain electrons decreases.

Question 19

Explain the trend of non metallic character in the modern periodic table.

Answer 19

• non metallic character increases from left to right of a period as the number of valence electrons start to increase and hence the tendency of gaining electrons also increases.
• non metallic character decreases down the group as now the force of the positive nucleus holding the valence electrons will be decrease as the distance between the electron and nucleus will increase, so it would be difficult to gain electrons.