Periodic Classification of Elements

One Marks Questions

Answer: When the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

Answer: Every eight-element has similar properties when the eighth element when the elements are arranged in the increasing order of their atomic masses.

Answer: The properties of elements are the periodic function of their atomic masses.

Answer: The properties of elements are the periodic function of their atomic number.

Answer: The vertical columns in the periodic table are called ‘groups’. The horizontal rows in the periodic table are called ‘periods’.

Answer: Dobereiner’sCould identifies only three triads from the elements known at that time.

Two Marks Questions

Answer:
The metallic character goes on decreasing along a period from left to right because atomic size goes on decreasing therefore, the tendency to lose electrons decreases.

Answer:
Atomic size decreases in moving from left to right along a period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atoms.

Answer:
Atomic size increases down the group. This is because new shells are being added as we go down the group.

Answer: The metallic character goes on increasing along with a group from top to bottom because atomic size goes on increasing therefore, the tendency to lose electrons increases.

Answer:
(a) It is because nuclear charge increases due to an increase in atomic number, therefore, the force of attraction between nucleus and valence electrons increases, i.e. effective nuclear charge increases, hence atomic radii decrease from Na to Cl.

(b) (i) Most metallic element is ‘Na’ as it can lose electrons easily due to larger atomic size.
(ii) Most non-metallic element is ‘Cl’ because it can gain electrons easily due to the smallest atomic size.

Answer:
(a) Li is bigger than Be as it has a lesser nuclear charge so there is less force of attraction between nucleus and valence electrons and less effective nuclear charge. It is, therefore, bigger in size.

(b) (i) Li is the most metallic as it can lose electrons easily due to its larger atomic size,
(ii) F is the most non-metallic element because it can gain electrons easily due to its smallest atomic size.

Answer:
* Mendeleev left some gaps in his periodic tables.
* When noble gases were discovered, they could be placed in a new group without disturbing the order.

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Three and Four Marks Questions

Answer:
* Law of octaves was applicable only up to calcium. After calcium, every eighth element did not possess properties similar to that of the first.
* Newland assumed that only 56 elements existed in nature. But later on, several new elements were discovered, whose properties didn't fit into the Law of Octaves.
* In order to fit elements into his table, Newlands adjusted two elements in the same slot, but some unlike elements under the same note.

Answer:
* No fixed position given to hydrogen.
* Isotopes of all elements posed a challenge.
* Atomic mass does not increase in a regular manner in going from one element to the next.

Answer:
(a) ‘Y’ is non-metal because it can gain electron easily as it has 7 valence electrons and forms a negative ion with a stable electronic configuration.

(b) It is because ‘X’ has a lesser atomic number, less nuclear charge, less force of attraction between valence electrons and nucleus, therefore, has a larger atomic size.

(c) ‘Z’ has 7 valence electrons. It can gain one electron to become stable. So, its valency is equal to one.

Answer:
i)Period:3
Group:1
Block:s

ii)Period:4
Group:8
Block:d

Answer:
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