Monday, November 12, 2012

CBSE NCERT 10th Periodic Properties of Elements in modern periodic tables

Solved Questions:CBSE :Chemistry: Chapter-Periodic Classifications


1. Define valency and describe its variation in a period and a group.

Ans: Valency of an element is its combining capacity.
Valency of an element = number of valence electrons (if equal to or less than 4)
Valency of an element = 8 — number of valence electrons (if more than 4)


2. What happens to the metallic character of the elements as we move in a period from left to right in the periodic table? Give reason.

Ans: Metallic character of the element decreases along a period due to decrease in atomic size along a period or outermost electrons are closer to nucleus. So they cannot be easily lost.
3. How were the positions of different isotopes decided in modern periodic table?

Ans: All the isotopes of an element have same atomic numbers. In modern periodic table, the elements have been allotted places based on their atomic numbers. Therefore, all the isotopes of an element have been assigned the same position in the modern periodic table
4. Where do you think hydrogen should be placed in the modern periodic table?

Ans: The modern periodic table is based on the atomic numbers of the elements and also upon their electronic distribution. Alkali metals as well as hydrogen have one electron in their valence shell. Therefore, hydrogen should be placed in the group of alkali metals. However, it has been assigned a separate position and is not a member of the group of alkali metals since it is a non-metal.
5. How were the positions of the elements Co and Ni resolved in the modern periodic table?

Ans: On the basis of atomic masses, Ni (58.7) should be placed before Co (58.9). This was defect in Mendeleev’s table because the element Co was placed before Ni. However, the atomic number of (Co = 27) is less than that of Ni ( 28). Therefore, they have been correctly arranged in the modern periodic table.
6. The two isotopes of chlorine have atomic masses 35 u and 37 u. Should they be placed in same slot in the periodic table?

Ans: Yes, they should be placed in the same slot because periodic table is based on the atomic numbers of the element and both the isotopes of chlorine have the same atomic number (Z = 17).
7. Which element is bigger in size, lithium (Z = 3) or Sodium (Z = 11) and why?

Ans: Lithium (Li) atom has only two shells K and L in it whereas a sodium (Na) atom has three shells K L and M. Since a sodium atom has one mote shell than a lithium atom, therefore sodium atom is bigger in size.
8. What were the criteria used by Mendeleev in creating his periodic table?

Ans: Ans. Mendeleev’s periodic table was based on the observation that properties of the elements are a periodic function of their atomic masses. This means that if elements are arranged in the increasing order of their atomic masses, then elements with similar properties get their place in the same group in a successive increasing order of atomic masses.
9. Why did Mendeleev leave some gaps in his periodic table?

Ans. Mendeleev left gaps in his periodic table to keep the elements with similar properties together. I-fe predicted dat new elements would be discovered later and they would occupy those gaps.
10. Why did Mendeleev treated the hydrides and oxides of element as the basic properties of elements for their classification?

Ans. During the period when Mendeleev gave his periodic table atomic numbers and electronic configurations were not known. Comparison of properties of elements could be best done by comparing their compounds. He selected oxides and hydrides because oxygen and hydrogen form compounds with most of the elements due to their high reactivity.
11. Why are the elements in the same group of the periodic table show close resemblance in their chemical behavior?


Ans: The chemical properties depend upon the number of valence electrons of an element. The elements in the same group of the periodic table show close resemblance in their chemical behavior as they have same number of valence electrons.
12. What happens to atomic radii in a group and period and why?

Ans: Atomic radius is the distance between the centre of atom and the outermost shell.
In a period, atomic radius generally decreases from left to right.
In a period there is a gradual increase in the nuclear charge. Since valence electrons are added in the same shell, they are more and more strongly attracted towards nucleus. This gradually decreases atomic radii.
Atomic radii increase in a group from top to bottom.
As we go down a group the number of shells increases and valence electrons are present in higher shell and the distance of valence electrons from nucleus increases. Also, the number of filled shells between valence electrons and nucleus increases Both the factors decrease the force of attraction between nucleus and valence electron. Therefore, atomic size increases on moving down a group.
13. Why are the ionization energy of elements increases in a period from left to right.

Ans: The minimum amount of energy required to remove an electron from a gaseous atom in its ground state to form a gaseous ion is called ionization energy.
The ionization energy increases in a period from left to right. This is because the force of attraction between valence electron and nucleus increases in a period from left to right.
14. What do you mean by Electron affinity: How it is vary in group and periods in modern periodic table?

Ans: The tendency of element to gain an additional electron. This ability is measured by electron affinity. It is the energy change when an electron is accepted by an atom in the gaseous state. Electron affinity is assigned a positive value when energy is released during the process. Greater the value of electron affinity, more energy is released during the process and greater is the tendency of the atom to gain electron.
In a group, the electron affinity decreases on moving from top to bottom as less and less amount of energy is released.
In a period, the electron affinity increases from left to right, as more and more amount of energy is released

15. What do you mean by Electro negativity?

Ans: Electro negativity is relative tendency of a bonded atom to attract the bond-electrons towards itself. It just compares the tendency of various elements to attract the bond-electrons towards themselves.
Electro negativity decreases in a group from top to bottom. Electro negativity increases in a period from left to right. Electro negativity is related to ionization energy. Electrons with low ionization energies have low electro negativity because their nuclei do not exert a strong attractive force on electrons. Elements with high ionization energies have high electro negativity due to the strong pull exerted on electrons by the nucleus.
Periodic Properties of Elements in modern periodic tables Class X (10th)
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Check your understanding:
1) What is the atomic radius of an atom?
2) What is the trend in atomic radius across a period?
3) The trend in atomic radius across a period is caused by _____.
4) What generally happens to atomic radii as one goes down a group or a family?
5) Write the equation for the ionization of an atom.
6) What is an ion? 
7) What is ionization energy?
8) Which group or family has the lowest ionization energy?
9) Group 18 or 8A, the noble gases, have the highest ionization energy (True or False).
10) Elements with a high ionization energy lose electrons easily (True or False).
11) The increase in ionization energy across a period is caused by _____.
12) Why does ionization energy generally decrease going down a group or family?
13) What is meant by electron shielding?
14) What is the second ionization energy of an atom?
15) What is electron affinity?
16) Write the equation for electron affinity for an exothermic process.
17) Write the equation for electron affinity for an endothermic process.
18) Which group or family gains electrons most easily?
19) What is a cat ion?
20) What is an anion?
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