Periodic Classification of Elements and Periodicity | Short Questions

 

Periodic Classification of Elements and Periodicity | Short Questions and Answers. This post is about FSc. Part-II Chemistry | Chapter No.1.

The following objective questions and answers are very important and conceptual for 2nd Year, Chemistry, for the final examinations of all Boards of Punjab as well as Pakistan:

Chapter No.1: Periodic Classification of Elements and Periodicity

Short Questions and Answers

Q.1: Why ionic radii of anions are larger than their parent atoms?
Answer:

There are the following reasons:

• Incoming electron is accommodated in the new shell thus increasing the orbit and therefore size increases.
• There is decrease in the nuclear charge.
• If the incoming electron enters the outermost orbit which has already electrons, the orbit will expand. For example, the atomic radius of F is 64 Pm while the ionic radius of `F^-` is 136 Pm.

Q.2: Why ionization energy decreases down the group?
Answer:

It is due to:

• There is increase in the atomic size down the group, the hold of the nucleus on the electrons decreases.
• Shielding effect also increases.

Q.3: Why ionization energy increases along the period?
Answer:

• Along the period, there is decrease in the atomic size, hold of the nucleus on the electrons increase.
• Nuclear charge increases.

Q.4: why d and f block elements are called transition elements?
Answer:

The elements of d and f block are in between s and p block elements. Their properties show a transition in systematically and they vary from left to right.

Q.5: What is law of octaves?
Answer:

Newland in 1864 classified 62 elements on the basis of their increasing atomic masses. According to Newland, “every eighth element has properties similar to the first one if they are arranged according to increasing atomic masses”.

Q.6: What is Mendleev’s periodic law?
Answer:

If the elements are arranged according to their ascending atomic masses, they show periodicity (repetition of physical and chemical properties after regular intervals) in the properties. It is called periodic law.

Q.7: What is modern periodic law?
Answer:

According to Mosley (1913), if elements are arranged according to their ascending order of atomic numbers then chemical properties repeat in the periodic manner. It is called modern periodic law.

Q.8: Why the size of cation is smaller than its parent element?
Answer:

• When an electron is removed from the valence shell there is increase in the nuclear charge.
• The size of atom reduces. For example, atomic radius of Na 157 Pm ionic radian of `Na^+` is 95 Pm.

Q.9: `Na^+`, `Mg^{+2}`, `AI^{+3}` are isoelectronic in nature. Do these ions have same ionic radii?
Answer:

`Na^+`, `Mg^{+2}`, `AI^{+3}` all have 10 electrons in their valence shells but their ionic radii are not same. Ionic radii decrease due to increase in the nuclear charge from left to right. `Na^+` has 11 P, `Mg^{+2}` has 12 P and `AI^{+3}` has 13 protons in it.

Q.10: Why second ionization energy is greater than first ionization energy?
Answer:

The amount of energy required to remove electron from an atom in the gaseous state is called first ionization energy.
The amount of energy required to remove second electron from mono-positive ion is called second ionization energy. Second ionization energy is greater than first ionization energy due to increase in the nuclear charge. For example:

`Mg\rightarrow Mg^{+1}+1\bar{e}`                  (I.E = 738 kJ/mol)
`Mg^{+1}\rightarrow Mg^{+2}+1\bar{e}`              (I.E = 1451 kJ/mol)

Q.11: How the classification of elements in different blocks helps in understanding their chemistry?

Answer:

This classification is based upon the valence orbital of the element involved in chemical bonding. So this classification is quite useful in understanding the chemical properties of elements especially the concept of valency as oxidation state.

Q.12: Why ionization energy decreases down the group and increases along period?
Answer:

As we know that:
Ionization energy `\propto` Nuclear charge `\propto\frac{1}{"Atomic Size"}`
As we go down the group the atomic size increases which results as the decrease in nuclear due to which ionization energy decreases down the group.
While moving from left to right in a period, the nuclear charge increases and atomic size decreases which makes the removal of electron difficult and hence the value of ionization energy increases.

Q.13: Why the second value of electron affinity of an element is usually shown with a positive sign?
Answer:

When a second electron is added to a uni-negative ion, the incoming electron is repelled by the already present negative charge and energy is absorbed in this process so, it is usually shown with a positive sign.

Q.14: Why metallic character increases from top to bottom in a group of metals?
Answer:

Metallic character increases from top to bottom in groups due to increase in size of atom and ease to remove electrons.

Q.15: Why the oxidation state of noble gases is usually zero?
Answer:

Noble gases consist of He, Ne, Ar, Kr, Xe and Rn. Their atomic numbers are 2, 10, 18, 36, 54 and 86 respectively. As the oxidation state of an element usually depends on the number of electrons which it contributes in bond formation. Noble gases have completely filled outermost shell and they have zero oxidation state.

NOTE: 
The remaining short questions and answers of this chapter will be published soon Insha Allah.

Attempt MCQs | Quiz of FSc. Part-II, Chemistry Chapter No.1: Periodic Classification of Elements and Periodicity