2nd Year NotesChemistry class 12th

2nd Year CHEMISTRY CHAPTER 1 (Periodic Classification of Elements)

2nd Year CHEMISTRY CHAPTER 1 (Periodic Classification of Elements) Question And Answer

Short And Simple Question And Answer

Q1: What are hydrides, and what is the trend in boiling points of group VI hydrides down the group?

Ans: Hydrides are binary compounds of hydrogen with other elements, such as NaH, H2O, and H2S. The trend in boiling points of group VI hydrides is an increase down the group, except for H2O, which, due to hydrogen bonding, has a higher boiling point than might be expected.

Group VIA (Hydrides)                     Boiling point (°C)
H2O                                                   100                                           
H2S                                                   -60.3                                           
H2Se                                                 -42                               
H2Te                                                 -2

Q2: Write name and symbol of an element from s block that has zero oxidation state. Also write its electronic configuration?

Ans: The element with zero oxidation state in the s block is Helium (He) with the electronic configuration 1s2.

Q3. Why melting and boiling points of elements belonging to groups VA to VIIA are lower? Discuss the trend of ionization energy in the periodic table?

Ans: As we move from left to right in a period of the periodic table, the number of electrons in the outermost shell increases, leading to an increase in the binding forces. This results in higher melting and boiling points up to group IVA. However, after group IVA, the pairing of electrons begins, leading to weaker binding forces and lower melting and boiling points for elements in groups VA to VIIA.

In terms of the trend in ionization energy:

Ionization energy increases from left to right in a period due to the increasing effective nuclear charge (proton number) and constant shielding effect.

Ionization energy decreases from top to bottom in a group due to the decreasing effective nuclear charge and increased shielding effect as the number of shells and atomic size increase down the group.

Q4. Two defects of Mendeleev’s periodic table?

Ans:Mendeleev did not assign a specific position for hydrogen in his table.

His table did not provide insights into the structure of the atom.

Q5. Definitions of Mendeleev’s and modern periodic laws?

Ans: Mendeleev’s Periodic Law: “If the elements are arranged in ascending order of their atomic masses, their chemical properties repeat in a periodic manner.”

Modern Periodic Law: “If the elements are arranged in ascending order of their atomic numbers, their chemical properties repeat in a periodic manner.”

Q6. Give four improvements made in Mendeleev’s periodic law?

Ans: Correct arrangement of some elements:

Argon (Ar) was placed before Potassium (K).

Cobalt (Co) was placed before Nickel (Ni).

Tellurium (Te) was placed before Iodine (I).

Position of rare earths:

The position of the rare earth elements of lanthanides and actinides was adjusted.

Position of isotopes:

The positions of isotopes were corrected.

Position of noble gases:

Noble gases were arranged in a separate group, Group VIIIA.

Q7. Why diamond is a non-conductor and graphite fairly a good conductor of electricity?

Ans: Diamond is a non-conductor:

In diamond, each carbon atom forms four strong sigma bonds with other carbon atoms, creating a tightly bonded crystal lattice. These bonds hold all the electrons tightly, leaving no free electrons for conducting electricity. Therefore, diamond is a non-conductor

Graphite is a good conductor:

In graphite, each carbon atom is bonded to three neighboring carbon atoms in a hexagonal planar structure. This leaves one electron relatively free to move within the layers of graphite. Hence, graphite is a good conductor of electricity within the layers but does not conduct perpendicular to the layers.

Q8. Define electron affinity. Discuss its trend in the periodic table?

Ans: Electron affinity is the energy change that occurs when a neutral atom gains an electron to form a negatively charged ion. It measures the atom’s ability to attract and hold an additional electron.

Trend in the periodic table:

Electron affinity generally increases from left to right across a period and decreases from top to bottom within a group. This trend is due to the increase in effective nuclear charge from left to right, which leads to a stronger attraction for electrons. Elements in the upper right of the periodic table, such as the halogens, have the highest electron affinities, while noble gases have very low electron affinities.

Q9: How do you justify the position of hydrogen at the top of alkali metals (group IA)?

Ans: Hydrogen can be placed at the top of the group IA because the properties of hydrogen resemble those of the elements in group IA in several ways:

• Hydrogen, like alkali metals, has one electron in its outermost shell (1s subshell).

• Both hydrogen and alkali metals readily combine with electronegative elements, such as halogens.

• Both exhibit an oxidation state of +1.

• Both form ionic compounds, where they lose or gain one electron.

Q10: How does hydrogen resemble alkali metals?

Ans: Hydrogen shares similarities with alkali metals, justifying its position at the top of group IA. Some resemblances include:

• Both hydrogen and alkali metals have one electron in the 1s subshell.

• They share a strong tendency to combine with electronegative elements like halogens.

• Both exhibit an oxidation state of +1.

• They both form ionic compounds when reacting with other elements.

Q11: Why can hydrogen be placed over group IVA of the periodic table?

Hydrogen can be situated at the top of group IVA because its properties are akin to those of group IVA elements. These resemblances include:

• Hydrogen, like group IVA elements, has a valence shell that is half-filled.

• Both hydrogen and group IVA elements form covalent bonds when combining with other elements.

• Similar to carbon, hydrogen exhibits notable reducing properties.

CuO + H2   →    Cu + H2O

SnO2 + C        Sn + CO2

Q12: Why are the oxidation states of noble gases usually zero?

The oxidation state of an element is tied to its valence electrons or vacancies in its valence shell. Noble gases have completely filled outermost shells with no vacancies, resulting in a usual oxidation state of zero, earning them the name “zero group elements.”

Q13: Why does metallic character increase from top to bottom in a group of metals?

Metallic character rises from top to bottom in a group because atomic size increases in that direction. This enlargement makes it easier to remove electrons from the outermost shell, reduces the effective nuclear charge, and enhances metallic character.

Q14: Why do metals conduct electricity?

Metals conduct electricity because each metal atom in a crystal loses all its valence electrons, resulting in positively charged atoms. When an electric field is applied, mobile electrons move toward the positive pole, creating space for incoming electrons from the negative pole. This movement of free electrons allows metals to conduct electricity.

Q15: Why do alkali metals form ionic hydrides?

Alkali metals are more electropositive than hydrogen, and they readily lose electrons to form unipositive ions. Hydrogen accepts the electron to create hydride ions (H-). These cations and anions then combine through ionic bonding, resulting in alkali metals forming ionic hydrides (e.g., Na+ H- and K+H-).

Q16: How was the confusion regarding Zn, Cd, Hg resolved in the Modern periodic table?

In the Modern periodic table, the groups were subdivided to segregate elements of different categories. Zn, Cd, and Hg were placed in subgroup IIB, distinct from the alkaline earth metals, addressing the inconsistency that had arisen from their placement with the alkaline earth metals in Mendeleev’s periodic table.

Q17: What is the Lanthanide contraction?

The gradual reduction in the atomic size of elements in the lanthanide series, as well as the actinide series, is known as the lanthanide contraction. This contraction is attributed to the limited shielding effect of the f sub-shell, which is progressively filled as you move across these series.

Q18: Name the various classes of hydrides?

Ans: There are three classes of hydrides:

  1. Ionic hydrides
  2. Polymeric or intermediate hydrides
  3. Covalent hydrides

Q19: Why do oxidation states usually remain the same in a group?

Ans: Oxidation states tend to remain the same in a group because the number of electrons in the outermost shell remains constant within a group. While the number of electrons in the outermost shell changes across a period from left to right, resulting in varying oxidation states, it remains consistent within a group. Nonetheless, the process of unpairing electrons can occur within a group, leading to changes in oxidation states.

Q20: Give two resemblances of hydrogen with IVA elements?

Ans: Hydrogen shares similarities with group IVA elements, which justify its placement at the top of group IVA:

  1. Hydrogen and group IVA elements have half-filled valence shells.

2. Both hydrogen and group IVA elements form chemical bonds through covalent bonding.

Q21: What are amphoteric oxides? Provide an example?

Ans: Amphoteric oxides are oxides that exhibit both acidic and basic properties. An example of an amphoteric oxide is beryllium oxide (BeO).

Q22:Explain the order of hydration energies for ions Al+3, Mg+2, and Na+1?

Ans: The order of hydration energies for these ions (Al+3 > Mg+2 > Na+1) is determined by the charge-to-size ratio. A higher charge-to-size ratio leads to greater hydration energy. In this specific order, Al+3 has a larger charge-to-size ratio than Mg+2 and Na+1, resulting in the following hydration energy order: Al+3 > Mg+2 > Na+1.

Q23: Why do ionization energies decrease down the group and increase from left to right in the periodic table?

Ans: In a period, ionization energy increases from left to right due to several factors:

  • An increase in proton number leads to a greater effective nuclear charge.
  • Shielding effect remains constant.
  • The number of electron shells remains the same.
  • Atomic size decreases.

Conversely, in a group, ionization energy decreases from top to bottom because of the following reasons:

A decrease in proton number results in a smaller effective nuclear charge.

Shielding effect increases.

The number of electron shells increases down the group.

Q24: Why is PbCl2 ionic, but PbCl4 is fairly covalent?

The ionic or covalent nature of lead halides depends on the oxidation state of lead. When a metal forms multiple halides, the halides with the lower oxidation state of the metal tend to be ionic, while those with a higher oxidation state are more likely to be covalent. In the case of lead, the high polarizing power of Pb+4 compared to Pb+2 makes PbCl2 predominantly ionic, while PbCl4 is fairly covalent.

Q25: What happens when acidic and basic oxides combine with each other?

Ans: When basic oxides and acidic oxides combine, they react to form salts. For example, when Na2O (a basic oxide) and SO3 (an acidic oxide) combine, they form Na2SO4, which is a salt.

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