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12th Class Punjab
Chemistry
Chemistry Short Question
Periodic Classification Of Elements And Periodicity Short Questions Preparation

Chapter 1 Periodic Classification Of Elements And Periodicity Chemistry 12th Class Punjab Short Questions

Question No. 1 - What is Newlands law of octaves?

Answer No. 1 - Newland who was English chemist in 1864, classified 62 elements, known at that time,in increasing order of their atomic masses. He noted that every eight element had some properties in common with the first one. The principle on which this classification is based was called the law of Octaves.

Question No. 2 - Lonic radii of negative ion is always bigger in size than its parent atom. Why?

Answer No. 2 - The reason is that addition of one or more electrons in the shell of a neutral atom enhances repulsion between the electrons causing expansion of the shell. For example the radius of fluorine atom is 72 pm and that of the fluoride ion F ion is 136 pm.

Question No. 3 - What is lanthanide contraction?

Answer No. 3 - Lanthanide contraction: The atomic number increases from left to right, the atomic radius decreases. This gradual decrease in the radius is due to increase in positive charge in the nucleus. As the positive charge increases, the negatively the outer most shell becomes gradually smaller. This effect is quite remarkable in the elements of longer periods in which d and f subshells are involved. For example the gradual reduction in the size of Lanthanides is significant and called Lanthanide Contraction.

Question No. 4 - Solution of Na2O in water is alkaline. Justify the statement.

Answer No. 4 - Alkali metal oxides dissolve in water to give alkaline solutions. For Example:
Na2O+H2O------------>2NaOH

Question No. 5 - d and f block elements are called transition elements. Give reasons.

Answer No. 5 - d and f block elements are located in between s and p block elements in the periodic table. Their properties are also in between s and p block elements and they vary from left to the right. It means that their properties show a transition in a systematic way. In d block elements d-d transition occur and elements exhibit colors. Similarly in f block elements d and f orbital show transition of elements hence these are called inner transition elements. That is why they are called as transition elements.

Question No. 6 - Give reason that Hydration Energy O Al3+ ions is more than Mg2+ ions.

Answer No. 6 - Hydration energy is highly depends upon charge to size ratio of the ions. Charge to size ratio increases from left to right in a period, the hydration energy also increase in the same fashion.

Question No. 7 - What improvements were made in Mendeleevs periodic table.

Answer No. 7 - Mendeleevs periodic law:- Mendeleevs arranged the elements in ascending order of their atomic masses and found that elements with similar properties appeared at regular intervals.

Question No. 8 - How the classification of elements in different blocks help in understanding their chemistry?

Answer No. 8 - Classification of elements in different blocks help in understanding chemistry of elements and predicting their properties especially the concept of valency or oxidation state.

Question No. 9 - M.P and B.P of short periods increase up to middle of period and then decreases. Why?

Answer No. 9 - Melting and boiling points of short periods: Across the short periods, the melting and boiling points of elements increase with the number of valance electrons up to group IVA and then decrease up to the noble gases. The melting points of groups IA element are low because each atom in them provides only one electron to for a bond with other atom. Melting point of group IIA elements are considerable higher than those of group IA elements because each atom in them provides two binding electrons.

Question No. 10 - Why the second value of electron affinity of an element is usually shown with a positive sign?

Answer No. 10 - Energy usually released when electronegative elements absord the first electron and E.A in such case is expressed in negative figure,as in the case of halogens.
When a second electron added to a uni-negative ion, the incoming electron is repelled by the already present negative charge and energy is absorbed in the process.

Question No. 11 - Describe 7th period of modern periodic table.

Answer No. 11 - This period is incomplete. This contains only two normal elements Fr and Ra, Ten transition elements and fourteen inner transition elements. The inner transition elements of this period are called Actinides, as they starts after Ac Actinium. The actinides are also shown at the bottom of the periodic table under the Lanthanides. Due to their scarcity, the inner transition elements are also called Rare Earth Elements.

Question No. 12 - Why the ionization energies decreases down the groups?

Answer No. 12 - Going down in the group, the nuclear charge increases but as the size of the atom and the number of electrons causing the shielding effect also increases therefore ionization energy decreases from top to bottom.

Question No. 13 - Melting points of Group VIIA elements increases down the group. Why?

Answer No. 13 - For elements of group VIIA, which exist in the form of molecules, the melting and boiling points increase down the group. This is because large molecules exert stronger force of attraction due to their higher polarizabilities.

Question No. 14 - Discuss the trend of ionization energy in periodic table.

Answer No. 14 - Trend of ionization energy in periodic table: In groups ionization energy decrease from top to bottom because size of atom and number of electrons causing the shielding effect also increases.

Question No. 15 - Why 1st electron affinity is negative and 2nd is positive?

Answer No. 15 - Energy is usually released when electronegative elements adsorb the first electron and E.A in such case is expressed in negative figure, as in the case of halogens, when a second electron is added to a uni negative ion, the incoming electrons is repelled by the already present negative charge and energy in this process is represented by +ve sign.

Question No. 16 - How this confusion is removed in the Modem Periodic Table?

Answer No. 16 - Removal of confusion: Zn, Cd, Hg were placed in a single vertical group, while according to their properties they belonged to two different categories. The same was true for so many other elements placed in the same vertical group in two types of vertical groups, A and B. In modern periodic table, Be, Mg, Ca, Sr and Ba are placed in group IIA and Zn, Cd, Hg in group IIB.

Question No. 17 - Why metallic character increases from top to bottom in a group of metals?

Answer No. 17 - Metallic character increases from top to bottom in a group because from top to bottom atomic size of atoms increases and it is easier to remove the electron of an atom bigger in size.

Question No. 18 - Define modern periodic table.

Answer No. 18 - Modern periodic table:The table of elements in which all the elements are arranged in ascending order of their atomic numbers having seven periods and eighteen groups is called modern periodic table.

Question No. 19 - How do member of group V!-A except oxygen show+4 and +6 oxidation state?

Answer No. 19 - Elements of group VI-Aexcept oxygen show covalency of +2, +4 and +6, +2 oxidation shown due to 2 unpaired electrons in the p orbitals. +4 oxidation state is shown when 1 electron from p-orbital is promoted to the next vacant d-orbital, while +6 oxidation state is shown when another electron from s-orbital is also promoted to the next vacant d-orbital.

Question No. 20 - Why the metals are good conductors?

Answer No. 20 - Metal are good conductors due to the presence of relatively loose electrons in the outermost shell of the element and ease of their movement in the solid lattice.

Question No. 21 - Give two defect of Mendeleevs periodic table.

Answer No. 21 - Two defect of Mendeleevs periodic table:
Position of Isotopes.
Electronic arrangement.

Question No. 22 - Ionization energy of Al3+ is greater than Mg2+. Give the reason.

Answer No. 22 - By moving from left to right in a period, the outer shell remains the same, while the nuclear charge increases effectively that makes the removal of an electron difficult and hence the value of ionization energy increases. Similarly the size of Al is smaller than Mg so Al3+ it has greater ionization energy than Mg2+.

Question No. 23 - The oxidation state vary in a period, but remain almost constant in a group. Give reason.

Answer No. 23 - The number of electrons in outermost shells goes on changing in periods from left to the right. So oxidation states go on changing. The number of electrons in the outermost shells remains the same in a group, so the oxidation states remains the same. Anyhow, the process of unpairing of electrons may happen in a group and oxidation states may change.

Question No. 24 - Define Metalloid.

Answer No. 24 - Metalloid: Some elements,especially lower members of groups, IIA, IVA and VA have properties of both metals as well as non-metals. These elements are called metalloids.

Question No. 25 - Why the oxidation state of noble gases is usually zero?

Answer No. 25 - Group VIII A elements, which are also called zero group elements usually show zero oxidation state because there is no vacancy in their outer most shell to accommodate more electrons.

Question No. 26 - How melting and boiling points varied in a period?

Answer No. 26 - Across the short periods, the melting and boiling points of elements increase with the number of valance electrons up to group IVA and then decrease up to the noble gases. The melting points of group IA element are low because each atom in them provides only one electron to for a bond with other atom. Melting points of group IIA elements are considerable higher than those of group IA elements because each atom in them provides two binding electrons. An important change occurs when we move from group IVA to group VA, VIA, VIIA as the lighter elements of these group exist as small covalent molecules. Rather than as three dimensional lattices.

Question No. 27 - Electrical conductance of metals in group IA generally increases from top to bottom. Explain

Answer No. 27 - The property in mainly due to the presence of relatively loose electrons in the outermost shell of the elements and ease of their movement in the solid lattice. In group IA, generally electrical conductance increases from top to bottom because from top to bottom loosely held electrons are more easily available.

Question No. 28 - Hydration energy depends on charge density of ion. Justify the statement.

Answer No. 28 - Hydration energy depends on charge to size ratio of the ion.For example of group IA, charge to size ratio decreases from top to bottom in a group, the hydration energy also decreases in the same fashion.

Question No. 29 - Why melting and boiling points of elements belonging to groups VA-VIIA are lower?

Answer No. 29 - Melting and boiling points of elements belonging to groups VA-VIIA are lower because elements of these groups exist as small, covalent molecules, rather than as three dimensional lattice having very weak inter molecular forces between them. Consequently, their melting and boiling points are extremely low.

Question No. 30 - The ionic radius of positive ion is smaller than its parent atom. Give reason.

Answer No. 30 - The removal of electrons causes an imbalance in proton-electron ratio. Due to greater attraction of the nuclear charge, the remaining electrons of the ion are drawn closer to the nucleus. Thus, a positive ion is always smaller than the neutral atom from which it is derived. The radius of Na is 157 pm and the radius of Na is 95 pm.

Question No. 31 - Write a note on blocks in periodic table.

Answer No. 31 - Blocks in periodic table:Elements in the periodic table can be classified into four blocks, this classification is based upon the valence orbital of the elements involved in chemical bonding. According to this classification, Elements of IA and IIA subgroup are called s-block elements. The elements for IIA to VIIIA subgroup are known as p-block elements as their valence electrons are present in p orbital. Similarly in transition elements, electrons are in d orbital are responsible for their valency hence they are called d-block elements. For Lanthanides and Actinides valence electrons are present in f orbital hence these elements are f-block elements.

Question No. 32 - What is Dobereiners Law of triads?

Answer No. 32 - Dobereiner a German chemist in 1829, arranged elements in groups called Triads, as each contained three elements with similar properties.

Question No. 33 - Why do metals conduct electricity?

Answer No. 33 - Metals conduct electricity: Metals conduct electricity because metals have loose free electrons in their outer most shell and ease of their movement in the solid lattice.

Question No. 34 - Define electron affinity. Give its trend in periodic table.

Answer No. 34 - Electron affinity: Energy released or absorbed, when an electron is added to a gaseous atom to form a negative ion is called electron affinity.

Question No. 35 - Why the ionic character of halids decrease from left to the right in periods?

Answer No. 35 - Within a period, isoelectronic positive ions show a decrease in ionic radius from left to right, because of the increasing nuclear charge. The same trend is observed for the isoelectronic negative ions of a period:ionic size decreases from left to right.

Question No. 36 - Ionization energy increases from left to right in a period. Justify the statement.

Answer No. 36 - In period by moving from left to right the outer shell remains the same while the nuclear charge increase effectively that makes the removal of an electron difficult and hence the value of ionization energy increases.

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