PERIODIC CLASSIFICATION OF ELEMENTS AND PERIODICITY
FILL IN THE BLANKS
Q.01: Fill in the blanks:
(i) Mendeleev in his periodic table, arranged the elements according to their atomic ___. (mass)
(ii) Vertical columns in modern periodic table are called _________ and horizontal rows are called _________. (groups, periods)
(iii) Members of group VIlA are called __________ and alkali metals is the family name of __________ group members. (halogens, IA)
(iv) Metals form ________ oxides and non-metals form _______ oxides. (basic, acidic)
(v) Hydrogen can be placed above the groups _______ of the periodic table. (IA, IVA & VIIA)
(vi) Shielding effect is actually the ____________ due to electrons in between the nucleus and the outermost shell. (repulsion)
(vii) Noble gases have the ____________ values of ionization energy due to their complete outermost shells. (highest)
(viii) When a second electron is added to a uni-negative ion, the incoming electron is __________ by the already present negative charge. (repelled)
(ix) Due to having partly filled d-orbitals ____ metals usually show variable valency. (transition)
(x) Melting and boiling points of halogens _________ down the group. (increase)
Q.02: Indicate True or False:
(i) In Mendeleev’s periodic table, elements Be, Mg, Zn and Cd are placed in the same group. (TRUE)
(ii) The second and third periods contain eighteen elements each. (FALSE)
CORRECT: The second period contains eight elements, while third period contains eighteen elements.
(iii) Alkaline earth metals are present in Group IIA. (TRUE)
(iv) Metals are present in the top right corner of the periodic table. (FALSE)
CORRECT: Non-metals are present in the top right corner of the periodic table.
(v) Metalloids are present in the lower half of Groups IVA, VA and VIA. (TRUE)
(vi) Hydrogen forms uninegative ion like halogens. (TRUE)
(vii) Oxidation state of an element is related to the number of period it belongs. (FALSE)
CORRECT: Oxidation state of an element is related to the number of group it belongs.
(viii) Diamond is a good conductor of electricity. (FALSE)
CORRECT: Diamond is a bad conductor of electricity.
(ix) Melting points of halogens decrease down the group. (FALSE)
CORRECT: Melting points of halogens increase down the group.
(x) Zinc oxide is an example of amphoteric oxide. (TRUE)
Q.03: Multiple choice questions. Encircle the correct answer.
(i) Keeping in view the size of atoms, which order is the correct one?
EXPLANATION: Magnesium occurs in 2nd period while Barium lies in 5th period in the same IIA group. As atomic size increase down the group, therefore Ba is greater in size than Mg.
(ii) Mark the correct statement?
(a) Na+ is smaller than Na atom.
(b) Na+ is larger than Na atom.
(c) Cl– is smaller than Cl atom.
(d) Cl– (ion) and Cl (atom) are equal in size.
EXPLANATION: The size of positive ion or cation is always smaller than the parent atom because the nuclear charge pulls the lesser number of valence electrons in positive ion more close to the nucleus.
(iii) Mark the correct statement.
(a) All lanthanides are present in the same group.
(b) All halogens are present in the same period.
(c) All the alkali metals are present in the same group.
(d) All the noble gases are present in the same period.
EXPLANATION: All the alkali metals have one electron in their valence s-orbital. Due to similar electronic configuration and similar properties, they are present in same IA group.
(iv) Which statement is incorrect?
(a) All the metals are good conductor of electricity.
(b) All the metals are good conductor of heat.
(c) All the metals from positive ions.
(d) All the metals from acidic oxides.
EXPLANATION: All the metals form basic oxides because their oxides generate O2- ions in water which combine with H+ to form OH—. Therefore, their oxides are basic in nature.
(v) Which statement is correct?
(a) Hydrogen resembles in properties with IA, IVA and VIIA elements.
(b) Hydrogen resembles in properties with IIIA, IVA and VA elements.
(c) Hydrogen resembles in properties with IIA, IVA and VIA elements.
(d) Hydrogen resembles in properties with IIA, IIIA and VIIA elements.
EXPLANATION: Hydrogen resembles in properties with IA because both have one electron in valence s-orbital, both form positive ions and combine with halogens. Hydrogen resembles with IVA because both hydrogen and carbon have half-filled valence shells, both form covalent bonds etc. Hydrogen resembles with VIIA because both hydrogen and halogen require one electron to complete their valence shells, both can form negative ions, both form compounds with alkali metals.
(vi) Mark the correct statement.
(a) The ionization energy of the calcium is lower than that of barium.
(b) The ionization energy of calcium is lower than that of magnesium.
(c) The ionization energy of calcium is higher than that of beryllium.
(d) The ionization energy of calcium is lower than that of strontium.
EXPLANATION: The greater is the size of the atom, the lower will be its ionization energy. Ca has greater atomic size than Mg, therefore it has lower I.E. than Mg.
(vii) Mark the correct statement.
(a) Electron affinity is a measure of energy required to remove the electron.
(b) Electron affinity is a measure of energy released by adding an electron.
(c) Electron affinity is a measure of energy required to excite an electron.
(d) Electron affinity is a measure of energy released by removing an electron.
EXPLANATION: When an electron enters an atom, it is attracted by the nucleus. Due to this attraction, it releases some of its energy which is called electron affinity.
(viii) Mark the correct statement.
(a) Metallic character increases down the group.
(b) Metallic character increases from left to right along a period.
(c) Metallic character remains the same from left to right along a period.
(d) Metallic character remains the same down the group.
EXPLANATION: Due to increase in atomic size down the group, the attraction on valence electrons decreases and they can easily be removed to form positive ions. That’s why, the metallic character increases down the group.
(ix) Mark the correct statement.
(a) Melting points of halogens decrease down the group.
(b) Melting points of halogens increase down the group.
(c) Melting points of halogens remain the same throughout the group.
(d) Melting points of halogens first increase and then decrease down the group.
EXPLANATION: Halogens exist as diatomic non-polar molecules having weaker London dispersion forces. Down the group, their atomic size increases. The polarizability of their molecules and the strength of London dispersion forces also increase. As a result, the melting points of halogens increase down the group.
(x) Mark the correct statement.
(a) Covalent character of metal halides increases from left to right in a period.
(b) Boiling points of Group IVA hydrides decrease down the group.
(c) Ionic character of hydrides increases from left to right in a period.
(d) The basicity of group IIA oxides decreases on descending down the group.
EXPLANATION: As we move from left to right in periodic table, the electronegativity difference between the metal atom and halogen decreases. So, the covalent character of metal halides also increases.
Q.04: What are the improvements made in the Mendeleev’s periodic table?
Improvements In Mendeleev’s Periodic Table: Following improvements were made in Mendeleev’s periodic table after Moseley’s modern periodic law:
- After the discovery of atomic number by Moseley in 1911, the elements were arranged in ascending order of atomic numbers instead of their atomic masses. This removed many confusions, like misfit pairs of elements, present in Mendeleev’s periodic table.
- An extra group VIIIA was introduced at the extreme right of the periodic table. This group contains noble gases, which had not been discovered in Mendeleev’s time.
- In Mendeleev’s periodic table, the elements like Be, Mg, Ca, Sr, Ba and Zn, Cd, Hg were placed in the same vertical group, though they have different properties. In modern periodic table, this confusion has been removed by dividing the elements into two sub-groups, A and B. So, Be, Mg, Ca, Sr & Ba, being normal elements, are placed in group IIA, while Zn, Cd & Hg, being transition elements, are placed in group IIB.
Q.05: How the classification of elements in different blocks helps in understanding their chemistry?
The classification of elements in different blocks helps in understanding their chemistry because block-wise classification is based on valence orbitals, involved in chemical bonding. Therefore, it is useful in understanding the chemical behavior of elements and predicting their properties, especially the concept of valency or oxidation state.
Q.06: How do you justify the position of hydrogen at the top of various groups?
Due to similarities in properties, hydrogen can be placed at the top of several groups in periodic table. For example:
With Alkali Metals in Group IA: Hydrogen can be placed at the top of group IA because it has many similarities with alkali metals. For example:
- Both hydrogen and alkali metals have one valence electron in s-subshell.
- Both can form mono-positive ions.
- Both combine with halogens.
- Both can form ionic compounds, which dissociate in water.
With Group IVA:
- Valence shell of both hydrogen and IVA elements is half-filled.
- Both hydrogen and IVA elements combine with other elements through covalent bonding.
- Both hydrogen and carbon have reducing properties.
CuO + H2 ⟶ Cu + H2O
SnO2 + C ⟶ Sn + CO2
With Halogens in Group VIIA: Hydrogen can be placed at the top of group VIIA because it has many similarities with halogens. For example:
- Hydrogen is a gas like most of the halogens.
- Both hydrogen and halogens exist in the form of stable diatomic molecules. i.e., H2, Cl2, Br2.
- Both hydrogen and halogens need one electron to complete their valence shells.
- Both hydrogen and halogens can accept electron to form negative ions. i.e., H–, F–, Cl–, etc.
- Both form stable ionic compounds with alkali metals.
Q.07: Why the ionic radii of negative ions are larger than the size of their parent atoms?
The size of a negative ion is always bigger than the parent atom. For example, atomic radius of F atom is 72 pm while ionic radius of F– ion is 136 pm. This is because the addition of one or more extra electrons in a neutral atom enhances repulsion between the valence electrons, causing expansion in the size of the anion.
Q.08: Why ionization energy decreases down the group and increases along a period?
Ionization energy increases from left to right in a period because due to the increase in nuclear charge and decrease in atomic size along a period, the attraction of the nucleus on the valence electrons increases, so the ionization energy also increases.
Ionization energy decreases from top to bottom in a group because due to increase in atomic size and shielding effect down the group, the attraction of the nucleus on the valence electrons decreases, so the ionization energy also decreases.
Q.09: Why the second value of electron affinity of an element is usually shown with a positive sign?
First electron affinity of an atom such as oxygen is usually negative because when the first electron enters a neutral atom, it is attracted by the nucleus, so energy is released. While, the second electron affinity is always positive because when second electron is added to an already formed uninegative ion, it is repelled by the negative charge of the anion. So, energy must be required to cancel this repulsion. For example, first electron affinity of oxygen is –141 kJ mol-1, while its second electron affinity is +780 kJmol-1. This means energy has been used to cancel the repulsion of the O– ion for the second incoming electron.
Q.10: Why metallic character increases from top to bottom in a group of metals?
Metallic character increases from top to bottom in a group because due to the increase in number of shells, atomic size & shielding effect down the group, the attraction of the nucleus on the valence electrons decreases and they can be easily removed easily from the atom.
Q.11: Explain the variation in melting points along the short periods.
The melting and boiling points of elements, across short periods, first increase with the increase in number of valence electrons up to group IVA and then decrease up to the noble gases. For example:
- The melting and boiling points of group IA elements are lower than that of group IIA because each atom in group IA provides one electron for metallic bonding, while each atom in group IIA provides two binding electrons.
- In the middle of the period in group IVA, carbon has maximum number of binding electrons. Thus, it has very high melting point in diamond, in which each carbon atom is bonded to four other carbon atoms, to form a giant molecule.
- Towards the end of the period, the lighter elements of group VA, VIA & VIIA exist as small, covalent molecules, having weak intermolecular forces, so they have very low melting and boiling points.
Q.12: Why the oxidation state of noble gases is usually zero?
Noble gases or the elements of group VIIIA have no vacancy in the valence shell due to their complete octet. Hence, they have usually zero oxidation state.
Q.13: Why diamond is a non-conductor and graphite is fairly a good conductor?
Diamond is a non-conductor of electricity, because in diamond each of the four valence electrons of carbon is used in making four covalent bonds with other carbon atoms, and no free electrons are available to conduct electricity.
Graphite is a good conductor of electricity parallel to the layers because in graphite, three of the four valence electrons of carbon are used in making three covalent bonds with other carbon atoms. While, the fourth electron becomes involved in delocalized pi bonding, and remains relatively free to move between the hexagonal layers of graphite to conduct electricity.