10th Science Unit 8 Periodic Classification of Elements Book Back Answers

1. a) 6,16
2. b) 7,17
3. c) 8,18
4. d) 7,18

1. The horizontal rows are periods and the vertical columns are groups.

2. The modern periodic table has 7 periods and 18 groups.

1. a) atomic number
2. b) atomic mass
3. c) isotopic mass
4. d) number of neutrons

1. Moseley discovered atomic number, which gave a better basis for arranging elements.

2. Modern periodic law: the physical and chemical properties of elements are periodic functions of their atomic numbers.

1. a) $17^{th}$
2. b) $15^{th}$
3. c) $18^{th}$
4. d) $16^{th}$

1. In the family-wise grouping, group 17 elements are called halogens.

2. Group 15 is nitrogen family, 16 is oxygen family and 18 is noble gases.

1. a) atomic radii
2. b) ionic radii
3. c) electron affinity
4. d) electronegativity

1. Electronegativity is the tendency of an atom to attract the shared pair of electrons towards itself in a covalent bond.

2. It is measured by comparing atoms on the Pauling scale, so it is a relative property; the others have absolute measured values.

1. a) $\mathrm{FeO.xH_2O}$
2. b) $\mathrm{FeO_4.xH_2O}$
3. c) $\mathrm{Fe_2O_3.xH_2O}$
4. d) $\mathrm{FeO}$

1. Rusting is the formation of a brown layer when iron is exposed to moist air.

2. Rust is chemically hydrated ferric oxide, $\mathrm{Fe_2O_3.xH_2O}$.

1. a) oxidizing agent
2. b) reducing agent
3. c) hydrogenating agent
4. d) sulphurising agent

1. In the aluminothermic process: $\mathrm{Fe_2O_3 + 2Al \rightarrow 2Fe + Al_2O_3 + Heat}$.

2. Aluminium removes oxygen from ferric oxide, so it acts as a powerful reducing agent.

1. a) painting
2. b) thinning
3. c) galvanization
4. d) electroplating

1. Coating zinc on iron sheets by using electric current is galvanization.

2. It is a surface coating method used to prevent corrosion.

1. a) He
2. b) Ne
3. c) Ar
4. d) Kr

1. Helium has only 2 electrons, both in its single outermost shell.

2. Ne, Ar and Kr have 8 electrons in their outermost shell.

1. a) stable arrangement of neutrons
2. b) stable configuration of electrons
3. c) reduced size
4. d) increased density

1. In noble gases like neon the outer s and p orbitals are completely filled.

2. This stable electron configuration means no more electrons can be added, so electron affinity is zero.

1. a) Ag
2. b) Hg
3. c) Mg
4. d) Al

1. An amalgam is an alloy of mercury (Hg) with another metal.

2. Example: silver–tin amalgam used for dental filling.

1. Pauling's rule: difference greater than 1.7 means the shared electrons are pulled almost completely to one atom.

2. So the bond is considered to be more ionic in nature.

1. The sixth period covers atomic numbers 55 to 86 (Caesium to Radon) and holds 32 elements — 8 representative, 10 transition and 14 inner transition (lanthanides).

1. Moseley's discovery of atomic number led to the modern periodic law: properties are periodic functions of atomic number.

2. Hence elements are arranged in the increasing order of atomic number.

1. Covalent radius = half the distance between the nuclei of two covalently bonded atoms of the same element.

2. $r = \dfrac{1.98}{2} = 0.99$ Å.

1. A cation is formed by loss of an electron, so the remaining electrons are held more strongly by the nucleus.

2. Hence a cation is smaller than the neutral atom, and the anion is the largest.

1. Henry Moseley (1912) discovered atomic number and modified the periodic law based on it.

1. Along a period the shell number is the same but nuclear charge increases, pulling the electron cloud inward.

2. So, like atomic radius, ionic radius decreases from left to right.

1. The two series placed separately below the table form part of Group 3.

2. They are the lanthanides and actinides.

1. Aluminium is extracted profitably from bauxite ($\mathrm{Al_2O_3 \cdot 2H_2O}$) by Baeyer's process followed by Hall's process.

1. Rust is the reddish brown scaly layer formed on iron in moist air.

2. It is chemically hydrated ferric oxide, $\mathrm{Fe_2O_3 \cdot xH_2O}$.

1. Galvanisation is the process of coating zinc on iron sheets using electric current → Coating with Zn.

2. Calcination is the strong heating of the concentrated ore in the absence (or limited supply) of air → Heating in the absence of air.

3. In the alumino thermic process $Fe_2O_3 + 2Al \rightarrow 2Fe + Al_2O_3 + Heat$, aluminium is oxidised and ferric oxide is reduced → Redox reaction.

4. Silver-tin amalgam, an alloy of mercury, is used for dental filling → Dental filling.

5. Group 18 elements have stable, completely filled valence shells and are unreactive → Noble gas elements.

1. Moseley discovered the atomic number and showed that it is the fundamental property of an element.

2. The modern periodic law states that properties of elements are periodic functions of their atomic numbers, not atomic masses.

1. Along a period the shell number stays the same but the nuclear charge (number of protons) increases.

2. The stronger pull shrinks the electron cloud, so atomic and ionic radii decrease from left to right.

1. A mineral is any compound or mixture of compounds of a metal found in the Earth.

2. An ore is a mineral from which the metal can be extracted readily and economically, e.g. bauxite is an ore of aluminium while clay is only a mineral.

1. Aluminium is used in the electrical cable industry because it is a good conductor of electricity, and it is also ductile and light (low density 2.7).

2. Silvery white colour is only an appearance; it has nothing to do with conduction of current.

1. An alloy is a homogeneous mixture of two or more metals, or of one or more metals with certain non-metallic elements.

2. Alloys behave like solid solutions, e.g. brass is a solid solution of zinc (solute) in copper (solvent).

1. a) A and R are correct, R explains the A.
2. b) A is correct, R is wrong.
3. c) A is wrong, R is correct.
4. d) A and R are correct, R doesn't explains A.

1. Electronegativity values given in the text: H = 2.1 and F = 4.0.

2. Difference $= 4.0 - 2.1 = 1.9$, so the Reason is correct.

3. Rule: if the electronegativity difference is greater than 1.7, the bond is more ionic. Since 1.9 > 1.7, the H–F bond is ionic, so the Assertion is correct.

4. The large electronegativity difference is exactly the cause of the ionic nature, so the Reason explains the Assertion.

1. a) A and R are correct, R explains the A.
2. b) A is correct, R is wrong.
3. c) A is wrong, R is correct.
4. d) A and R are correct, R doesn't explains A.

1. Assertion is correct: magnesium is coated/attached to steel as a sacrificial metal in cathodic protection to prevent rusting.

2. Reason is correct: magnesium is more reactive than iron.

3. Being more reactive, magnesium corrodes first (acts as anode) and saves the iron, which acts as cathode.

4. So the Reason correctly explains the Assertion.

1. a) A and R are correct, R explains the A.
2. b) A is correct, R is wrong.
3. c) A is wrong, R is correct.
4. d) A and R are correct, R doesn't explains A.

1. Assertion is correct: in the presence of $CO_2$ and moisture, copper gets covered with a green layer of basic copper carbonate — $2Cu + O_2 + CO_2 + H_2O \rightarrow CuCO_3.Cu(OH)_2$.

2. Reason is also correct: copper is not attacked by alkalis.

3. But the green layer is formed by the action of air, moisture and carbon dioxide, not by any reaction with alkali.

4. Hence both statements are true, but the Reason does not explain the Assertion.

1. A is a reddish brown metal with high lustre, high density and high melting point (1356 °C) — so A is copper (Cu).

2. On heating with oxygen below 1370 K, copper forms black copper(II) oxide, CuO, which is B: $2Cu + O_2 \xrightarrow{below\ 1370K} 2CuO$ (black).

3. On heating with oxygen above 1370 K, copper forms red copper(I) oxide, $Cu_2O$, which is C: $4Cu + O_2 \xrightarrow{above\ 1370K} 2Cu_2O$ (red).

4. Thus A = Cu, B = CuO (copper II oxide, black), C = $Cu_2O$ (copper I oxide, red).

1. A is a silvery white metal whose alloys (duralumin, magnalium) are used in making aircraft — so A is aluminium (Al).

2. Aluminium is not affected by dry air, but on heating at 800 °C it burns brightly forming its oxide.

3. $4Al + 3O_2 \rightarrow 2Al_2O_3$ (aluminium oxide) — so B is $Al_2O_3$.

4. (At the same temperature it also forms aluminium nitride: $2Al + N_2 \rightarrow 2AlN$.)

1. When iron is exposed to moist air, a scaling reddish brown layer is formed on its surface; this is rust.

2. Rust is chemically hydrated ferric oxide, $Fe_2O_3 \cdot xH_2O$, and the phenomenon of its formation is called rusting.

3. Equation: $4Fe + 3O_2 + xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O$ (rust).

4. Rusting is a type of corrosion, in which iron loses its metallic character.

1. Presence of air (oxygen): oxygen is needed to oxidise the iron surface.

2. Presence of moisture (water): rusting is a wet or electrochemical corrosion that occurs only in moist air.

3. When both are present together, iron forms a brown layer of hydrated ferric oxide (rust) on its surface.

(a)

(b)

1. Element in period 3, group 13 has atomic number 13 — so A is aluminium (Al).

2. Red hot aluminium reacts with steam to give aluminium oxide and hydrogen, so B is aluminium oxide ($Al_2O_3$): $2Al + 3H_2O \rightarrow Al_2O_3 + 3H_2 \uparrow$

3. Aluminium reacts with a strong alkali (caustic soda) to form sodium meta aluminate, so C is sodium meta aluminate ($NaAlO_2$): $2Al + 2NaOH + 2H_2O \rightarrow 2NaAlO_2 + 3H_2 \uparrow$

4. Hydrogen gas is liberated in both reactions.

1. Nitric acid ($HNO_3$), both dilute and concentrated, renders aluminium passive.

2. Nitric acid does not attack aluminium; instead it oxidises the surface of the metal.

3. A thin, continuous and non-porous oxide film ($Al_2O_3$) is formed on the surface.

4. This film prevents further contact of the acid with the metal below, so the metal becomes chemically inactive (passive).

(a)

1. Electronegativity values: $F = 4.0$ and $H = 2.1$.

2. Difference in electronegativity $= 4.0 - 2.1 = 1.9$.

3. Since the difference is greater than 1.7, the bond is considered to be more ionic in character.

4. Fluorine, being highly electronegative, pulls the shared pair of electrons strongly towards itself.

(b)

1. The property used is electronegativity.

2. Electronegativity is the measure of the tendency of an atom to attract the shared pair of electrons towards itself in a covalent bond.

3. The Pauling scale of electronegativity is used to predict the nature of the bond: difference = 1.7 means 50% ionic and 50% covalent; less than 1.7 means more covalent; greater than 1.7 means more ionic.

(c)

1. Along a period (left to right): electronegativity increases, because the nuclear charge increases and the atom attracts the shared electrons more strongly.

2. Down a group (top to bottom): electronegativity decreases, because the number of valence shells increases and the shared electrons are farther from the nucleus.

3. Hence fluorine, at the top right of the table, is the most electronegative element (4.0).