Atomic Structure and Bonding Class 9 ICSE Important Questions with Answers

Answer: (b) $18$18.

By the Bohr-Bury rule the maximum number of electrons in a shell is $2n^2$2n². For the $M$M shell, $n=3$n=3, so $2(3)^2 = 18$2(3)² = 18.

Answer: (b) The same atomic number but different mass number.

Isotopes have the same number of protons (same $Z$Z) but a different number of neutrons, so their mass numbers differ.

Answer: (b) A single covalent bond.

Each chlorine atom needs one electron to complete its octet, so the two atoms share one pair of electrons, forming a single covalent bond, $Cl-Cl$Cl-Cl.

Answer: (b) $MX_2$MX₂.

$X$X (config $2,8,7$2,8,7) gains one electron to form $X^{-}$X^-; $M$M (config $2,8,2$2,8,2) loses two electrons to form $M^{2+}$M^2+. To balance charges, one $M^{2+}$M^2+ combines with two $X^{-}$X^-, giving $MX_2$MX₂.

Answer: (c) J. J. Thomson, from his study of cathode rays: streams of negatively charged particles, all identical whatever the gas or metal used.

Answer: (a) Conduction needs free-moving ions. In the solid the ions are locked in the lattice, but on melting or dissolving they become mobile and carry current; so R correctly explains A.

(i) Protons = atomic number = $17$17.

(ii) Neutrons = mass number $-$- atomic number $= 35 - 17 = 18$= 35 - 17 = 18.

(iii) Electrons = protons = $17$17 (neutral atom).

Electronic configuration: $2,8,7$2,8,7 (this element is chlorine).

Hydrogen: protium $^1_1H$¹₁H ($0$0 neutrons), deuterium $^2_1H$²₁H ($1$1 neutron) and tritium $^3_1H$³₁H ($2$2 neutrons). All three have $1$1 proton.

Carbon: carbon-12 ($^{12}_{6}C$¹²₆C, $6$6 neutrons) and carbon-14 ($^{14}_{6}C$¹⁴₆C, $8$8 neutrons).

A free radical is an atom or group of atoms with an unpaired electron. It is formed when a covalent bond breaks and each atom keeps one electron of the shared pair. Free radicals are very reactive.

In sunlight (UV light), the shared pair in $Cl-Cl$Cl-Cl splits equally: $Cl_2\xrightarrow{\text{UV light}}Cl^\bullet+Cl^\bullet$Cl₂→ (UV light)Cl^•+Cl^•. Each chlorine atom has one unpaired electron, so each is a chlorine radical.

Magnesium (config $2,8,2$2,8,2) loses its two outer electrons to form $Mg^{2+}$Mg^2+; each chlorine (config $2,8,7$2,8,7) gains one electron to form $Cl^{-}$Cl^-. Two chlorine atoms are needed for one magnesium atom.

Magnesium transfers $2$2 electrons (valency $2$2); each chlorine accepts $1$1 electron (valency $1$1). The bond is electrovalent (ionic).

Oxygen (config $2,6$2,6) needs $2$2 more electrons to complete its octet, and each hydrogen (config $1$1) needs $1$1 electron to attain the helium duplet. Oxygen therefore shares one electron with each of two hydrogen atoms.

Two shared pairs (the two $O-H$O-H single bonds) and two lone pairs remain on the oxygen atom. Water is a covalent molecule.

Bohr-Bury rules:

1. The maximum number of electrons in a shell is $2n^2$2n², where $n$n is the shell number ($K=2$K=2, $L=8$L=8, $M=18$M=18).

2. The outermost shell can hold a maximum of $8$8 electrons (and the next-to-outermost not more than $18$18).

3. Electrons fill the shells one after another from the innermost outward; a new shell begins only after the previous outer shell has $8$8 electrons.

For $19$19 electrons: $K=2$K=2, $L=8$L=8, then $M$M can begin filling but the outermost cannot exceed $8$8 before the next shell starts, so $M=8$M=8 and the remaining $1$1 goes to $N$N. Configuration: $2,8,8,1$2,8,8,1 (potassium).

1. Most alpha particles passed straight through the foil. Conclusion: most of the space inside an atom is empty.

2. A few were deflected through small angles. Conclusion: the positive charge of the atom is concentrated in a very small region, which repels the positive alpha particles that pass close to it.

3. Very few (about $1$1 in $20\,000$20 000) bounced back. Conclusion: almost all the mass and all the positive charge are packed in a tiny, dense centre, the nucleus.

(a)

(i) An electrovalent bond is formed by the complete transfer of one or more electrons from a metal atom to a non-metal atom, forming oppositely charged ions. A covalent bond is formed by the mutual sharing of one or more pairs of electrons between two non-metal atoms.

(ii) Electrovalent compounds have high melting/boiling points, are usually soluble in water and conduct electricity when molten or in solution. Covalent compounds have low melting/boiling points, are usually insoluble in water (soluble in organic solvents) and are generally non-conductors.

(b)

(i) Sodium chloride: sodium ($2,8,1$2,8,1) transfers its one outer electron to chlorine ($2,8,7$2,8,7); $Na^{+}$Na^+ and $Cl^{-}$Cl^- ions form and are held by electrostatic attraction, $Na^{+}Cl^{-}$Na^+Cl^-.

(ii) Methane: carbon ($2,4$2,4) shares each of its four outer electrons with one electron from each of four hydrogen atoms, forming four $C-H$C-H single covalent bonds so that carbon attains an octet and each hydrogen a duplet.

(a) Chlorine exists as a mixture of two isotopes, $^{35}_{17}Cl$³⁵₁₇Cl and $^{37}_{17}Cl$³⁷₁₇Cl, in the ratio $3:1$3:1. The value $35.5\ u$35.5 u is the weighted average mass of these isotopes, not the mass of any single atom.

Average atomic mass $= \dfrac{(3\times 35) + (1\times 37)}{3+1} = \dfrac{105 + 37}{4} = \dfrac{142}{4} = 35.5\ u$= (3× 35) + (1× 37)/(3+1) = (105 + 37)/4 = 142/4 = 35.5 u.

This confirms the observed value of $35.5\ u$35.5 u.

(b) Each nitrogen atom (config $2,5$2,5) has $5$5 outer electrons and needs $3$3 more for an octet, so the two atoms share three pairs of electrons, forming a triple covalent bond, $N\equiv N$N≡ N.

(a) Merit: it discovered the nucleus and showed that the atom is mostly empty space, with electrons outside a tiny, dense, positively charged nucleus.

Demerits: (i) an electron moving in a circle should continuously lose energy, spiral into the nucleus and make the atom collapse, but atoms are stable; (ii) it said nothing about how the electrons are arranged around the nucleus.

(b) Bohr's model (simplified):

1. Electrons revolve around the nucleus only in certain fixed circular paths called orbits or shells (energy levels K, L, M, N).
2. Each orbit has a fixed energy; while an electron stays in its orbit it neither loses nor gains energy.
3. An electron absorbs energy to jump to a higher orbit and gives out energy when it falls back to a lower one.

Because an electron in a permitted orbit does not lose energy, it does not spiral into the nucleus, so the atom is stable. This removed Rutherford's main demerit.

(i) P and Q are isotopes of the same element (oxygen), because they have the same atomic number ($8$8 protons) but different numbers of neutrons, and hence different mass numbers.

(ii) Mass number of Q $=$= protons $+$+ neutrons $= 8 + 10 = 18$= 8 + 10 = 18.

(iii) Atom R has $12$12 electrons, configuration $2,8,2$2,8,2; its outermost shell has $2$2 electrons, so its valency is $2$2 (it is magnesium).

(iv) R (a metal, config $2,8,2$2,8,2) loses $2$2 electrons and P (oxygen, config $2,6$2,6) gains $2$2 electrons, so they combine by transfer of electrons to form the ionic (electrovalent) compound magnesium oxide, $MgO$MgO ($Mg^{2+}O^{2-}$Mg^2+O^2-).