Atomic Structure and Chemical Bonding — ICSE Class 9 Chemistry Important Questions

Answer: (b) $18$18.

By the Bohr-Bury rule the maximum number of electrons in a shell is $2n^2$2n^2. 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) $MX_2$MX_2.

$X$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 $MX_2$MX_2.

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).

Isotopes are atoms of the same element that have the same atomic number but different mass numbers (i.e. the same number of protons but a different number of neutrons).

Uses:

• $U$U-$235$235 (uranium-235) is used as a fuel in nuclear reactors and atomic energy.

• $Co$Co-$60$60 (cobalt-60) is used in radiotherapy for the treatment of cancer.

(Iodine-131 is used to treat goitre/thyroid disorders.)

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$2n^2, 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).

(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, $^{35}_{17}Cl$^35_17Cl and $^{37}_{17}Cl$^37_17Cl, 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.

(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-).