Journey Inside the Atom — CBSE Class 9 Science Important Questions

Answer: (c) Electron.

The electron carries a charge of $-1$-1 and has a mass about $\frac{1}{2000}$1/2000 that of a proton, which is taken as negligible. Protons are positive, neutrons are neutral.

Answer: (b) Protons in the nucleus.

The atomic number $Z$Z is defined as the number of protons in the nucleus, which also equals the number of electrons in a neutral atom.

Answer: (b) Most of the space inside an atom is empty.

Since the great majority of $\alpha$-particles were undeflected, the atom must be mostly empty space, with the positive charge and mass concentrated in a very small nucleus that deflected only a few particles.

Answer: (b) $19$19 and $1$1.

Total electrons $=2+8+8+1=19$=2+8+8+1=19, so the atomic number is $19$19. The outermost (N) shell has $1$1 electron, so the atom loses it easily, giving a valency of $1$1 (it is potassium).

Answer: (a) Both A and R are true and R is the correct explanation of A. Isotopes have equal protons (same $Z$Z) but different neutron numbers, so their mass numbers ($A=$A= protons $+$+ neutrons) differ. The difference in neutrons is exactly why the mass numbers differ.

Valency is the combining capacity of an atom — the number of electrons it gains, loses or shares to complete its outermost shell.

Magnesium ($Z=12$Z=12) has the configuration $2,8,2$2,8,2. Its outermost shell has $2$2 electrons, which it loses to attain a stable octet, so the valency of magnesium is $2$2.

For $^{35}_{17}\text{Cl}$^35_17Cl, the atomic number $Z=17$Z=17 and the mass number $A=35$A=35.

• Number of protons $=Z=17$=Z=17.
• Number of electrons $=Z=17$=Z=17 (neutral atom).
• Number of neutrons $=A-Z=35-17=18$=A-Z=35-17=18.

Sodium has $11$11 electrons. Filling shells using the $2n^2$2n^2 rule: K shell $=2$=2, L shell $=8$=8, M shell $=1$=1.

Electronic configuration: $2,8,1$2,8,1. The nucleus contains $11$11 protons, giving a charge of $+11$+11.

The innermost K shell holds $2$2 electrons, the L shell holds $8$8, and the single outermost M-shell electron gives sodium a valency of $1$1.

The main rules for filling electron shells are:

1. The maximum number of electrons a shell can hold is given by $2n^2$2n^2, where $n$n is the shell number. So K$(n=1)=2$(n=1)=2, L$(n=2)=8$(n=2)=8, M$(n=3)=18$(n=3)=18, N$(n=4)=32$(n=4)=32.

2. The outermost shell can hold a maximum of $8$8 electrons.

3. Electrons are not filled in a new shell unless the inner shells are filled — shells are filled step by step from the innermost outward (that is, only after a shell reaches its capacity, subject to rule 2, does the next shell begin to fill).

The average atomic mass is the weighted mean of the isotopic masses according to their proportions.

Out of every $4$4 atoms, $3$3 are Cl-$35$35 and $1$1 is Cl-$37$37.

$\text{Average mass}=\frac{(35\times3)+(37\times1)}{3+1}=\frac{105+37}{4}=\frac{142}{4}$Average mass=(35×3)+(37×1)/3+1=105+37/4=142/4

$=35.5\ \text{u}.$=35.5 u.

This is why the atomic mass of chlorine is $35.5$35.5 u, a fractional value, even though no single atom has that mass.

Experiment: Rutherford bombarded a very thin gold foil with fast-moving, positively charged $\alpha$-particles and recorded where they struck a fluorescent screen placed around the foil.

Observations:

1. Most of the $\alpha$-particles passed straight through the foil without any deflection.
2. A few were deflected through small angles.
3. A very small number (about $1$1 in $12{,}000$12,000) bounced back through large angles.

Conclusions:

1. Since most particles went straight through, most of the atom is empty space.
2. Since a few were deflected, there is a small, dense, positively charged region — the nucleus — that repels the positive $\alpha$-particles.
3. Since only very few bounced back, the nucleus is very small compared to the size of the atom, yet it carries nearly all the mass and positive charge.

From this Rutherford proposed the nuclear model: a tiny positive nucleus at the centre with electrons revolving around it.

Drawback: According to classical physics, an electron revolving in a circular orbit is accelerating and would continuously radiate energy, spiral inward and fall into the nucleus, making the atom unstable. This contradicts the observed stability of atoms.

Thomson's model: The atom is a sphere of positive charge with electrons embedded in it, like seeds in a watermelon (the 'plum pudding' model). The atom is electrically neutral. Limitation: it could not explain the results of the $\alpha$-scattering experiment.

Rutherford's model: Improved this by placing all the positive charge and mass in a tiny central nucleus, with electrons revolving around it in the empty space, based directly on the scattering observations. Limitation: revolving electrons should lose energy and collapse into the nucleus, so it could not explain the atom's stability.

Bohr's model: Improved this by proposing that electrons revolve only in certain fixed stable orbits (energy levels/shells) without radiating energy, and that energy is absorbed or released only when an electron jumps between orbits. This explained the stability of the atom.

Isotopes: atoms of the same element with the same atomic number but different mass numbers, e.g. $^{35}_{17}\text{Cl}$^35_17Cl and $^{37}_{17}\text{Cl}$^37_17Cl. Use: an isotope of uranium ($\text{U-}235$U-235) is used as fuel in nuclear reactors; an isotope of iodine ($\text{I-}131$I-131) is used to treat goitre.

Isobars: atoms of different elements with different atomic numbers but the same mass number, e.g. $^{40}_{20}\text{Ca}$^40_20Ca and $^{40}_{18}\text{Ar}$^40_18Ar. Use: studying isobars helps in understanding nuclear reactions and in comparing nuclei of equal mass.

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

(ii) An isotope of iodine, $\text{I-}131$I-131, is used to treat goitre (an enlarged thyroid gland).

(iii) The chemical behaviour of an atom is decided by the number and arrangement of its electrons. All isotopes of an element have the same number of electrons and the same electronic configuration, so they react identically. Their differing numbers of neutrons do not affect chemistry.

(iv) For $^{60}_{27}\text{Co}$^60_27Co: neutrons $=A-Z=60-27=\mathbf{33}$=A-Z=60-27=33.