Structure of the atom
Multiple Choice Questions
1. Which of the following correctly represent the electronic distribution in the $Mg$ atom?
(a) 3, 8, 1
(b) 2, 8, 2
(c) $1,8,3$
(d) 8, 2, 2
Answer Answer is 2, 8, 2 Explanation Atomic number of $Mg$ is 12 hence electronic distribution will be $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2}$.Show Answer
(a) Electron
(b) Proton
(c) Nucleus in the atom
(d) Atomic mass
Answer Answer is (c) Nucleus in the atom Explanation: Rutherford’s ‘alpha $(\alpha)$ particles scattering experiment’ experiment concludes that alpha particles returned to their original path. This showed the presence of nucleus in the centre.Show Answer
(a) ${ }^{31}X_{15}$
(b) ${ }^{31}X_{16}$
(c) ${ }^{16}X_{15}$
(d) ${ }^{15}X_{16}$
Answer Answer is (a) ${ }^{31}X_{15} $ Explanation: Number of protons in an element depicts atomic number. Number of protons and electrons are equal in an element. Hence atomic number is written in subscript whereas mass number is written in the subscript b\times
\times
\timesore the symbol of the element.Show Answer
(i) Law of conservation of mass
(ii) Law of constant composition
(iii) Law of radioactivity
(iv) Law of multiple proportion
(a) (i), (ii) and (iii)
(b) (i), (iii) and (iv)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iv)
Answer Answer is (d) (i), (ii) and (iv) Explanation: Dalton’s theory explains Law of conservation of mass, Law of constant composition, Law of multiple proportion. But it never give any details of Law of radioactivity .Show Answer
(i) considered the nucleus as positively charged
(ii) established that the $\alpha$-particles are four times as heavy as a hydrogen atom
(iii) can be compared to solar system
(iv) was in agreement with Thomson’s model
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) only (i)
Answer Answer is (a) (i) and (iii) Explanation: Positively charged alpha particles were deflected by nucleus. This shows nucleus is positively charged. Rutherford also postulated that electrons are arranged in an atom around the nucleus like planets arranged around sun.Show Answer
(i) Atomic number $=$ number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic mass $=$ number of protons $=$ number of neutrons
(iv) Atomic number $=$ number of protons $=$ number of electrons
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)
Answer Answer is (d) (ii) and (iv) Explanation: Atomic number $Z$ is the number of proton present in an electron which is also equal to number pf electron in an atom. Since the mass of neutron is negligible. Number of protons and electron are added to obtain mass number of an element.Show Answer
(i) the mass of the atom is assumed to be uniformaly distributed over the atom
(ii) the positive charge is assumed to be uniformaly distributed over the atom
(iii) the electrons are uniformaly distributed in the positively charged sphere
(iv) the electrons attract each other to stabilise the atom
(a) (i), (ii) and (iii)
(b) (i) and (iii)
(c) (i) and (iv)
(d) (i), (iii) and (iv)
Answer Answer is(a) (i), (ii) and (iii) Explanation: Thomson proposed that negatively charged electron are stabilised by positively charged protons in the nucleus. Hence option II) the positive charge is assumed to be uniformly distributed over the atom is wrong statement and other statements are part of Thomson’s model of atom.Show Answer
(i) electrons have negative charge
(ii) the mass and positive charge of the atom is concentrated in the nucleus
(iii) neutron exists in the nucleus
(iv) most of the space in atom is empty Which of the above statements are correct?
(a) (i) and (iii)
(b) (ii) and (iv)
(c) (i) and (iv)
(d) (iii) and (iv
Answer Answer is (b) (ii) and (iv) Explanation: An atom consists of a positively charged, dense and very small nucleus which have all the protons and neutrons. Positive charge is due to protons, as neutrons have no charge. Here the space is empty because alpha particles pass straight through the gold foil without any deflection. Thomson explained that electron s have negative charge. Existence of neutron was discovered by Chadwick.Show Answer
(a) 13
(b) 10
(c) 14
(d) 16
Answer Answer is (b) 10 Explanation Mass number $(A)$ of the atom $=27$ Number of neutron in the atom $=14$ Number of Electrons=Mass number-Number of neutrons=27-14 Number of electrons $=13$ Since ions of the element has 3 positive charges number of electron in the ion is 13-3 which equal 10. Hence the answer is 10Show Answer
(a)
(b)
(c)
(d)
Answer Answer is d) Explanation: Number of protons in $Mg$ atom $=2+8+2=12$ Number of neutrons in $Mg$ atom $=24-12=12$ [as mass number of $Mg$ atom = 24 and number of neutrons = mass number - number of protons] Therefore, option (d) is the correct answerShow Answer
(a) One of the oxygen atoms has gained electrons
(b) One of the oxygen atoms has gained two neutrons
(c) The two oxygen atoms are isotopes
(d) The two oxygen atoms are isobars.
Answer Answer is(c) The two oxygen atoms are isotopes Explanation Two Oxygen atoms in $CH_3 COOC_2 H_5$ can have different number of neutrons only if the two O-atoms are isotopes. Isotopes of an element have same number of protons (and electrons) but different number of neutrons.Show Answer
(a) always metals
(b) always metalloids
(c) either metals or non-metals
(d) always non-metals
Answer Answer is (c) either metals or non-metals. Explanation If element shows positive valency it is a metal and if the element shows negative valency it will be a non-metal.Show Answer
(a) N. Bohr
(b) E. Goldstein
(c) Rutherford
(d) J.J. Thomson
Answer Answer is (d) J.J. ThomsonShow Answer
(a) 3
(b) 7
(c) 1
(d) 4
Answer Answer is (c) 1Show Answer
(a) 2, 8, 3
(b) 2, 8, 2
(c) 8, 2, 3
(d) 2, 3, 8
Answer Answer is (a) 2, 8, 3 Explanation Atomic number of Aluminium is 13, First shell can have maximum of 2 electron and second shell holds a mximum of 8 electrons. Hence option a is right answer.Show Answer
(a)
(b)
(c)
(d)
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (ii) and (iv)
(d) (i) and (iv)
Answer Answer is ii and iv Explanation First shell can have maximum of 2 electron and second shell can have maximum of 8 electron hence ii and iv do not represent Bohr’s model of an atom correctly.Show Answer
(a) An atom has equal number of electrons and protons.
(b) An atom has equal number of electrons and neutrons.
(c) An atom has equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutrons.
Answer Answer is (a) An atom has equal number of electrons and protons. Explanation An atom is electrically neutral because the number of protons are always equal to number of electrons. Hence option a) is right.Show Answer
(i) Rutherford’s atomic model
(ii) Thomson’s atomic model
(iii) Bohr’s atomic model
(a) (i), (ii) and (iii) (b) (ii), (iii) and (i)
(c) (ii), (i) and (iii)
(d) (iii), (ii) and (i)
Show Answer
Answer
Answer is (c) (ii), (i) and (iii)
Explanation:
Thomson’s atomic model was proposed in the year 1904
Rutherford’s atomic model was proposed in the year 1911
Bohr’s atomic model was proposed in the year 1913
Short Answer Questions
19. Is it possible for the atom of an element to have one electron, one proton and no neutron. If so, name the element.
Answer Yes, Hydrogen is the element which is having only 1 proton and 1 electron and no neutron hence there is no repulsive force in the nucleus hence it is stable.Show Answer
AnswerShow Answer
Answer $35 Cl$ and $37 Cl$ cannot have different valencies because they are the isotopes of same element.Show Answer
Answer Rutherford used gold for his scattering experiment because gold is the most malleable metal and he wanted the thinnest layer as possible. Therefore, Rutherford selected a Gold foil in his alpha scatttering experiment.Show Answer
(a)
(b)
Answer Atom (a) has zero valency as it has 8 electron in its valence shell making the configuration stable. Atom (b) has valency of +1 as it has 7 electrons in it outermost shell . It can accept 1 electron to achieve octet configuration.Show Answer
Answer If an electron is removed from the outermost shell a cation will be formed and the charge of the element will be +1 .Show Answer
Answer Atomic number of chlorine atom $=17$ So, its electronic configuration is K L M 2 8 7 L shell of chlorine contains 8 electrons.Show Answer
Answer In this atom 6 electrons are already present in its outermost orbitals. In order to attain noble gas configuration element has to accept two electron hence its charge is -2 .Show Answer
$(x)$
(y)
$(z)$
Answer Atomic number, mass number and valency of atoms X, Y and ZShow Answer
Atomic no
Mass no
Valency
$X$
5
11
3
$Y$
8
18
2
$Z$
15
31
3.5
Answer Statement is wrong because number of protons can never be greater than number of neutrons. Number of protons will always be less than or equal to number of neutrons. Number of electrons and protonsare always equal in a neutral atom.Show Answer
Answer Mass number $=$ No. of protons + No. of neutrons $=31$ $\therefore$ Number of neutrons $=31-$ number of protons $=31-15$ $=16$Show Answer
(A) (B)
(a) Ernest Rutherford - (i) Indivisibility of atoms
(b) J.J.Thomson - (ii) Stationary orbits
(c) Dalton - (iii) Concept of nucleus
(d) Neils Bohr - (iv) Discovery of electrons
(e) James Chadwick - (v) Atomic number
(f) E. Goldstein - (vi) Neutron
(g) Mosley - (vii) Canal rays
Answer (A) - (B) (a) Ernest Rutherford - (iii) Concept of nucleus (b) J.J.Thomson - (iv) Discovery of electrons (c) Dalton - (i) Indivisibility of atoms (d) Neils Bohr - (ii) Stationary orbits (e) James Chadwick - (vi) Neutron (f) E. Goldstein - (vii) Canal rays (g) Mosley - (v) Atomic numberShow Answer
Answer Elements with different atomic numbers but same mass numbers are known as isobars. Calcium and argon are isobars.Show Answer
$ \begin{array}{|l|l|l|} \hline \text{Element} \hspace{20 mm} & n_p \hspace{10 mm}& n_n \hspace{10 mm}\\ \hline & & \\ \hline & & \\ \hline & & \\ \hline \end{array} $
Answer $ \begin{array}{|l|l|l|}
\hline \text{Element} \hspace{20 mm} & n_p \hspace{10 mm}& n_n \hspace{10 mm}\\
\hline Cl & 17 & 18 \\
\hline C & 6 & 6 \\
\hline Br & 35 & 46 \\
\hline
\end{array}
$Show Answer
Answer Helium has 2 electrons in its outermost shell thereby completing duplet configuration. Hence it has no valence shell left empty making its valency 0 .Show Answer
(a) Rutherford’s $\alpha$ - particle scattering experiment led to the discovery of the _ _ _
(b) Isotopes have same _ _ _ but different _ _ _ .
(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be _ _ _ and _ _ _ respectively.
(d) The electronic configuration of silicon is _ _ _ and that of sulphur is
Answer (a) Rutherford’s $\alpha$-particle scattering experiment led to the discovery of the atomic nucleus (b) Isotopes have same atomic number but different mass number (c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be $\mathbf{0}$ and $\mathbf{1}$ respectively. (d) The electronic configuration of silicon is 2.8.4 and that of sulphur is 2.8.6Show Answer
Show Answer
Answer
Mass number $=4$
Atomic number $=2$
$X$ is Helium.
It has 0 valency and it will not react with any other atom because it has its outer shell filled.
Long Answer Questions
36. Why do Helium, Neon and Argon have a zero valency?
Answer Helium has 2 electron in its outermost orbit thus filling shell 1 and forming duplet configuration in valence shell .Neon has 8 electron in their valence orbit hence completing duplet configuration. In the same way Argon and Neon has 8 electron in its outermost shelling completing octet configuration. As these elements have maximum electron in their valence shell thus reach stable electron configuration and they will not take part in any sort of chemical reactions.Show Answer
(i) What will be the ratio of their sizes?
(ii) If atom is represented by planet earth ’ $Re$ ’ $=6.4 \times 106 m$, estimate the size of the nucleus.
Answer Assuming the atom and the nucleus to be spherical. (a) Atomic size is represented in terms of atomic radius, $\dfrac{r_H}{r_n}=10^{5}$ As volume of sphere $=\dfrac{4}{3} \pi r^{3}$, therefore, $V_H=\dfrac{4}{3} \pi r_H^{3}$ and $V_n=\dfrac{4}{3} \pi r_n^{3}$ Thus, the ratio of volumes $\dfrac{V_H}{V_n}=\dfrac{\dfrac{4}{3} \pi r_H^{3}}{\dfrac{4}{3} \pi r_n^{3}}=\dfrac{r_H^{3}}{r_n^{3}}=(\dfrac{r_H}{r_n})^{3}=(10^{5})^{3}=10^{15}$ (b)
$
\dfrac{V_n}{V_H}=10^{-15} \text{ or } V_n=10^{-15} \times V_H
$ If atom is represented by planet earth with $R_e=6.4 \times 10^{6} m$ Then, volume of atom $.N_H)=\dfrac{4}{3} \pi R_e^{3}=\dfrac{4}{3} \times 3.14 \times(6.4 \times 10^{6} m)^{3}$ $
\begin{aligned}
& =1097.5 \times 10^{18} m^{3}=1.0975 \times 10^{21} m^{3} \\
\therefore \quad \text{ Volume of nucleus } & =10^{-15} \times(1.0975 \times 10^{21}) m^{3} \\
& =1.0975 \times 10^{6} m^{3}
\end{aligned}
$Show Answer
Answer Rutherford drawn following conclusion from his $\alpha$-ray scattering experiment.Show Answer
Answer Rutherford proposed that electron revolve around the nucleus in well differentiated orbits. Nucleus is the centre which is positively charged. Rutherford proposed that nucleus is very small and nearly all the mass of an atom is centred in the nucleus. Thompson proposed that electron are scattered positively charged spheres like a Christmas pudding and the mass of the atom was supposed to be uniformly distributed.Show Answer
Answer Rutherford’s model could not explain the stability of the atom. Revolving electrons would lose energy as they are the charged particles and due to acceleration, charged particles would radiate energy. Orbit of the revolving electron will reduce in size, following a spiral path as shown in figure and ultimately the electron should fall into the nucleus. In other words, the atom should collapse. Continuous loss of energy by a revolving electronShow Answer
Answer Postulates of Bohr’s model of an atom are (i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom. (ii) While revolving in discrete orbits the electrons do not radiate energy. These orbits or shells are called energy levels. These orbits or shells are represented by the letters K,L,M,N,… or the numbers, $n=1,2,3,4, \ldots$.Show Answer
Answer Atomic number of sodium $(Z)=11$ Mass number of sodium $(A)=23$ Number of protons in the nucleus $=11$ Number of neutrons in the nucleus $=23-11=12$ Number of electrons $=11$ Electronic configuration of $Na$-atom $=2,8,1(K, L, M)$ $Na+$ ion is formed from s,odium atom by loss of an electron (present in the outermost shell). Hence, its electronic configuration is $2,8(K, L)$. However, number of protons and neutrons remains the same. Sodium atom Sodium ionShow Answer
Show Answer
Answer
Total number of $\alpha$ particles used for bombardment $=1$ mole
1 mole $=6.022 \times 10^{23}$ particles
number of $\alpha$ particles deflected at angles greater than $50^{\circ}(>50^{\circ})=1 \%$
Number of $\alpha$ particles deflected at angles greater than $50^{\circ}=100-1=99 \%$
Actual number of $\alpha$ particles deflected at angles less than $50^{\circ}=\dfrac{99}{100} \times 6.022 \times 10^{23}$
$=5.96 \times 10^{23}$
