Mix Example - STRUCTURE OF THE ATOM Questions in English

(A) The atomic number of Magnesium $(Mg)$ is $12$.
According to the Bohr-Bury scheme,the maximum number of electrons that can be accommodated in a shell is given by the formula $2n^2$,where $n$ is the orbit number.
The first shell ($K$-shell,$n=1$) can hold a maximum of $2(1)^2 = 2$ electrons.
The second shell ($L$-shell,$n=2$) can hold a maximum of $2(2)^2 = 8$ electrons.
The remaining electrons go into the third shell ($M$-shell,$n=3$).
For $Mg$ $(Z=12)$,the distribution is $2$ in the $K$-shell,$8$ in the $L$-shell,and $2$ in the $M$-shell.
Thus,the electronic configuration is $2, 8, 2$.

(B) Rutherford's alpha particle scattering experiment showed that most of the alpha particles passed straight through the gold foil,but a very small fraction was deflected by large angles,and some even bounced back. This observation led to the conclusion that the positive charge and most of the mass of the atom are concentrated in a very small,dense region at the center,which was named the nucleus.

(C) The atomic number $(Z)$ of an element is equal to the number of protons, which is equal to the number of electrons in a neutral atom. Here, the number of electrons is $15$, so $Z = 15$.
The mass number $(A)$ is the sum of the number of protons and neutrons. $A = Z + \text{number of neutrons} = 15 + 16 = 31$.
The standard representation of an element is written as $_{Z}^{A}X$, where $Z$ is the atomic number (subscript) and $A$ is the mass number (superscript).
Therefore, the correct representation is $_{15}^{31}X$.

(D) Dalton's atomic theory was based on the laws of chemical combination.
It successfully explained the $Law$ $of$ $conservation$ $of$ $mass$,the $Law$ $of$ $constant$ $composition$ (or definite proportions),and the $Law$ $of$ $multiple$ $proportions$.
However,Dalton's theory did not explain the $Law$ $of$ $radioactivity$,as radioactivity involves the decay of atomic nuclei,which was not known at the time of Dalton.
Therefore,the correct statements are $(i), (ii),$ and $(iv)$.

(A) Rutherford's model of the atom proposed that the positive charge and most of the mass of the atom were densely concentrated in a very small region called the nucleus,which is positively charged $(i)$.
He compared the model to the solar system,where the nucleus acts like the Sun and electrons act like planets revolving around it $(iii)$.
While $\alpha$-particles are indeed four times as heavy as a hydrogen atom,this was a property of the particles used in the experiment,not a specific postulate of the model itself $(ii)$.
Rutherford's model contradicted Thomson's model because Thomson's model suggested a uniform distribution of positive charge,whereas Rutherford's model proved it was concentrated in the center $(iv)$.

(B) For a neutral atom,the atomic number is defined as the number of protons present in the nucleus,which is also equal to the number of electrons orbiting the nucleus. Thus,statement $(iv)$ is correct.
The mass number of an element is defined as the sum of the number of protons and the number of neutrons present in the nucleus of an atom. Thus,statement $(ii)$ is correct.
Statement $(i)$ is incorrect because the atomic number is not the sum of protons and electrons.
Statement $(iii)$ is incorrect because atomic mass is not equal to the number of protons or neutrons.
Therefore,statements $(ii)$ and $(iv)$ are true.

(C) Thomson's model of the atom,often called the 'plum pudding' model,proposes that:
$1$. The atom consists of a positively charged sphere with electrons embedded in it.
$2$. The mass of the atom is considered to be uniformly distributed throughout the atom.
$3$. The positive charge is also considered to be uniformly distributed throughout the atom.
$4$. The electrons are embedded in this positive sphere like seeds in a watermelon or plums in a pudding,meaning they are uniformly distributed.
$5$. Statement $(iv)$ is incorrect because the model suggests that the positive charge balances the negative charge of the electrons to stabilize the atom,not that electrons attract each other.
Therefore,statements $(i)$,$(ii)$,and $(iii)$ are correct.

(D) Rutherford's $\alpha$-particle scattering experiment led to the following conclusions:
$1$. Most of the $\alpha$-particles passed straight through the gold foil,which indicated that most of the space inside the atom is empty (Statement $iv$ is correct).
$2$. $A$ small fraction of $\alpha$-particles were deflected by small angles,indicating the presence of a positively charged center called the nucleus.
$3$. $A$ very few $\alpha$-particles rebounded,which showed that the entire mass and positive charge of the atom are concentrated in a very small volume called the nucleus (Statement $ii$ is correct).
$4$. The experiment did not provide information about the negative charge of electrons or the existence of neutrons.
Therefore,statements $(ii)$ and $(iv)$ are correct.

(A) The mass number $(A)$ is the sum of protons $(p)$ and neutrons $(n)$. Given $A = 27$ and $n = 14$.
Therefore,the number of protons (atomic number) is $p = A - n = 27 - 14 = 13$.
In a neutral atom,the number of electrons equals the number of protons,so the neutral atom has $13$ electrons.
The ion has $3$ positive charges,which means it has lost $3$ electrons.
Thus,the number of electrons in the ion $= 13 - 3 = 10$.

(C) The atomic number of Magnesium $(Mg)$ is $12$,which means it has $12$ protons $(p=12)$.
The electronic configuration of a neutral $Mg$ atom is $2, 8, 2$.
When $Mg$ forms a $Mg^{2+}$ ion,it loses $2$ electrons from its outermost shell,resulting in an electronic configuration of $2, 8$ (a total of $10$ electrons).
Therefore,the $Mg^{2+}$ ion must have $p=12$ and $10$ electrons in its shells.
Looking at the provided figures:
- Figure $C$ shows $p=12$ and $10$ electrons (arranged as $2$ in the inner shell and $8$ in the outer shell).
Thus,figure $C$ represents the $Mg^{2+}$ ion.

(C) Isotopes are atoms of the same element that have the same atomic number (number of protons and electrons) but different mass numbers.
The difference in mass number arises because isotopes have a different number of neutrons in their nuclei.
Since oxygen atoms in a sample can exist as different isotopes (e.g.,$^{16}O$,$^{17}O$,or $^{18}O$),they will have the same number of electrons but a different number of neutrons.

(D) Elements with a valency of $1$ can be metals or non-metals.
For example,Sodium $(Na)$ is a metal with a valency of $1$,while Chlorine $(Cl)$ is a non-metal with a valency of $1$.
Therefore,elements with valency $1$ can be either metals or non-metals.

(A) The first model of an atom was proposed by $J.J. Thomson$ in $1904$. This model is known as the $Plum$ $Pudding$ model or $Watermelon$ model. According to this model,an atom consists of a positively charged sphere with electrons embedded in it,similar to seeds in a watermelon.

(B) The number of protons in an atom is equal to its atomic number. Here,the atomic number is $3$,which corresponds to the element Lithium $(Li)$.
Since the atom is neutral,the number of electrons is equal to the number of protons,which is $3$.
The electronic configuration of this atom is $2, 1$.
Since the number of electrons in the outermost shell (valence shell) is $1$,the atom will lose $1$ electron to achieve a stable octet configuration.
Therefore,the valency of the atom is $1$.

(C) The atomic number of aluminium $(Al)$ is $13$.
According to the Bohr-Bury scheme,the maximum number of electrons that can be accommodated in a shell is given by the formula $2n^2$,where $n$ is the orbit number.
For the first shell $(n=1)$,the capacity is $2(1)^2 = 2$ electrons.
For the second shell $(n=2)$,the capacity is $2(2)^2 = 8$ electrons.
For the third shell $(n=3)$,the remaining electrons are placed. Since $13 - 2 - 8 = 3$,the third shell contains $3$ electrons.
Thus,the electron distribution is $2, 8, 3$. Therefore,option $(C)$ is correct.

(D) According to Bohr's model,the maximum capacity of the $K$ shell (first shell) is $2$ electrons.
The maximum capacity of the $L$ shell (second shell) is $8$ electrons.
Figure $(ii)$ shows $4$ electrons in the $K$ shell,which violates the rule as the $K$ shell can hold only $2$ electrons.
Figure $(iv)$ shows $9$ electrons in the $L$ shell,which violates the rule as the $L$ shell can hold a maximum of $8$ electrons.
Therefore,figures $(ii)$ and $(iv)$ do not represent Bohr's model correctly.

(A) An atom is electrically neutral because it contains an equal number of positively charged protons and negatively charged electrons. While the number of neutrons can vary (leading to isotopes),the number of protons and electrons must be equal for the atom to remain neutral.

(B) The chronological order of the development of atomic models is as follows:
$1$. Thomson's atomic model (also known as the Plum Pudding model) was proposed in $1904$.
$2$. Rutherford's atomic model (based on the alpha-particle scattering experiment) was proposed in $1911$.
$3$. Bohr's atomic model (which introduced quantized orbits) was proposed in $1913$.
Therefore,the correct chronological order is $(ii), (i), (iii)$.

(N/A) Yes,it is possible. The atom of the element hydrogen $(H)$ consists of one proton and one electron,but it contains no neutrons. It is represented as $_1^1H$.

(A) The fact that atoms are divisible is supported by the following observations:
$1$. Discovery of subatomic particles: The discovery of electrons by $J$.$J$. Thomson and protons by $E$. Goldstein proved that atoms are composed of even smaller particles.
$2$. Existence of isotopes and isobars: The existence of different forms of the same element (isotopes) and different elements with the same mass number (isobars) suggests that the internal structure of the atom is complex and divisible.

(B) The atoms $^{35}Cl$ and $^{37}Cl$ are isotopes of chlorine.
Isotopes are atoms of the same element that have the same atomic number but different mass numbers.
Since they have the same atomic number $(Z = 17)$,they possess the same number of electrons ($17$ electrons) and the same electronic configuration $(2, 8, 7)$.
Valency is determined by the number of valence electrons in the outermost shell.
Since both isotopes have the same number of valence electrons,their valencies are identical.

(B) Gold is highly malleable,which means it can be beaten into very thin sheets.
In his $\alpha$-ray scattering experiment,Rutherford required a target material that was as thin as possible to ensure that the $\alpha$-particles would pass through with minimal interference from multiple atomic layers.
Therefore,gold was the ideal choice because it could be hammered into a foil of approximately $1000$ atoms in thickness.

(N/A) In Fig. $(a)$,the atom has $2$ electrons in the first shell,$8$ in the second,and $8$ in the third shell. Since the outermost shell is completely filled (octet rule),the valency is $0$.
In Fig. $(b)$,the atom has $2$ electrons in the first shell and $7$ in the second shell. To complete its octet,it needs $1$ electron,therefore its valency is $1$.

(B) When an atom loses an electron,it loses a negative charge,resulting in an excess of protons relative to electrons.
Since the element $X$ has one electron in its outermost shell,it will lose this single electron to achieve a stable octet configuration.
By losing one electron,the atom gains a net positive charge of $+1$.
Therefore,the nature of the charge is positive and the value is $+1$.

(A) The atomic number of chlorine is $17$.
According to the Bohr-Bury scheme,the electrons are distributed in shells as follows:
$K$ shell $(n=1)$: $2$ electrons
$L$ shell $(n=2)$: $8$ electrons
$M$ shell $(n=3)$: $7$ electrons
Thus,the electron distribution is $2, 8, 7$.
The $L$ shell contains $8$ electrons.

(B) The outermost shell of an atom requires $8$ electrons to achieve a stable noble gas configuration (octet rule).
Since the element $X$ has $6$ electrons in its outermost shell,it needs to gain $8 - 6 = 2$ electrons to complete its octet.
When an atom gains electrons,it becomes a negatively charged ion (anion).
Since it gains $2$ electrons,the charge on the resulting ion will be $-2$.

(N/A) The atomic number is equal to the number of protons. The mass number is the sum of protons and neutrons. The valency is determined by the number of electrons in the outermost shell (valence shell).

Atom	Atomic No.	Mass No.	Valency
$X$	$5$	$11$	$3$
$Y$	$8$	$18$	$2$
$Z$	$15$	$31$	$3$

(N/A) The student's statement is incorrect.
$1$. In a neutral atom,the number of protons is always equal to the number of electrons.
$2$. The number of protons is not necessarily greater than the number of neutrons. In many stable isotopes,the number of neutrons is equal to or greater than the number of protons.
$3$. Therefore,the proposed relationship (protons > neutrons > electrons) is scientifically invalid.

(A) The mass number $(A)$ of an element is the sum of the number of protons and neutrons in its nucleus.
Given the representation $^{31}_{15}X$,the mass number $A = 31$ and the atomic number $Z$ (number of protons) $= 15$.
The formula for the number of neutrons $(n)$ is: $n = A - Z$.
Substituting the values: $n = 31 - 15 = 16$.
Therefore,the number of neutrons present in the nucleus is $16$.

(A) $(a)-(iii)$: Ernest Rutherford proposed the nuclear model of the atom, introducing the concept of the nucleus.
$(b)-(iv)$: $J$.$J$. Thomson discovered the electron through his cathode ray experiments.
$(c)-(i)$: John Dalton proposed the atomic theory, which stated that atoms are indivisible.
$(d)-(ii)$: Neils Bohr proposed that electrons revolve in discrete stationary orbits around the nucleus.
$(e)-(vi)$: James Chadwick discovered the neutron.
$(f)-(vii)$: $E$. Goldstein discovered canal rays (protons) in a discharge tube.
$(g)-(v)$: Henry Moseley discovered the concept of atomic number.

(B) Elements that have the same mass number but different atomic numbers are known as isobars.
In this case,both calcium $(Ca)$ and argon $(Ar)$ have a mass number of $40$,but their atomic numbers are $20$ and $18$ respectively.
Therefore,they are classified as isobars.

(N/A) To find the number of protons $(n_{(p)})$ and neutrons $(n_{(n)})$, we use the notation ${ }_{Z}^{A} X$, where $Z$ is the atomic number (number of protons) and $A$ is the mass number (protons + neutrons).
$1$. For ${ }_{17}^{35} Cl$: $n_{(p)} = Z = 17$. $n_{(n)} = A - Z = 35 - 17 = 18$.
$2$. For ${ }_{6}^{12} C$: $n_{(p)} = Z = 6$. $n_{(n)} = A - Z = 12 - 6 = 6$.
$3$. For ${ }_{35}^{81} Br$: $n_{(p)} = Z = 35$. $n_{(n)} = A - Z = 81 - 35 = 46$.

Element	$n_{(p)}$	$n_{(n)}$
$Cl$	$17$	$18$
$C$	$6$	$6$
$Br$	$35$	$46$

(N/A) The helium atom has $2$ electrons in its outermost shell,which represents a complete duplet. Since its outermost shell is already full and stable,it does not need to lose,gain,or share electrons to achieve stability. Therefore,its valency is $0$.

(N/A) The experiment led to the discovery of the atomic nucleus.
$(b)$ Isotopes are atoms of the same element having the same atomic number but different mass numbers.
$(c)$ Neon $(Z=10)$ has an electronic configuration of $2, 8$,so its valency is $0$. Chlorine $(Z=17)$ has an electronic configuration of $2, 8, 7$,so it needs $1$ electron to complete its octet,making its valency $1$.
$(d)$ Silicon $(Z=14)$ has an electronic configuration of $2, 8, 4$. Sulphur $(Z=16)$ has an electronic configuration of $2, 8, 6$.

(0) The atomic number of element $X$ is $2$.
This means it has $2$ electrons in its $K$ shell.
Since the $K$ shell can hold a maximum of $2$ electrons,it is completely filled.
Therefore,the element is stable and does not need to gain,lose,or share electrons.
Hence,the valency of this element is $0$.

(B) Helium has $2$ electrons in its only energy shell (duplet),while Neon and Argon have $8$ electrons in their valence shells (octet).
Since these elements have the maximum number of electrons in their outermost shells,their valence shells are completely filled.
This stable electronic configuration means they have no tendency to gain,lose,or share electrons with other elements.
Therefore,their combining capacity,or valency,is equal to $0$.

(B) $(i)$ The volume of a sphere is given by $V = \frac{4}{3} \pi r^3$.
Let $R$ be the radius of the atom and $r$ be the radius of the nucleus.
Given the ratio $\frac{R}{r} = 10^5$,we have $R = 10^5 r$.
The ratio of their volumes is $\frac{V_{atom}}{V_{nucleus}} = \frac{\frac{4}{3} \pi R^3}{\frac{4}{3} \pi r^3} = \left( \frac{R}{r} \right)^3$.
Substituting the value,we get $(10^5)^3 = 10^{15}$.
$(ii)$ If the atom is represented by the Earth,then $R_{atom} = R_e = 6.4 \times 10^6 \, m$.
Since $r_{nucleus} = \frac{R_{atom}}{10^5}$,we have $r_{nucleus} = \frac{6.4 \times 10^6 \, m}{10^5} = 6.4 \times 10^1 \, m = 64 \, m$.

(N/A) Rutherford concluded from the $\alpha -$ particle scattering experiment that:
$(i)$ Most of the space inside the atom is empty because most of the $\alpha -$ particles passed through the gold foil without getting deflected.
$(ii)$ Very few particles were deflected from their path,indicating that the positive charge of the atom occupies very little space.
$(iii)$ $A$ very small fraction of $\alpha -$ particles were deflected by $180^o$,indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
From the data,he also calculated that the radius of the nucleus is about $10^5$ times smaller than the radius of the atom.

(N/A) Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits. There is a positively charged centre in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centred in the nucleus.
Whereas,Thomson proposed the model of an atom to be similar to a Christmas pudding. The electrons are studded like currants in a positively charged sphere like Christmas pudding and the mass of the atom was supposed to be uniformly distributed.

(A) According to classical electromagnetic theory,any charged particle moving in a circular orbit would undergo acceleration and radiate energy.
As the electron revolves,it would continuously lose energy.
Due to this loss of energy,the electron would spiral inward and eventually fall into the nucleus.
If this were true,the atom would be highly unstable,and matter as we know it would not exist.

(N/A) The postulates put forth by Neils Bohr regarding the model of an atom are as follows:
$(i)$ Only certain special orbits,known as discrete orbits of electrons,are allowed inside the atom.
$(ii)$ While revolving in these discrete orbits,the electrons do not radiate energy. These orbits are called energy levels or shells. Energy levels in an atom are represented by circles. These orbits are denoted by the letters $K, L, M, N, \dots$ or by the numbers $n = 1, 2, 3, 4, \dots$.

(N/A) Since the atomic number of a sodium atom is $11$,it has $11$ electrons.
$A$ positively charged sodium ion $(Na^+)$ is formed by the removal of one electron from a sodium atom.
So,a sodium ion has $11 - 1 = 10$ electrons in it.
Thus,the electronic distribution of a sodium ion will be $2, 8$.
The atomic number of an element is equal to the number of protons in its atom.
Since a sodium atom and a sodium ion contain the same number of protons,the atomic number of both is $11$.

(C) The percentage of $\alpha$-particles deflected at angles greater than $50^o$ is $1\%$.
Therefore,the percentage of $\alpha$-particles deflected at angles less than $50^o$ is $100\% - 1\% = 99\%$.
One mole of $\alpha$-particles contains $6.022 \times 10^{23}$ particles (Avogadro's number).
The number of $\alpha$-particles that deflect at angles less than $50^o$ is calculated as:
$\text{Number} = \frac{99}{100} \times 6.022 \times 10^{23}$
$\text{Number} = 0.99 \times 6.022 \times 10^{23}$
$\text{Number} = 5.96178 \times 10^{23} \approx 5.96 \times 10^{23}$ particles.

(B) The mass of an atom is primarily determined by the sum of the number of protons and neutrons present in the nucleus.
These particles are collectively known as nucleons.
Electrons have negligible mass compared to protons and neutrons,so they do not contribute significantly to the atomic mass.

(C) An atom consists of protons,neutrons,and electrons.
Protons carry a positive charge,while electrons carry a negative charge.
In a neutral atom,the number of protons is equal to the number of electrons.
Since the magnitude of the positive charge of a proton is exactly equal to the magnitude of the negative charge of an electron,they cancel each other out.
Therefore,the atom as a whole is electrically neutral.

(N/A) The particle discovered by $J. Chadwick$ is the neutron.
It is located in the nucleus of the atom along with protons.

(B) According to Bohr's postulate,electrons revolve around the nucleus in specific,discrete orbits known as stationary orbits. While revolving in these discrete orbits,electrons do not radiate energy. This explains the stability of the atom,as the electrons do not lose energy and fall into the nucleus.

(N/A) The valence shell of a helium atom is the $K$-shell,which can hold a maximum of $2$ electrons. Since the $K$-shell is completely filled with $2$ electrons,the atom is stable and does not need to gain,lose,or share electrons to achieve a stable configuration. Therefore,its combining capacity or valency is $0$.

(N/A) An octet of electrons in the valence shell refers to the presence of $8$ electrons in the outermost shell of an atom. Atoms with $8$ electrons in their valence shell (except for Helium,which has $2$) are considered to have a stable electronic configuration,similar to noble gases. This state is known as the octet rule,which explains why atoms tend to gain,lose,or share electrons to achieve stability.

(N/A) Elements exist in nature as a mixture of different isotopes,each with a specific relative abundance. Because these isotopes have different atomic masses,a single atomic mass value cannot represent the element accurately. Therefore,the average atomic mass is calculated by taking the weighted average of the masses of all naturally occurring isotopes based on their percentage abundance.