Chemistry · Physics

Atomic Structure and Mass

277 Questions

Atomic structure and mass focus on the composition of atoms, including protons, neutrons, electrons, and isotopes. This topic is essential for the chemistry sections in engineering and civil services competitive exams. Review these questions to practice calculating atomic mass, neutron count, and isotopic distributions.

Atomic mass unit calculationsNeutron and proton countsIsotopic distribution averagesLaw of triadsRest energy of atoms

Atomic Structure and Mass Questions

Multiple choice

1 a.m.u. is equivalent to
  1. $\frac{1}{12}$th of mass of carbon atom
  2. $\frac{1}{6} th$ of mass of carbon atom
  3. $\frac{1}{4}th$ of mass of carbon atom
  4. $\frac{1}{16}th$ of mass of carbon atom
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

1 a.m.u. is equivalent to $\frac{1}{12}$th of mass of carbon atom.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae
Boron has two stable isotopes, $^{10}B (19\%)$ and  $^{11}B(81\%)$. Average atomic weight for boron in the periodic table is:

  1. 10.8

  2. 10.2

  3. 11.2

  4. 10.0
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Average atomic weight $=\dfrac{ \sum \%abundant \times atomic\ mass}{100}$
$=\dfrac{ 19 \times 10 + 81 \times 11}{100}=10.81$

Option B is the answer.
Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

An element $X$ have three isotopes $X^{20}, X^{21}$ and $X^{22}$. The percentage abundance of $X^{20}$ is $90\%$ and its average atomic mass of elements is $20.18$. The percentage abundance of $X^{21}$ should be ______________.

  1. $2\%$
  2. $8\%$
  3. $10\%$
  4. $0\%$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Average atomic mass $=\dfrac {\displaystyle \sum \%\ abundance\ \times \ A.M }{100}$

$20.18=\dfrac {20\times 90+x\times21+(10-x)22}{100}$

$2018=1800+21x+220-22x$

$x=2020-2018$

$x=2\%$

Option $(A).$

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

The fractional abundance of $Cl^{35}$ in a sample of chlorine containing only $Cl^{35}$ (atomic weight $=34.9$) and $Cl^{37}$ (atomic weight $=36.9$) isotopes, is $0.6$. The average mass number of chlorine is  _______________.

  1. $35.7$
  2. $35.8$
  3. $18.8$
  4. $35.77$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$\dfrac {C1^{35}}{C1^{37}}\Rightarrow \dfrac {0.6}{(1-0.6)}=\dfrac {0.6}{0.4}$

Average atomic mass $=\dfrac {34.9\times 0.6+36.9\times 0.4}{1}$
Average Atomic mass $=35.7$

Option $A$ is the answer.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

The isotopes of chlorine with mass numbers $35$ and $37$ whose average mass is 35.5 exist in the ratio of:

  1. $1:1$
  2. $3:1$
  3. $3:4$
  4. $3:2$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let the fraction of $Cl^{35}$ isomer be $p$. The fraction of $Cl^{37}$ isomer will be $1-p$.

Average atomic mass $=\displaystyle {p\times m _{1}+(1-p)m _{2}}$
$\therefore 35.5 = p \times 35 + (1 - p) \times 37$

So, $p = 0.75$

Hence, the ratio of two isotopes will be $p: (1-p) = 0.75 : (1-0.75) = 0.75:0.25 = 3:1$

Option B is correct.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

The relative atomic mass of naturally occurring chlorine is not a whole number. What is the reason for this ?

  1. Chlorine atoms can have different number of neutrons

  2. Naturally occurring chlorine cannot be obtained pure

  3. Chlorine is unstable

  4. The mass of the electrons has been included
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

 The relative atomic mass of an element is the average of the atomic masses of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance. 


Chlorine has two isotopes available in Nature. 

One has At.mass = 35 which forms roughly $75\%$ of the naturally available Chlorine. 

Another has At.mass = 37 which forms roughly $25\%$ of the naturally available Chlorine. 

So they have fractional atomic mass.
Hence, the correct option is $\text{A}$

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

Among the following quantities:
(i) mass number
(ii) average mass of a Carbon atom in amu
(iii) the charge of nucleus in amu and
(iv) mass of a Carbon -12 atom in grams

  1. only (i) is whole number

  2. only (i) and (iii) are whole number

  3. only (i) and (iv) are whole numbers

  4. only (iii) and (iv) are whole numbers
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

(1) The mass number is always the whole number because the mass number is the total number of neutrons and protons in the nucleus of the atom and an atom never contains fractional or an integer number of protons or neutrons.

(2) Average mass of carbon atom in $amu$ is $12.011u$. It is in decimal and can be written as fractional form.
(3) Charge on the nucleus in $amu$:- Charge on the nucleus is equal to the number of protons and the number of protons are always whole.
(4) Mass of carbon atom= $\cfrac {Mass\quad of\quad carbon}{Avogadro\quad number}$
$=\cfrac {12g}{6.022\times 10^{23}}$
$=1.99\times 10^{-23}$ grams
It is not a whole number.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

What is the atomic mass of sodium?

  1. $22\ g/mol$
  2. $23\ g/mol$
  3. $11\ g/mol$
  4. $20\ g/mol$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A relative atomic mass is a measure of how heavy atoms are. It is the ratio of the average mass per atom of an element from a given sample to 1/12 the mass of a carbon-12 atom.

The relative atomic mass in 1 mole of isotopes of sodium atoms is 23 g/mol.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

What is the relative atomic mass of cadmium?

  1. 112.411 amu

  2. 20.00 amu

  3. 21.00 amu

  4. 20.43 amu
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The relative atomic mass is an average of the atomic masses of all the different isotopes in a sample, with each isotope's contribution to the average determined by how big a fraction of the sample it makes up.
The relative atomic mass of Cadmium is 112.411 amu.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

Which of the following is/are correct?

  1. One atomic mass unit is a mass unit equal to exactly one -twelfth (1/12th) the mass of one atom of carbon-12.

  2. One atomic mass unit is a mass unit equal to exactly one - sixteenth (1/16th) the mass of one atom of oxygen-16.

  3. The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12h the mass of one carbon-12 atom.

  4. None of the above.
Reveal answer Fill a bubble to check yourself
A,B,C Correct answer
Explanation

The following are correct. One atomic mass unit is a mass unit equal to exactly one -twelfth (1/12th) the mass of one atom of carbon-12.
One atomic mass unit is a mass unit equal to exactly one - sixteenth (1/16th) the mass of one atom of oxygen-16.
The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12h the mass of one carbon-12 atom.
For example, the mass of one carbon-12 atom is 12 amu. The mass of one magnesium-24 atom is 24 amu. The mass of one calcium-40 atom is 140 amu.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

Oxygen occurs in nature as a mixture of isotopes $^16O$, $^17O$ and $^18O$ having atomic masses of 15.995 u, 16.999 u and 17.999 u and relative abundance of 99.763%, 0.037%, and 0.200% respectively. What is the average atomic mass of oxygen?

  1. 15.999 u

  2. 16.999 u

  3. 17.999 u

  4. 18.999 u
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
 Average atomic mass of an element existing in different isotopes is given by:
$M _{ avg }=\dfrac { \sum _{ i=1 }^{ n }{ { M } _{ i }{ A } _{ i } }  }{ \sum _{ i=1 }^{ n }{ A _{ i } }  } $
where $M _i=$atomic mass of an isotope with relative abundance of $A _i$
Given:$M _1=15.995 u,A _1=99.763,M _2=16.999 u, A _2=0.037,M _3= 17.999 u, A _3=0.200$
on subtitutiing we get:
${ M } _{ avg }=\dfrac { 15.995\times 99.763+16.999\times 0.037+17.999\times 0.200 }{ 99.763+0.037+0.200 } $
${ M } _{ avg }=15.999\ u$
option A is correct
Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

For every ,one $^{37}Cl$ isotope there are three $^{35}Cl$ isotopes, in a sample of chlorine. What will be the average atomic mass of chlorine?

  1. 35

  2. 37

  3. 35.5

  4. 35.6
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Average atomic mass of an element existing in different isotopes is given by:
$M _{ avg }=\dfrac { \sum _{ i=1 }^{ n }{ { M } _{ i }{ A } _{ i } }  }{ \sum _{ i=1 }^{ n }{ A _{ i } }  } $
where $M _i=$atomic mass of an isotope with relative abundance of $A _i$
Given:$M _1=37 u,A _1=1,M _2=35 u, A _2=3$
on subtitutiing we get:
${ M } _{ avg }=\dfrac { 37\times 1+35\times 3 }{ 1+3 } $
${ M } _{ avg }=35.5\ u$
option C is correct
Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

One gm metal $M^{3+}$ was discharged by the passage of $1.81\times 10^{23}$ electrons. What is the atomic mass of metal?

  1. $8g/mol$
  2. $9g/mol$
  3. $10g/mol$

  4. None of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Applying faraday's first law-
$ \dfrac{Q}{F} = \dfrac{Wt}{Mwt}\times V.f.$
$ \dfrac{ne}{F}= \dfrac{1}{Mwt}\times 3$
$  Mwt         = \dfrac{1\times F\times 3}{ne}$
$  Mwt         = 10\dfrac{gm}{mol}$