Chemistry · Physics

Atomic Structure and Mass

308 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

Which of the following elements tend to have their atomic masses in decimals?

  1. Elements having different isotopes

  2. Elements having different isobars

  3. Elements with variable valencies

  4. Elements with tendency to form cations

  5. Elements with tendency to form anions
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

An element may have different isotopes. The mass of a natural element is taken as the average mass of all naturally occurring isotopes of that element.

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 $O^{18}/O^{16}$ ratio in some meteorites is greater than the used to calculate the average atomic mass of oxygen one earth. The average mass of an atom of oxygen in these meteorites is ______ that of terrestrial oxygen atom?

  1. equal to

  2. greater than

  3. less than

  4. none of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Oxygen-18 is heavier than Oxygen-16. Therefore, meteorites enriched in the heavier isotope O-18 compared to standard terrestrial oxygen will have a higher average atomic mass of oxygen than typical terrestrial samples.

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 $\text{A}$