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 determination of atomic and isotopic mass some basic concepts of chemistry chemistry

There are two isotopes of an element with atomic mass $z$. Heavier one has atomic mass $z+2$ and lighter one has $z-1$, then an abundance of lighter one is:

  1. $66.6\%$
  2. $96.7\%$
  3. $6.67\%$
  4. $33.3\%$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Abundance of an isotope can be calculated as :


Let abundance of isotope with atomic mass $Z+2$ is x
Then abundance of isotope $Z-1$ is $1-x$

$(Z+2)x+(Z-1)(1-x)\quad =Z$

On solving this, we get
$x=\dfrac { 1 }{ 3 } $
Therefore,% abundance of isotope with atomic no.$Z+2$ is =$=\dfrac { 1 }{ 3 } \times 100=33.33$%
Therefore,% abundance isotope with atomic no. $Z-1$ is =$100-33.33=67.66%$%
Therefore isotope with atomic $Z-1$ is higher in % abundance

Multiple choice determination of atomic and isotopic mass some basic concepts of chemistry chemistry

$ _{ 17 }^{ 35 }{ Cl }$ and $ _{ 17 }^{ 37 }{ Cl }$ are two isotopes of chlorine. If average atomic mass is $35.5$ then ratio of these two isotopes is:

  1. $35 : 37$
  2. $1 : 3$
  3. $3 : 1$
  4. $2 : 1$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Solution:- (C) $3 : 1$

Let $x$ and $y$ be the fraction of ${ _{17}^{35}{Cl}}$ and ${ _{17}^{37}{Cl}}$ in ${ _{17}^{35.5}{Cl}}$.
Therefore,
Average at. mass $= \cfrac{x \times 35 + y \times 37}{x + y}$
$35.5 = \cfrac{35x + 37y}{x+y}$
$\Rightarrow 35.5 x + 35.5 y = 35x + 37y$
$\Rightarrow 35.5 x - 35x = 37 y - 35.5 y$
$\Rightarrow 0.5 x = 1.5 y$
$\Rightarrow \cfrac{x}{y} = \cfrac{1.5}{0.5} = \cfrac{3}{1}$
Hence the ratio of ${ _{17}^{35}{Cl}}$ and ${ _{17}^{37}{Cl}}$ in ${ _{17}^{35.5}{Cl}}$ is $3 : 1$.

Multiple choice determination of atomic and isotopic mass some basic concepts of chemistry chemistry

If isotopic distribution of $ C-12 $ and $ C-14 $ is  98 %  and  2 %  respectively, what would be the number of $ C-14 $ isotope in $ 12 gm $  carbon sample?

  1. $ 1.032 \times 10^{22} $
  2. $ 3.01 \times 10^{23} $
  3. $ 5.88 \times 10^{23} $
  4. $ 6.02 \times 10^{23} $
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

1 mole of carbon contains $N _A$ molecules.


$14g$ of $C-14$ contains $6.023 \times 10^{23}$ atoms

$1g$ of $C-14= \dfrac{6.023 \times 10^{23}}{14}$


$12g$ of $C-14$ atom contains $=\cfrac {6.023 \times 106{23}\times 12}{14}$

                                                  $=5.16 \times 10^{23}$ atoms

Now $2$% of $5.16 \times 10^{23}=\cfrac {5.16 \times 10^{23}\times 2}{100}$

                                         $=1.032 \times 10^{22}$ atoms .

Multiple choice determination of atomic and isotopic mass some basic concepts of chemistry chemistry

Chlorine has two naturally occurring isotopes, $^{35}Cl$ and $^{37}Cl$. If the atomic mass of Cl is 35.5 the ratio of natural abundance of $^{35}Cl$ and $^{37}Cl$ is closest to :

  1. $ 3 : 5 $
  2. $3 : 1$
  3. $2 : 5 $
  4. $4 : 1$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let x be the fraction of Cl-35. Then 35x + 37(1-x) = 35.5. 35x + 37 - 37x = 35.5. -2x = -1.5. x = 0.75. The ratio of Cl-35 to Cl-37 is 0.75 / 0.25 = 3:1.

Multiple choice determination of atomic and isotopic mass some basic concepts of chemistry chemistry

The average atomic mass of copper is $63.546$ amu .Natural copper consists of two iostopes: $^{63} Cu$ and $^{65} Cu$.Their natural abundances are $69.09\%$ and $30.91\%$ respectively. If the mass of $^{63} Cu$ isotope is $62.9298$ amu ,What is the mass of $^{65} Cu$ isotope?

  1. 64.9000

  2. 65.1233

  3. 64.9233

  4. 65.1933
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Average mass = (mass1 * abundance1) + (mass2 * abundance2). 63.546 = (62.9298 * 0.6909) + (mass2 * 0.3091). 63.546 = 43.4779 + 0.3091 * mass2. 20.0681 = 0.3091 * mass2. mass2 = 64.9243.

Multiple choice determination of atomic and isotopic mass some basic concepts of chemistry chemistry

 Rhenium (Re) consists of $37.1$% $185$ Re and $62.9$% $187$ Re. Calculate the relative atomic mass?

  1. $185.6$
  2. $185.9$
  3. $186.3$
  4. $186.1$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Relative atomic mass is calculated as (abundance1 * mass1 + abundance2 * mass2) / 100. For Rhenium: (37.1 * 185 + 62.9 * 187) / 100 = (6863.5 + 11762.3) / 100 = 18625.8 / 100 = 186.258, which rounds to 186.3.

Multiple choice determination of atomic and isotopic mass some basic concepts of chemistry chemistry

The relative atomic mass of an atom is:

  1. measured in atomic mass units (u)

  2. based on the mass of 1 atom of carbon-12

  3. different for different isotopes of an element

  4. all of the above are true
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Relative atomic mass is the mass of an atom measured relative to 1/12th the mass of 1 atom of C-12 isotope which is also known as atomic mass unit or amu(u).

Multiple choice determination of atomic and isotopic mass some basic concepts of chemistry chemistry

Boron found in nature has an atomic weight of 10.811 and is made up of the isotopes $\displaystyle { B }^{ 10 }$ (mass 10.013 amu) and $\displaystyle { B }^{ 11 }$ (mass 11.0093). What percentage of naturally occurring boron is made up of $\displaystyle { B }^{ 10 }$ and $\displaystyle { B }^{ 11 }$, respectively?

  1. 30 : 70

  2. 25 : 75

  3. 20 : 80

  4. 15 : 85
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let the abundance of $B^{11}$ be $x$% and $B^{10}$ be $(100-x)$%

Average atomic mass= [Atomic mass of $B^{11} \times$ abundance + Atomic mass of $B^{10}\times$ abundance]$/100$ 
$\Rightarrow 10.811=\cfrac { 11.0093\times x(percent)+10.013\times (100-x)(percent) }{ 100 } $
$\Rightarrow 10.811\times 100= 11.0093x$%$+1001.3-10.013x$%
$\Rightarrow 1081.1=0.9963x$%$+1001.3$
$\Rightarrow 1081.1-1001.3=0.9963x$%
$\Rightarrow 79.8=0.9963x$%
$\Rightarrow x$%=$\cfrac {79.8}{0.9963}$
$\therefore x$%=$80$
$(100-x)$%=$20$
$\therefore$ Natural abundance of $B^{10}=20$
    Natural abundance of $B^{11}=80$
Ratio= $20:80$

Multiple choice ionic bond atomic structure and chemical bonding chemistry

A neutral atom of an element has a nucleus with nuclear charge $11$ times and mass $23$ times that of hydrogen. The element can form a stable charged ion by:

  1. losing 1 electron

  2. losing 2 electrons

  3. gaining 1 electron

  4. gaining 2 electrons
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Atomic number of hydrogen $= 1$.

$\therefore$ The atomic number of neutral atom $= 11$ & electronic configuration $= 2, 8, 1$. 
Therefore, on losing one electron, the atom forms the stable noble gas configuration as $2, 8$.

So, the correct option is A.

Multiple choice nanoscience and nanotechnology nanotechnology logic gates

"Fullerene" is made up of how many atoms ?

  1. 40

  2. 60

  3. 20

  4. 10
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
  1. Fullerene is another form of carbon allotropes.
    2. It can be identified as $C-60$ since, it is made of $60$atoms, arranged in the shape of a football.
    3. Since, this look like the geodesic dome designed by the US archited Buckminster Fuller, the molecule was named Fullerence.