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 chemistry atom fundamental particles of an atom the structure of atoms discovery of subatomic particles

The molar mass of an electron is:

  1. $6.023\times10^{23}$ g/mol
  2. $5.486\times10^{-4}$ g/mol
  3. $9.108\times10^{-28}$ kg/mol
  4. $9.108\times10^{-24}$ g/mol
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Mass of an electron $=9.1\times 10^{-28}$g
1 mole $=6.02\times 10^{23}$ electrons
$\therefore$ molar mass of electron $=(9.1\times 10^{-28}) \times (6.02\times 10^{23}) = 5.48\times 10^{-4}$ g/mole

Multiple choice chemistry atom fundamental particles of an atom the structure of atoms discovery of subatomic particles

How many moles of electron weighs one kilogram?

  1. $6.023\times 10^{23}$
  2. $\dfrac {1}{9.108}\times 10^{31}$
  3. $\dfrac {6.023}{9.108}\times 10^{54}$
  4. $\dfrac {1}{9.108\times 6.023}\times 10^8$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Mass of one electron is $9.108\times 10^{-31} kg$.


Mass of one mole of electrons is ${9.108 \times 10^{-31} \times 6.023}\times 10^{23}={9.108\times 6.023}\times 10^{-8}$.

Thus, $1$ kg corresponds to $\dfrac {1}{9.108\times 6.023}\times 10^8$ moles of electrons.

Hence, the correct option is $D$

Multiple choice chemistry atom fundamental particles of an atom the structure of atoms discovery of subatomic particles

How many moles of electrons weigh one kilogram? 

(Mass of electron = $\displaystyle 9.108\times 10^{-31}kg $; Avagadro number = $\displaystyle 6.023\times 10^{23}kg $)

  1. $\displaystyle \frac{1}{9.108\times 6.023}\times 10^{8} $
  2. $\displaystyle 6.023\times 10^{23}$
  3. $\displaystyle \frac{1}{9.108}\times 10^{31}$
  4. $\displaystyle \frac{6.023}{9.108}\times 10^{54}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Mass of electron $\displaystyle 9.108\times 10^{-31}kg$.
The number of electrons that weigh 1 kg will be $\frac {1}{\displaystyle 9.108\times 10^{-31}}$
The Avagadro number is $\displaystyle 6.023\times 10^{23}$.    
The number of moles of electrons that weigh 1 kg will be
$\frac {1}{\displaystyle 9.108\times 10^{-31} \times \displaystyle 6.023\times 10^{23}}=\displaystyle \frac{1}{9.108\times 6.023}\times 10^{8}$

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. Such an arrangement of elements called as :

  1. Dobereiner triad.

  2. Mendeleev triad.

  3. Modern triad.

  4. none of the above.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Dobereiner observed in few atoms that mass of one atom is equal to the average of two atoms. He defined almost all properties basing on atomic weight.Later on it was proved wrong any-ways. Hence, this is called Dobereiner triad. Lithium has 7, sodium has 23 where as potassium has 39 as atomic masses.
$ 23 = (7+39)/2 = 23$

Here, we could observe that sodium atomic mass is average of potassium and lithium masses. 

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

Which of the following sets of atomic masses do not correspond to Dobereiner's triad?

  1. 7, 23, 39

  2. 20, 38, 56

  3. 47, 91, 178

  4. 54, 95, 183
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation

The atomic mass of the middle element should be the average of the atomic masses of the first and third elements. In case of C and D options, the atomic masses do not correspond to that rule.

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

Atomic wt. of P is 31 and Sb is 120. What will be the atomic wt. of As, as per Dobernier triad rule?

  1. $151$
  2. $75.5$
  3. $89.5$

  4. Unpredictable
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Atomic wt. of P is 31 and Sb is 120. The atomic wt. of As, as per Dobernier triad rule will be average of two. It will be $\displaystyle \frac {31+120}{2} = \frac {151}{2} =75.5 $

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

If three elements X, Y, and Z form a Dobereiner's triad and atomic weights of X and Z are 9 and 40 respectively, then the atomic weight of the element Y is approximately:

  1. 24.5

  2. 49

  3. 34.5

  4. 29
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

According to Dobereiner, the atomic weight of the central element of the triad is the arithmetic mean of the atomic weights of the other two members. 


Therefore, atomic weight of Y = $\cfrac{9 + 40}{2} = 24.5\ g$


Hence, option A is correct.

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

Three elements X, Y and Z form a Dobereiner triad. The ratio of the atomic weight of X to that of Z is 7: 25.


 If the sum of the atomic weights of X and Z is 160, find the atomic weights of X, Y and Z.

  1. X $\rightarrow$ 35, Y $\rightarrow$ 80, Z $\rightarrow$ 125
  2. X $\rightarrow$ 125, Y $\rightarrow$ 80, Z $\rightarrow$ 35
  3. X $\rightarrow$ 80, Y $\rightarrow$ 35, Z $\rightarrow$ 125
  4. X $\rightarrow$ 80, Y $\rightarrow$ 125, Z $\rightarrow$ 35
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Let weight of X be $7a$  $\therefore$ Atomic weight of $Z=25a$

$\therefore7a+25a=160$

$\Rightarrow a=5$

Weight of X$=35$ and of Z is $125$

Atomic weight of Y is arranged of X and $2=\cfrac { 35+125 }{ 2 } =80$

Hence, the correct option is $\text{A}$
Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

Which of the following sets of atomic masses do not correspond to Dobernier traid?

  1. $7, 23, 39$
  2. $20, 38, 56$
  3. $47, 91, 178$
  4. $54, 95, 183$
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation

The following sets of atomic masses do not correspond to Dobernier traid.
(C) $\displaystyle 47, 91, 178$
(D) $\displaystyle 54, 95, 183$
In these sets, the atomic mass of middle element is not equal to the mean of atomic masses of first and last element.
$\displaystyle 91 \neq \dfrac {47+178}{2}$
$\displaystyle 95 \neq \dfrac {54+183}{2} $

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

In a triad of $A, B, C$ elements if the atomic masses of $A$ and $C$ respectively are $100$ and $200$, then the atomic mass of $B$ is:

  1. $300$
  2. $175$
  3. $125$
  4. $150$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

According to Dobereiner's law of traid, the average of the masses of the first and the last element is equal to the atomic mass of the middle element. So, $\dfrac{100+200}{2}$ = 150. Hence, the atomic mass of B is 150.


Hence, the correct option is $\text{D}$

Multiple choice chemistry the periodic table dobereiner's triads development of the modern periodic table into the history

When a Dobereiner's triad is considered, the sum of atomic weights of the terminal elements $X$ and $Z$ is 162.5. The atomic weight of $Z$ is 50 units less than 5 times that of $X$. Identify the periods of $X, Y$ and $Z$ respectively in the present periodic table.

  1. $3,4,5$
  2. $4,5,6$
  3. $2,3,4$
  4. $3,5,4$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Let the atomic weights (in g/mol) of elements X, Y and Z be x, y and z respectively.
The atomic weight of Z is 50 units less than 5 times that of X.

$\displaystyle z = 5x-50$.....(1)

The sum of atomic weights of the terminal elements X and Z is 162.5
$\displaystyle x+z=162.5$
$\displaystyle z = 162.5-x$......(2)

Substitute (2) into (1)
$\displaystyle 162.5-x=5x-50$
$\displaystyle 162.5=6x-50$
$\displaystyle 6x=212.5$
$\displaystyle x=35.4$ g/mol

Substitute this in equation (2)
$\displaystyle z = 162.5-x = 162.5-35.4=127.1$ g/mol.

The atomic weight of Y is the arithmetic mean of the atomic weights of X and Z.
$\displaystyle y = \dfrac {x+z}{2}$
$\displaystyle y = \dfrac {35.4+127.1}{2}=81.3$ g/mol.

From the periodic table, we see that the elements with atomic weights 35.4, 81.3 and 127.1 belong to 3 (chlorine), 4 (bromine) and 5 (iodine).

Hence, the option (A) is the correct answer.

Multiple choice chemistry structure of the atom neutrons discovery of neutrons structure of atoms

Number of neutrons present in $1.7$ g of ammonia is:

  1. $0.8N _A$
  2. $0.7N _A$
  3. $0.652N _A$
  4. $0.52N _A$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
Let us consider the problem.
For, moles of $N{H _3} = \frac{{1.7}}{{17}} = 0.1$
Let the Neutrons in 1 molecules of $N{H _3} = 7 + 3 \times 0 = 7$
Hence,
Number of Neurons $= 0.1 \times {N _A} \times 7$
The correct answer is $0.7{N _A}$
Multiple choice chemistry structure of the atom neutrons discovery of neutrons structure of atoms

Calculate number of neutrons present in $12 \times 10^{25}$ atoms of oxygen $( _8O^{17})$:
(Given : N$ _A = 6 \times 10^{23}$)

  1. $1800$
  2. $1600$
  3. $1800\ N _A$
  4. $3200\ N _A$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Atomic number =$ P =8$ 


Atomic Mass =$ N+P=17$


                         $N=9$
 Total no. of Neutrons = $9\times 12\times 10^{25}$

                                     =$\dfrac{9\times 12\times 10^{25}\times N _{A}}{6\times 10^{23}}$ 

                                 $=1800N _{A}$

Hence, the correct option is C.