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 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

The total number of electrons present in 1.8 g of water is :

  1. $6.023\times 10^{22}$
  2. $10.8576\times 10^{23}$
  3. $10.8576\times 10^{22}$
  4. $6.023\times 10^{23}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

We know that, 1 molecule of water consists of 10 electrons (2 electrons each of two hydrogen atoms and 8 electrons of one oxygen atom).
The number of molecules present in 1.8 g of water.
$18 : g : of : H _2O\xrightarrow[]{contains}6.023\times 10^{23}molecules$
$\Rightarrow1.8 : g : of : H _2O : contains : '! x' : molecules$
$x=\dfrac{1.8\times 6.023\times 10^{23}}{18}$
$=\dfrac{ 6.023\times 10^{23}}{10}=6.023\times 10^{22}molecules$.
$\therefore$ Number of electrons in 1.8 g of water $=6.023\times 10^{22}\times10=6.023\times 10^{23}$

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 structure of the atom neutrons discovery of neutrons structure of atoms

Which of the following has the greatest mass?

  1. Electron

  2. Proton

  3. Neutron

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

Neutron has greatest mass. Proton is a subatomic particle with symbol p and positive electric charge of +1 elementary charge and mass slightly less than that of neutron. Mass of neutron is $1.6749\times10^{-27}  kg$.

Multiple choice chemistry what is inside atom neutrons discovery of neutrons structure of atoms

Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

  1. 4

  2. 2

  3. 8

  4. 6
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Helium has 2 neutrons. The mass of an atom is given by the sum of the masses of protons and neutrons present in the nucleus. Since, the number of protons present in helium is 2 and given mass is 4. Therefore, the number of neutrons will be equal to $2$ i.e. $4-2.$

Multiple choice chemistry what is inside atom neutrons discovery of neutrons structure of atoms

Carbon has six protons and six neutrons in its nucleus. What is the atomic mass of carbon?

  1. 4

  2. 12

  3. 8

  4. 6
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The atomic mass of carbon is $12$. The mass of an atom is given by the sum of the masses of protons and neutrons present in the nucleus. Since, the number of protons and neutrons present in carbon is $6$.
Therefore, the atomic mass of carbon is $12$ i.e. $6 + 6=12$.

Multiple choice chemistry what is inside atom neutrons discovery of neutrons structure of atoms

Oxygen atom has an atomic mass of 16 u and eight neutrons in its nucleus. What is its atomic number?

  1. 4

  2. 12

  3. 8

  4. 6
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The atomic number of oxygen is 8. The mass of an atom is given by the sum of the masses of protons and neutrons present in the nucleus. Also, atomic number is the number of protons present in the nucleus. Since, the number of neutrons present in oxygen is 8 and given mass is 16. Therefore, the number of neutrons will be equal to $8$ i.e. $16-8$.

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

Which of the following atoms contains the least number of neutrons?  

  1. $ _{92}^{235}{U}$
  2. $ _{92}^{238}{U}$
  3. $ _{93}^{239}{Np}$
  4. $ _{93}^{240}{Np}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

 The number of neutrons is the difference between the mass number and atomic number.
   
$\displaystyle _{92}^{235}{U}$ contains the least number of neutrons

The number of neutrons in $\displaystyle _{92}^{235}{U}$ $\displaystyle = 235-92=143$

The number of neutrons in $\displaystyle _{92}^{238}{U} $ $\displaystyle =238-92=146$

The number of neutrons in $\displaystyle _{93}^{239}{Np}$ $\displaystyle = 239-93=146$

The number of neutrons in  $\displaystyle _{93}^{240}{Np}$ $\displaystyle =240-93=147$

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

The mass of neutron is of the order of: 

  1. ${10}^{-27} kg$
  2. ${10}^{-26} kg$
  3. ${10}^{-25} kg$
  4. ${10}^{-24} kg$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The mass of neutron is of the order of $\displaystyle {10}^{-27} kg$

It is equal to $\displaystyle 1.674 \times {10}^{-27} kg$ or $\displaystyle 1.674 \times {10}^{-24} g$. It is also equal to 1.0087 amu.

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

If A = Atomic mass, Z = Atomic number, then number of neutrons is equal to ?

  1. $A - Z$
  2. $A + Z$
  3. $Z - A$
  4. $\frac{Z}{A}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

If A = Atomic mass, Z = Atomic number then the number of neutrons is equal to: 


$ A$ = Atomic mass = Number of protons + Number of neutrons

$Z$ = Atomic Number = Number of protons$

$A$ = $Z$ + Number of neutrons

Number of neutrons = $Z-A$

Multiple choice chemistry p- block elements-ii group 17 elements - general properties group 17 elements group 17 elements - properties

Chlorine has two naturally occuring isotopes $ _{ 17 }^{ 37 }{ Cl }$ and $ _{ 17 }^{ 35 }{ Cl }$ and average atomic mass of chlorine is $35.5$. The percentage of $ _{ 17 }^{ 37 }{ Cl }$ is:

  1. 25%

  2. 75%

  3. 40%

  4. 60%
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Let x be the percentage of Cl-37. Then (100-x) is the percentage of Cl-35. The average atomic mass is (37x + 35(100-x))/100 = 35.5. Solving this gives 37x + 3500 - 35x = 3550, so 2x = 50, x = 25%.

Multiple choice chemistry basic concepts of chemistry law of multiple proportion law of multiple proportions laws of chemical combination

The ratio between the different weights of oxygen in different compounds which combine with the same weight of nitrogen ($14$ parts) is $8:16:24:32:40$ or $1:2:3:4:5$.
This is simple whole number which supports the law of:

  1. conservation of mass

  2. multiple proportion

  3. constant composition

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

The answer is multiple proportion.
When two elements combine to from two or more than two compounds, the weights of one of the element which combine with a fixed weight of the other, bear a simple whole number ratio is the law of multiple proportion and the given example supports the law.

Multiple choice chemistry basic concepts of chemistry law of multiple proportion law of multiple proportions laws of chemical combination

Carbon and oxygen combine to form two oxides, carbon monoxide and carbon dioxide in which the ratio of the weights of carbon and oxygen is respectively 12 : 16 and 12 : 32. These figures illustrate the:

  1. Law of multiple proportions

  2. Law of reciprocal proportions

  3. Law of conservation of mass

  4. Law of constant proportions
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
When two elements combine to form two or more chemical compounds, then the masses of one of the elements which combined with a fixed mass of the other, bear a simple ratio to one another. For eg, Carbon combines with oxygen to form two compounds namely carbon dioxide and carbon monoxide.

In carbon dioxide, 12 parts by mass of carbon combine with 32 parts by mass of oxygen while in carbon monoxide, 12 parts by mass of carbon combine with 16 parts by mass of oxygen. The masses of oxygen which combined with a fixed mass of carbon in carbon monoxide and carbon dioxide are 16 and 32. These masses of oxygen bear a simple ratio of 16:32 or 1:2 to each other.

Hence, the correct option is $A$.