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 ratio of the neutrons present in nitrogen atom and a silicon atom is: 

(Mass number of nitrogen and silicon is 14 and 28 respectively)

  1. 7 : 3

  2. 3 : 7

  3. 1 : 2

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

For nitrogen; atomic number = 7 and mass number = 14 

$\therefore$ Number of neutrons $= 14-7 = 7$
For silicon; atomic number = 14 and mass number = 28 
$\therefore$ number of neutrons $= 28-14= 14$
$\therefore$ Ratio of number of neutrons in nitrogen and silicon $= 7:14 = 1:2$

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

The ratio of mass of proton to electron is about:

  1. $1.8\times 10^{-3}$
  2. $1.8\times 10^{3}$

  3. 1

  4. 0
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Mass of proton = $1.6\times 10^{-27}$kg and mass of electron = $9.1\times 10^{-31}$kg
$\therefore\dfrac{\text{mass of proton}}{\text{mass of electron}} = \dfrac{1.6\times 10^{-27}}{9.1\times 10^{-31}} = 1.8\times 10^{3}$

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

The electronic configuration of an ion $M^{2+}$ is 2, 8, 14. If its mass 56, the number of neutrons in its nucleus is :

  1. 30

  2. 32

  3. 34

  4. 42
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$M^{2+}=2, 8, 14;\Rightarrow $ Electronic Configuration of $M=2, 8, 16$
So. total electrons in M will be $=26$
i.e., atomic number $(Z)=26$, Mass number (given) $=56$
$\therefore$ Number of neutrons$=A-Z=56-26=30$

Hence, the correct option is A.

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

The approximate atomic weight of electron or positron per mol is:

  1. $5.48\times 10^{-6} g mol^{-1}$
  2. $ 5.48\times 10^{-8} g mol^{-1}$
  3. $5.48\times 10^{-4} g mol^{-1}$

  4. None of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Mass of one electron $=9.1\times 10^{-27} g $

So, mass of one mole of electron $=m\times N _o$ $=5.48\times 10^{-4} g mol^{-1}$ as  $N _o = 6.023\times 10^{23}$.

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

Rest mass of an electron is $9.11\times 10^{-31}$ kg. Molar mass of the electron is:

  1. $1.50\times 10^{-31}kg$ $mol^{-1}$
  2. $9.11\times 10^{-31}kg$ $mol^{-1}$
  3. $5.5\times 10^{-7}kg$ $mol^{-1}$
  4. $6.02\times 10^{23}kg$ $mol^{-1}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Rest mass of an electron is $9.11\times 10^{-31}kg$.
So, mass of one mole of electron is $9.11\times 10^{-31}\times6.023\times10^{23} = 5.5\times 10^{-7}kg/mole$

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

Find the mass number of sodium having $12$ neutrons.

  1. $22$
  2. $23$
  3. $11$
  4. $12$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The atomic weight is basically a measurement of the total number of particles in an atom's nucleus. Mass number can be found by rounding off an elements atomic mass to the nearest whole number.
Mass Number $=$ (Number of Protons) $+$ (Number of Neutrons)
$= 11 + 12$
$= 23$

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

The number of protons is equal to ________ in a neutral atom.

  1. atomic weight

  2. atomic number

  3. number of electrons

  4. both B and C
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The atomic number is the number of protons in an atom of an element. Atoms are electrically neutral. That means that there must be a balance between the positively charged protons and the negatively charged electrons.


Thus, atoms must have equal numbers of protons and electrons.


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

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

The mass of electron, proton and neutron in grams are :

  1. $1.108\times 10^{-28}, 1.6726\times 10^{-24}, 1.675 \times 10^{-24}$
  2. $9.108\times 10^{28}, 1.6726\times 10^{24}, 1.675 \times 10^{24}$
  3. $9.108\times 10^{-24}, 1.6726\times 10^{-24}, 1.675 \times 10^{24}$
  4. $9.108\times 10^{-28}, 1.6726\times 10^{-24}, 1.675 \times 10^{-24}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

An electron is a tiny particle with a mass of $9.108\times 10^{-28}$ g and a negative charge. All neutral atoms contain electrons. The mass of a proton is $1.6726\times 10^{-24}$ g, or about 1836 times the mass of an electron. The proton carries a positive electrical charge, that is equal in magnitude to the charge of the electron but opposite in sign. Mass of a neutron is $1.675 \times  10^{-24}$ g. It is very close to that of the proton. A neutron carries no charge.

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

Find the number of neutrons in silver having atomic mass of $107.86$.

  1. $65$
  2. $71$
  3. $61$
  4. $47$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The atomic weight is basically a measurement of the total number of particles in an atom's nucleus. Mass number can be found by rounding off an elements atomic mass to the nearest whole number.
$Mass\ Number = (Number\ of\ protons) + (Number\ of\ neutrons)$
Number of neutrons $= mass\ number- number\ of\ protons$
Atomic mass of silver $= 107.86$
Mass number $= 108$
Number of protons $= 47$
Number of neutrons $= 108 - 47$
$= 61$
$\therefore$ Number of neutrons in a silver $=61$

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. $\dfrac{1}{9.108} \times 10^{32}$
  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 $ \displaystyle =9.108 \times 10^{-31}$ kg.

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

The number of moles of electrons that weigh one kilogram $ \displaystyle = \dfrac {1}{9.108   \times 6.023 \times 10^{-8}}= \dfrac {1}{9.108   \times 6.023}  \times 10^{8}$

Hence, the correct option is $D$
Multiple choice chemistry atom fundamental particles of an atom the structure of atoms discovery of subatomic particles

Mass of 1 mole neutrons $(m _n= 1.675 \times 10^{-27}\, kg)$ is:

  1. $1.8 \times 10^{-3}\, kg$
  2. $1.008 \times 10^{-4}\, kg$
  3. $1.08 \times 10^{-3}\, kg$
  4. $1.008 \times 10^{-3}\, kg$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Mass of one neutron $= 1.675 \times 10^{-27}$ kg


1 mole contains $6.02 \times 10^{23}$ neutrons

mass of 1 mole neutrons $= 1.675 \times 6.02 \times  10^{-27} \times 10^{23}$

                                         $=1.008 \times 10^{-3}$ kg