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 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 limitations of bohr model and explanation of bohr's second postulate by matter waves bohr's model atoms atomic nuclei physics

Maximum energy is evolved during which of the following transitions?

  1. $n=1$ to $n=2$
  2. $n=2$ to $n=6$
  3. $n=2$ to $n=1$
  4. $n=6$ to $n=2$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

When transition is from upper state to lower state, the energy is evolved.
Energy is emitted only when an electron jumps from an outer stationary orbit of energy ${ E } _{ 2 }$ to inner stationary orbit of energy ${ E } _{ 1 }$. The energy emitted $E={ E } _{ 2 }-{ E } _{ 1 }$
In given options only (C) and (D) are practicable.
In option (C),
$E=-Rhc\left( \dfrac { 1 }{ { 1 }^{ 2 } } -\dfrac { 1 }{ { 2 }^{ 2 } }  \right) $
     $=-\dfrac { 3 }{ 4 } Rhc$
In option (D),
$E=-Rhc\left( \dfrac { 1 }{ { 2 }^{ 2 } } -\dfrac { 1 }{ { 6 }^{ 2 } }  \right) $
    $=-\dfrac { 2 }{ 9 } Rhc$
Thus, maximum energy is evolved in option (C).
Note: The process in which absorption of energy by an electron takes the electron from an inner orbit to some outer orbit of higher energy is called excitation.

Multiple choice physics dual nature of matter and radiation davisson and germer experiment and its conclusion matter waves wave nature of matter

In Davisson-Germer experiment, electron gun is coated with oxide of _______ .

  1. Barium

  2. Sodium

  3. Potassium

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

The Davisson and Germer experiment consisted of Nickel crystal (target), electron gun and a detector. The electron gun consisted of tungsten filament with high and low tension batteries. The work function of uncoated tungsten filament is 4.5 eV. The coating of Barium oxide on this filament reduces its work function so that electron can be emitted at low temperature.

Multiple choice chemistry atoms, molecules and radicals history of atomic model atomic theory of dalton daltons atomic theory

Dalton's theory of same element have same mass and properties was proved wrong with the invention of:

  1. molecules

  2. isotopes

  3. neutrons

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

According to Dalton Atomic Theory, atoms of an element are identical in mass, size and many other chemicals or physical properties. 


But, practically we observe that atoms of several elements differ in their densities and masses. 

These atoms with the different masses are known as isotopes. For example, Chlorine (Cl) has 2 isotopes with the mass numbers of 35 and 37.

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

Multiple choice chemistry atoms, molecules and radicals history of atomic model atomic theory of dalton daltons atomic theory

Which of the following is not a postulate of Dalton's atomic theory? 

  1. Different elements have atoms of different masses.

  2. Atoms combine in simple whole number ratios to form compounds..

  3. Atoms are not created or destroyed in chemical change.

  4. Atoms of a single element can have different masses.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Postulates of Dalton's atomic theory:

  • All matter consists of indivisible particles called atoms.
  • Atoms of the same elements are similar in shape and mass but differ from the atoms of other elements.
  • Atoms cannot be created or destroyed.
  • Atoms of different elements may combine with each other in a fixed, simple, whole number ratios to form compound atoms.
  • The atom is the smallest unit of matter that can take part in a chemical reaction.

Multiple choice physics nuclei gamma decay change in nucleus due to radioactive decay alpha, beta and gamma particles (rays) and their properties

When writing electron configurations,electrons are represented in their lowest possible energy state.Which of the following state is this ?

  1. excited state.

  2. lowest state.

  3. configuration state.

  4. ground state.

  5. unenergetic state.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

According to Aufbau's principle, electrons are filled in the atomic orbitals in order of lower energy to the higher energy starting from the orbital of lowest possible energy known as the ground state.

Multiple choice gamma decay change in nucleus due to radioactive decay alpha, beta and gamma particles (rays) and their properties

The atomic masses of the hydrogen isotopes are
$m _1H^1=1.007825\ amu$
$m _1H^2=2.014102\ amu$
$m _1H^3=3.016049\ amu$
The energy released in the reaction
$ _1H^2+ _1H^2 \rightarrow _1H^3+ _1H^1$
is nearly:

  1. $1\ MeV$
  2. $2\ MeV$
  3. $4\ MeV$
  4. $8\ MeV$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Mass defect: $\Delta M = 2m _1H^2 -(m _1H^1+m _1H^3)$
$\therefore$   $\Delta M = 2(2.014202)  -(1.007825+3.016049) = 4.33\times 10^{-3}$  amu
Energy released, $E = \Delta M\times 931$  $MeV$         
$\implies$   $E = 4.33\times 10^{-3}\times 931 \approx 4MeV$
Multiple choice chemistry what is inside atom discovery of neutrons structure of atoms discovery of subatomic particles

The mass of an atom is : 

  1. the sum of the masses of protons and electrons present in the nucleus.

  2. equal to the mass of neutrons in the nucleus.

  3. the sum of the masses of protons and neutrons present in the nucleus.

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

The mass of an atom is given by the sum of the masses of protons and neutrons present in the nucleus.

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

Who determined the mass of a neutron?

  1. John Dalton (early 1800's)

  2. Ernest Rutherford (1911)

  3. Niels Bohr (1913)

  4. James Chadwick (1932)
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The presence of neutron and its mass was determined by James Chadwick. Neutron was discovered by bombarding $\alpha-$particles on $Be$. On doing so, highly energetic beam of radiation was observed by Chadwick. Thus mass of neutron was determined by James Chadwick.

Multiple choice physics some natural phenomena how lightning occurs? the story of lightning advanced of lightning

There are $4\times 10^{23}$ atoms in a metal sphere. If one electron is removed from 0.1% atom. what will be the charge on the sphere?

  1. $64C$
  2. $6.4C$
  3. $0.64C$
  4. $-6.4C$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Total atoms = 4 x 10^23. Atoms with an electron removed = 0.1% of 4 x 10^23 = 4 x 10^20. Charge = (number of electrons) x (charge per electron) = (4 x 10^20) x (1.6 x 10^-19 C) = 64 C.

Multiple choice chemistry lattice energy born-haber cycle born-haber cycles energy cycles

The ionization potential of ithium is 520 KJ/ mole .The energy required to convert 70 mg of lithum atoms in gaseous state into $Li^{+}$ ions is ______________.

  1. 52 KJ

  2. 5.2 KJ

  3. 520 KJ

  4. 52 J
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The ionization potential is 520 kJ/mol. Molar mass of Li is 7 g/mol. 70 mg is 0.07 g, which is 0.01 moles. Energy required = 0.01 moles * 520 kJ/mol = 5.2 kJ.