Atoms And Nuclei, Important Details and Top MCQs

This section deals with atoms and nuclei. It has also some important MCQS on the topic. However, before jumping to solve those MCQs, we urge you to revise the topic first.

Atomic And Nuclear Physics

Atom

An atom is the smallest unit of ordinary matter that forms a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. There have been various theories that have been formulated from time to time to understand the nature and structure of atom. Some of those theories are listed below.

Theory of the universe by Democritus:

Democritus, who was a Greek philosopher, was the first person to use the term atom (atomos: meaning indivisible). He said that Atom is the fundamental or basic unit of matter. According to his theory:

1. All matter consists of very tiny structures called atoms, which are too small to be seen by a naked eye.
2. Atoms are completely solid and there is no empty space in an atom.
3. Atoms have no internal structure.
4. Each atom (of a different substance) is different in size, weight and shape.

John Dalton’s Atomic Model:

According to Dalton’s atomic model,

1. All matter consists of tiny particles called atoms. This is same as the above theory.
2. Atoms are indestructible and unchangeable.
3. When elements react, it is their atoms that combine to form new compounds.

Thomson’s Plum Pudding Model

After carrying out a cathode ray tube experiment, Thomson put forwards his plum pudding model.

According to this model;

1. Each atom is a sphere filled consisting of positive and negative charges.
2. The positive charge is uniformly distributed inside this sphere, where as negative charged particles (which he called corpuscles and were later named as electrons) are arranged just like there are plums in a cake pudding, or the seeds in a watermelon.

Rutherford’s Model Of Atom

After performing his famous scattering experiment, Rutherford also formulated an atomic model called Rutherford’s model of atom.

According to this model:

1. An atom has both negative charge as well as the positive charge.
2. The positive charge is not unifromly distributed inside an atom. It is situated in a very small sphere located at the core of an atom, and he called it nucleus.
3. Rutherford stated that the atom was like a mini solar system and that the electrons orbited the nucleus in a wide orbit. That is why it is also known as the planetary model. These orbits can be of any energy and radius.
4. Most of the space inside an atom is empty, and almost its entire mass is located inside the nucleus. That is nucleus is very heavy.

Rutherford’s model of atom could not explain the stability of an atom and neither could it explain the existence of the emission spectrum. Hence, a new model was proposed by Bohr.

Bohr’s Model Of Atom

According to this model;

1. Electrons orbit the nucleus in orbits, but not in all orbits. Electrons can orbit the nucleus in only such orbits that have a particular size (radius) and energy.
2. The lower the energy of the electron, the lower the orbit.
3. This means that as electrons fill up the orbitals, they will fill the lower energy level first.
4. An electron emits energy in the form of radiation, when it jumps from any higher orbit to a lower orbit. Conversely, we need to supply energy to an electron if it wants to go from a lower orbit to a higher orbit.

This model also had some flaws. It could be applied only to Hydrogen atom and Hydrogen like atoms. In adition, it was later found that it could not explain Zeeman and Stark effect etc. Hence, a new model was put forth by Schrodinger. We will not discuss that model here.

Electrons, Protons and Neutrons

The details about the neutron, proton and electron are given below;

Electron

• Electron was discovered by J.J. Thomson in 1886 while he was doing cathode ray experiment.
• It is negatively charged and its charge is represented by -e, which corresponds to a charge of -1.602 × 10^-19 C.
• It is the lightest of the three particles.

Proton

• Proton was discovered by Ernst Rutherford.
• It is a positively charged subatomic particles. The charge of a proton is represented as +e, which corresponds to approximately 1.602 × 10^-19 C
• The mass of a proton is approximately 1.672 × 10^-24
• Protons are over 1800 times heavier than electrons.
• No of protons in an atom is known as its Atomic Number.
• In an atom, number of protons is always equal to the number of electrons, and hence an atom is always neutral in nature.
• It resides inside the nucleus of an atom.

Neutron

• It was discovered by james Chadwick.
• Its mass is almost the same as that of a proton i.e. 1.674×10^-24
• Neutron is an electrically neutral particle.
• It resides inside the nucleus of an atom.

Since neutrons and protons stay in nucleus, they are therefore together known as nucleons. The total number of nucleons in an atom is known as its Mass Number.

Isotopes and Isobars

The atoms that have the same number of protons but different number of neutrons are known as isotopes. Example ₁H¹, ₁H² and ₁H³ are the three isotopes of Hydrogen. Isotopes have same atomic numbers but different mass numbers.

The atoms that have same mass numbers but different atomic numbers are known as isobars. Example ;₁₈Ar⁴⁰ and ₁₉K⁴⁰ are isobars. Isobars have same number of nucleons but different number of protons.

Radioactivity

The property of certain atoms, by virtue of which they emit energy and subatomic particles spontaneously, is known as radioactivity. The atoms that show radioactivity are known as radioactive atoms. Example. An isotope of Hydrogen known as Protium (_1H³).

Radioactivity was discovered by Henry Becquerel.

During radioactive emission, atoms releases three kinds of radiations. They are Alpha particles, Beta Particles and Gamma Rays.

Alpha particles are doubly ionised Helium atoms and hence positively charged and heavier in nature.

Beta particles are actuallt fast moving electrons and hence negatively charged.

Gamma rays are simply photons and hence neutral in nature.

Half life in radioactivity

Half life is the time during which the number of radioactive nuclei reduces to half. If initially we have 100 radioactive nuclei, and after 5 minutes, the number of undecayed nuclei is 50, then the half life for such a material is 5 minutes.

Half life of Radioactive Carbon (_6C¹⁴) is 5,730 years. This radioactive carbon is used in estimating the age of fossils and it, as a technique is known as Radioactive Carbon Dating.

Radiocarbon dating is used to determine the age of carbon bearing materials like wood, animal fossils etc. It is owing to the reason that plants take in radioactive carbon during photosynthesis, and the same radioactive carbon reaches animals through feeding upon such plants.

Uranium dating is used to determine the age of Earth, minerals and rocks.

Nuclear Fission

The phenomenon of splitting of a heavier nuclei into two or more lighter but stabler nuclei is known as Nuclear Fission Reaction or simple Nuclear Fission.

Nuclear fission was first demonstrated by Halin and Fritz Strassmann.

The materials that can undergo fission reaction are known as fissionable or fissile materials.

The minimum amount of fissile material required to sustain a chain reaction is known as Critical Mass. The critical mass of Uranium-235 is about 47Kg.

During fission reaction, a lot of energy is released. It is employed in Atom Bombs and Nuclear Reactors.

First nuclear reactor was established in Chicago University under the supervision of Prof Enrico Fermi.

Nuclear Fusion

When two or more light nuclei combined together to form a heavier nucleus is called as nuclear fusion.

For the nuclear fusion, a temperature of the order of 10⁸ K is required.

During Nuclear fusion, a lot of energy is required. This reaction is employed in Sun and Hydrogen Bomb.

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MCQs On Atomic and Nuclear Physics

This set contains many important MCQs on atoms and nuclei for competitive exams. Solve these important MCQs on atoms and nuclei to check and enhance your level of preparation for competitive exams.

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Who discovered neutron?

1. Rutherford
2. Chadwick
3. J J Thomson
4. Fermi

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Which of the following is are neutral in nature.

1. Alpha particle and neutron
2. Gamma ray and neutron
3. Neutron and Beta particle
4. All of the above

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What is critical mass?

1. Minimum mass required to show radioactivity.
2. Minimum mass required to emit cathode rays.
3. Minimum mass required to sustain a chain reaction.
4. Minimum mass of a hydrogen bomb to emit energy.

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Nuclear fission reaction is employed in:

1. Nuclear reactors
2. Sun
3. Hydrogen bomb
4. None

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Nucleons refer to:

1. protons
2. neutrons
3. Both
4. None

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Atom literally means:

1. Indivisible
2. Divisible
3. Invisible
4. Injectible

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Hydrogen bomb is based upon the principle of:

1. Nuclear fission
2. Nuclear fusion
3. Controller nuclear reaction
4. None of these

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Who discovered Radioactivity?

1. Henry Becqurel
2. Roentgen
3. Rutherford
4. Thomson

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The half life of radioactive carbon is:

1. 5730 years
2. 6730 years
3. 4730 years
4. 7730 years

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The half-life of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?

1. 12.5 g
2. 25 g
3. 33.3 g
4. 20 g

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A fission chain reaction can be controlled and sustained if:

1. some neutrons are absorbed at each step.
2. All the neutrons emiited are absorbed.
3. All the emitted neutrons are allowed to do their work.
4. none

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Which among the following is not true about Rutherford’s model of atom.

1. It said that most of the space in an atom is empty.
2. It said that almost all the mass of an atom resides in its nucleus.
3. It said that electrons revolve around nucleus in orbits with particular energy and radius.
4. It could not explain the emission spectrum of atoms.

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Which atomic model said that electrons revolve in orbits of some particular energy and radius?

1. Thomsons plum pudding model
2. Bohrs model of atom
3. Rutherfords model of atom
4. Democritus model of atom

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