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Atomic Nucleus

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Atomic Nucleus is defined as the central region of the atom that has all its mass concentrated. Through Rutherford’s experiment, we know that all the mass of the atom is concentrated in a very small part inside the center of the atom. The concentrated part in the center is called the Nucleus of the atom. The nuclei of the atom hold the Protons and Neutrons of the atoms that are responsible for the mass of the atom as the mass of electrons is negligible as compared to the mass of Protons and Neutrons.

The electrons revolve around the nucleus in a fixed orbit. The nucleus only occupies 10-14 times the volume of the atom and contains 99.99% mass of the atom. In this article, we will learn about the Structure of the Atom, the Structure of the Nucleus, Atomic Mass, and others in detail.

Before learning about the Nucleus we first need to learn about the Structure of atoms in brief so let’s start with that.

Structure of Atom

An atom is a complex arrangement of negatively charged electrons organized in defined shells around a positively charged nucleus. The image of the electrons is shown below showing positively charged nucleus and negatively charged electrons revolving about nucleus in their fixed orbit.

Structure-of-Atom

This nucleus, which is made up of protons and neutrons, contains the majority of the atom’s mass (except for common hydrogen which has only one proton). Every atom is roughly the same size. The Angstrom (Å), defined as 1 x 10-10 m, is a useful unit of length for measuring atomic sizes. An atom has a diameter of about 2-3 Å.

Learn more about, Atomic Structure

Discovery of Nucleus

Thomson started experimenting with cathode ray tubes. Cathode ray tubes are vacuum-sealed glass tubes with the majority of the air removed. At one end of the tube, a high voltage is applied across two electrodes, causing a beam of particles to flow from the cathode (negatively charged) to the anode (positively charged). Because the particle beam, or ‘cathode ray,’ originates at the cathode, the tubes are known as cathode ray tubes. 

Thomson used two oppositely charged electric plates to surround the cathode ray. The cathode ray was redirected from the negatively charged to the positively charged electric plate. This meant that the cathode ray was made up of negatively charged particles. Cathode rays are particles that are negatively charged. Scientists gradually accepted Thomson’s discoveries. Cathode ray particles became well-known and were given the name electrons.

Rutherford’s Atomic Model demonstrated that the atom is mostly empty space with a tiny, densely packed, positively charged nucleus. Rutherford proposed the nuclear model of the atom based Rutherford’s Gold Foil Experiment that involved firing an alpha beam at a thin sheet of gold foil. The majority of the particles passed through the gold foil without being deflected, but a small number were slightly deflected and an even smaller fraction was deflected more than 90 degrees from their path. 

This prompted Rutherford to propose the nuclear model, which states that an atom is made up of a very small, positively charged nucleus surrounded by negatively charged electrons. Based on the number of alpha particles deflected in his experiment, Rutherford calculated that the nucleus took up a tiny fraction of the volume of the atom.

Composition of a Nucleus

An atom’s nucleus is made up of a densely packed arrangement of protons and neutrons. Because these are the two heavy particles in an atom, 99.9% of the mass is concentrated in the nucleus. Because protons have a net positive charge, the nucleus of an atom is positively charged overall, while negatively charged electrons revolve around the central nucleus. Because the mass concentration at an atom’s nucleus is enormous, the nuclear forces that hold protons and neutrons together are also enormous. 

Since, the protons are so close to each other inside the tiny nucleus, electrostatic forces of repulsion also act inside the nucleus but it is very weak as compared to nuclear force of attraction that we neglect the same. Because the total number of protons in a nucleus equals the total number of electrons revolving around the nucleus, the atom as a whole is electrically neutral.

Characteristics of Nucleus

Various characteristics of the Nucleus are,

  • Nucleus is located in the centre of the atom and contains protons and neutrons.
  • Because of the presence of positively charged protons, the nucleus determines the overall charge on the atom.
  • It is also in charge of the chemical properties of the element, such as the valency of the atom, the element’s reactivity to other elements, and the atomic number and mass number of the atom.
  • While the neutron is electrically neutral, it contributes to the atom’s mass.
  • The numeric sum of protons and neutrons is the atomic mass. The mass of an atom determines physical properties such as melting point, boiling point, density, and so on.
  • The nucleus remains stationary at the centre of the atom, while the electron revolves around it due to the attraction between the positively charged nucleus and the negatively charged electron.
  • The electron spins on its axis as it moves around the nucleus. An electron’s mass is considered negligible in comparison to the nucleus, but it is responsible for the valency of the atom and determines the atom’s ability to participate in any chemical reaction.
  • Protons and Neutrons are combinedly called the Nucleons as they reside inside the Neutrons.

Mass of a Nucleus

Nucleus contains almost all of an atom’s mass, with only a minor contribution from the electron cloud. The mass of an atom is related with its atomic mass number, which is the total number of protons and neutrons in its nucleus. An isotopes of an element have same atomic number but distinct mass number. The mass number is written after the element name or as a superscript to the left of an element’s symbol. Carbon-12, or 12C, is the most common carbon isotope.

Atoms are so small in size and mass that using standard measuring units, while possible, is frequently inconvenient. On atomic scale, units of mass and energy have been defined to make measurements easier to express. The atomic mass unit is the unit of mass measurement (amu).

1 amu = 1.66 x 10-24 Grams

Also, Read

FAQs on Nucleus of Atom

1. What is an Atom?

An atom is the smallest particle with which all the matter in the universe is made up of. It is believed that an atom can not be further broken down but recent studies have shown that Atoms can be further divided into smaller parts.

2. What is Atomic Nucleus?

A Nucleus is the part of the atom that contains all its mass it is at the centre of the atom and occupies very very tiny space of atom but contain 99.99% mass of atom. It contains all the protons and neutrons of the atom and electrons revolve around it in an fixed orbit.

3. Whom Discovered the Atomic Nucleus?

Atomic Nucleus was discovered by Rutherford.

4. What does the Atomic Nucleus Contain?

The atomic nucleus contains protons and neutrons

5. What is the Density of Atomic Nucleus?

The average density of Atomic Nucleus is 2.5 × 1017 kg/m3

6. What is the Radius of Atomic Nucleus?

The radius of atomic nucleus is in the order of 10-13

7. What are Subatomic Particles?

Subatomic particles are the particles that make up an atom. As the name suggest they are smaller than Atom. The three subatomic particle are,

  • Electrons
  • Protons
  • Neutrons

8. What are Quarks?

The subatomic particles of Protons and Neutrons are further made up of even smaller particles called the Quarks. There are six different kinds of quarks.

9. What are Isotopes?

Atoms having same atomic number but different atomic mass are called Isotopes. Example C-12 and C-14 are isotopes both have atomic number of 6 but mass number or atomic mass of C-12 is 12 and C-14 is 14.

10. What are Isobars?

Atoms having same atomic mass but different atomic numbers are called Isobars. Example 18Ar40 and 20Ca40 both are isobars both have same atomic mass of 40 but different atomic number. Atomic number of Ar is 18 and of Ca is 20.


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Last Updated : 22 Sep, 2023
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