Nucleus of an Atom and Atomic Mass: Explanation, Composition and Elements

The nucleus of an atom is the central region, where the majority of its mass is concentrated. Rutherford's alpha particle scattering experiment led us to the discovery that the nucleus of an atom holds a greater portion of its mass.

• Numerically, the nucleus of an atom has almost 10-14 times the volume of the atom while containing 99.99% of its mass.
• The nucleus of an atom is so small that if you expanded it to fill a room, it would be no larger than a pinhead!
• The nucleus of an atom comprises a proper and tight arrangement of protons as well as neutrons.
• Since protons are positively charged particles and neutrons are neutral, therefore a nucleus as a whole is positively charged.
• The electrons are negatively charged and rotate around the center of the nucleus.
• The atomic mass is the mass of an atom i.e. the quantity of matter contained in an atom of an element.
• After the English chemist, John Dalton, the atomic mass unit came to be known as the Dalton (Da).

Key Terms: Atom, Nucleus, Protons, Electrons, Positive charge, Negative charge, Neutron, Radioactive decay, Isobars, Isotones, Isotopes, Fundamental forces, Positrons, Atomic mass unit

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Nucleus of an Atom

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Ernest Rutherford discovered in 1911 that every atom has a nucleus. The nucleus of an atom consists of electrically positive protons and electrically neutral neutrons. These are bonded together by the strongest known fundamental force, known as the "strong force."

• The nucleus accounts for less than 0.01% of the volume of the atom but often comprises more than 99.9% of its mass.
• The chemical characteristics of a material are determined by the negatively charged electrons that surround the nucleus.
• The number of electrons is generally equivalent to the number of protons in the nucleus.
• Some of the nuclei are unstable and can suffer radioactive decay before stabilizing.
• This decay can be the emission of photons (gamma decay), the emission or capture of electrons or positrons (beta decay), and the emission of helium nuclei (alpha decay).
• The majority of nuclei are spherical or ellipsoidal, although others have unusual forms.
• When other particles strike nuclei, they vibrate and rotate.
• Some are unstable, breaking apart or changing the amount of protons and neutrons.

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Atomic Mass

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Atoms are incredibly small, and therefore their mass is also relatively small.

• A unit of measuring mass such as Kilogram or Kg cannot be used for weighing something as small as an atom.
• In order to address this issue, scientists created a new unit of mass which is known as the Atomic Mass Unit (u) or (amu).
• The reference of the unit is taken as Carbon-12 and 1 Atomic Mass unit is equal to 1/12^th the weight of one atom of Carbon 12.

The mass of a hydrogen atom is given as follows:

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Composition of Nucleus of an Atom

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The nucleus of an atom involves the tightly packed arrangement of neutrons and protons. 99% of the mass is concentrated in the nucleus so the protons and the neutrons are the heavy particles in the atom.

• Protons have a positive charge and for this reason, the nucleus is positively charged on the whole.
• The electrons that are negatively charged are usually found revolving around the central nucleus. 
• As the mass concentration at the nucleus of an atom is immense, the force that holds protons and neutrons together is also immense.
• The protons are in close vicinity to each other inside the tiny nucleus.
• Due to this reason, the electrostatic force of repulsion acts inside the nucleus.
• Nuclear energy is reliant on the release of the energy trapped in the nucleus of an atom.
• The total number of protons in a nucleus is equal to the number of electrons.
• These electrons revolve around the nucleus and hence the atom is electrically neutral as a whole.

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Isotopes, Isotones, and Isobars

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The main differences between the isotopes, isotones, and isobars are given below

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What is a Neutron?

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In 1932, another subatomic particle was discovered by J. Chadwick.

• According to James Chadwick, neutrons are basic particles that are electrically neutral.
• He calculated the mass of the neutron with high accuracy using the laws of energy and momentum conservation.
• The mass of a neutron was very similar to that of a proton.

M_N = 1.00866 u = 1.6749 X 10^-27 kg

So the nucleus now has another resident, a proton-neutron pair known as a Nucleon.

• Atomic Number (Z) = number of protons/ electrons
• Neutron Number (N) = Number of Neutron
• Atomic Mass Number (A) = Z + N = Total number of protons and neutrons

For example, the nucleus of a Uranium atom is represented by a ²³⁵₉₂U, indicating that one atom of Uranium 235 has 235 nucleons, 92 of which are protons and the remaining 143 neutrons.

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Some Interesting Facts about an Atom

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By 1900, it was known that an atom was a particle that was indivisible but contained at least one subatomic particle, which is the electron.

• The electron was identified by J.J. Thomson.
• Before the electron was identified, the presence of new radiations in a gas discharge was discharged by E. Goldstein in 1886.
• He called them canal rays. These rays were radiations that were positively charged which led to the eventual discovery of another subatomic particle.
• This subatomic particle had a charge that was equal in magnitude but did not share the sign the electron had.
• Its mass is 2000 times that of the electron approximately. It was given the name proton.
• Protons are generally represented as p and electrons are represented as e.
• The mass of a proton is taken as one unit and its charge is taken as plus one.
• An electron’s mass is considered to be negligible and its charge is minus one.

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Things to Remember

• The nucleus of an atom comprises protons as well as neutrons.
• The nucleus of an atom has almost 10-14 times the volume of the atom while containing 99.99% of its mass.
• The atomic mass is the quantity of matter contained in an atom of an element.
• 1 u = 1.660539 10-27 kg.
• In a neutral total number of protons in a nucleus is equal to the number of electrons.
• Isotopes have different numbers of neutrons.
• Isotones have the same number of neutrons but different proton numbers.
• Isobars have the same number of nucleons.

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