Nucleus | Definition, History, Composition, Size

Definition of Nucleus


In chemistry, a nucleus is the positively charged centre of the atom consisting of protons and neutrons. It’s also known as the “atomic nucleus”. 


history of nucleus

The word “nucleus” comes from the Latin word nucleus, a form of the word nux, which means nut or kernel. The term was coined in 1844 by Michael Faraday to describe the centre of an atom. The sciences involved in the study of the Nucleus, its composition, and characteristics are called nuclear physics and nuclear chemistry. Discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. 

Composition of Nucleus of an Atom

composition of Nucleus of an Atom

The Nucleus of an atom consists of a tightly packed arrangement of protons and neutrons. These are the two heavy particles in an atom, and hence 99.9% of the mass is concentrated in the Nucleus.

Of the two, the protons possess a net positive charge. And hence the Nucleus of an atom is positively charged on the whole, and the negatively charged electrons revolve around the central Nucleus.

Since the mass concentration at the Nucleus of an atom is immense, the nuclear forces holding the protons and the neutrons together are also large. The protons are in such close vicinity to each other inside the tiny Nucleus. And therefore the electrostatic forces of repulsion also act inside the Nucleus.

Nuclear energy relies on nothing but releasing the energy trapped in the Nucleus of an atom. The total number of protons in a nucleus is equal to the number of electrons revolving around the Nucleus, and hence the atom, on the whole, is electrically neutral.

Nuclear Model of an Atom

Nuclear Model of an Atom

Rutherford needed to develop an entirely new model of the atom to explain his results. Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space.

In contrast, the highly deflected particles must have experienced a tremendously powerful force within the atom. He concluded that all of the positive charge and the majority of the atom’s mass must be concentrated in a very small space in the atom’s interior, which he called the Nucleus.

The Nucleus is the tiny, dense, central core of the atom and is composed of protons and neutrons.

Rutherford’s atomic model became known as the nuclear model. In the nuclear atom, the protons and neutrons, which comprise nearly all of the mass of the atom, are located in the Nucleus at the centre of the atom.

The electrons are distributed around the Nucleus and occupy most of the volume of the atom. It is worth emphasizing just how small the Nucleus is compared to the rest of the atom. If we could blow up an atom to be the size of a large professional football stadium, the Nucleus would be about the size of a marble.

Rutherford’s model proved to be an important step towards a full understanding of the atom. However, it did not completely address the nature of the electrons and the way in which they occupied the vast space around the Nucleus.

For this and other insights, Rutherford was awarded the Nobel Prize in Chemistry in 1908. Unfortunately, Rutherford would have preferred to receive the Nobel Prize in Physics because he considered physics superior to chemistry. In his opinion, “All science is either physics or stamp collecting.”

Size of the Nucleus

Size of the Nucleus

The Nucleus of an atom is much smaller than the overall diameter of the atom because the electrons can be distant from the atom’s centre.

A hydrogen atom is 145,000 times larger than its Nucleus, while a uranium atom is around 23,000 times larger than its Nucleus. The hydrogen nucleus is the smallest Nucleus because it consists of a lone proton.

It is 1.75 femtometers (1.75 x 10-15 m). The uranium atom, in contrast, contains many protons and neutrons. Its Nucleus is about 15 femtometers.

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