Discovery of the Structure of an Atom: Atoms are the basic unit of chemical elements that contain subatomic particles like electrons, protons, and neutrons. Atoms have a nucleus at the centre and electron cloud surrounding the nucleus, which is composed of protons and neutrons in the nucleus, and electrons revolving around the nucleus in different orbits or electron clouds. 

In the early days, scientists believed that the atom was the tiniest particle of matter that could exist, but gradually, it was inferred that the atom was almost empty inside. The fact that something was inside the atom was noticed in \(1886\) when some radiation was noticed coming out of the atom. By \(1900\) scientists were pretty sure about a charged particle ‘electron’ present inside the atom. Over the years, this structure of the atom was rigorously studied, and finally, it was confirmed that each atom is composed of subatomic particles inside it. These subatomic particles in the atom are negatively charged electrons, positively charged protons, and neutrons without any charge. With time many scientists came up with their atomic models, and in this article, we will extensively study the evolution of the structure of an atom.

What is Atomic Structure?

Atomic structure is the concept of the structure of an atom as a positively charged nucleus at the centre consisting of protons and neutrons surrounded by several electrons that revolve in definite orbits or shells. If the number of electrons is equal to the number of protons, then the whole atom is electrically neutral.

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Discovery of Charged Sub-atomic Particles in an Atom

In \(1803,\) John Dalton stated the atom is the smallest and indivisible particle of matter. Later, with the discovery of sub-atomic particles, this theory was knocked out. Let’s learn about the discoveries of these sub-atomic particles (electron, proton, and neutron) one by one:

1. Discovery of Electron

In \(1897,\) based on cathode rays’ experiment, J. J. Thomson discovered electrons as negatively charged sub-atomic particles. 

In this experiment, when a high voltage was applied across two electrodes at one end of the sealed glass tube, a beam of particles began to flow from the negatively charged cathode to the positively charged anode. Based on the origin, these rays were known as “cathode rays”. Based on the deflection of these cathode rays across the charged plate, this is composed of negatively charged particles that were later named “electrons”.

Soon after the discovery of electrons, J.J. Thomson, in \(1904,\) proposed the Plum Pudding Model of an atom where he assumed that the electrons were embedded in the sea of positively charged matter resembling ‘plum’ as electrons scattered in positively charged ‘pudding’.

2. Discovery of Proton

Eugen Goldstein discovered positively charged particles in an atom through a series of experiments in which high voltage electric current was passed through a cathode tube consisting of gas at low pressure. These new rays were known as canal rays, positive rays, or anode rays.  In \(1909,\) Rutherford discovered protons as positively charged particles through his famous gold foil experiment. In this experimental set-up, a gold foil was bombarded with positive alpha particles and noticed that most of these particles penetrated through the atom, and some scattered in different directions, and some of the alpha particles near to the centre of the atom, even completely deflected back towards the source. This proved that positively charged particles are present at the centre of the atom. Rutherford named these particles as a proton, which was derived from the Greek word “protos,” meaning “first.” 

3. Discovery of Neutron

Sub-atomic particles that do not have any charge at all are known as neutrons, meaning they are neutral. After the discovery of protons, by \(1920,\) scientists confirmed that most of the mass of the atom was concentrated inside the nucleus at the centre of an atom. In \(1932,\) James Chadwick finalised that the core of an atom also contained a new uncharged particle along with the proton, which he termed as neutron. Thus, after the discovery of the neutron, a complete picture of an atom was quite clear.

Evolution of Atomic Models

1. Ancient Theory – The word ‘atom’ actually was coined by Ancient Greek that means ‘indivisible’. They outlined the basic idea that everything in the universe is made of atoms, which are invisible and indivisible spheres of matter of endless type and numbers.

2. Dalton’s Atomic Model – In \(1803,\) an English chemist John Dalton developed a more scientific definition of an atom. He worked on the ideas of the Ancient Greeks in describing atoms as small spheres that are indivisible and that atoms of an element are identical to each other. He came up with a theory popularly known as “Dalton’s Atomic Theory”. 

3. The Plum Pudding Model – Soon after the discovery of cathode rays “electrons”, J.J. Thomson in \(1904\) proposed the Plum Pudding Model of an atom where he assumed that the electrons were embedded in the sea of positively charged matter resembling ‘plum’ as electrons scattered in positively charged ‘pudding’. This discovery of electrons made him win the Nobel Prize in \(1906.\)

4. Rutherford’s Nuclear Model – Ernest Rutherford discovered the nucleus while working upon a gold foil alpha-scattering experiment. He proposed an atomic model where the electrons move in orbit around the positively charged nucleus.

5. Planetary Model – Rutherford’s model didn’t explain what factor kept the electrons orbiting around instead of simply spiralling into the nucleus. Niels Bohr brought a solution to this problem. He invoked quantum theory to explain the arrangement of electrons. He postulated the existence of energy levels or shells of electrons in an atom. Each electron could only be found in a specific energy level, and their energy was quantised and couldn’t take just any random value.

6. Quantum Model – Though Bohr’s proposal of stable energy levels in an atom resolved the problem of electrons spiralling into the nucleus to a certain extent, but not entirely, thus to overcome this in \(1926,\) Schrödinger proposed that, instead of electrons moving in fixed orbits or shells, they behave as waves. Based on numerous mathematical equations, he proposed the quantum model that shows the nucleus surrounding the electron cloud.

Atomic Structure of Isotopes

Atoms of the same element with the same atomic numbers and different mass numbers are called isotopes. 

Isotopes are atoms of the same element that possess an identical number of protons but the different number of neutrons. For example, the three isotopes of the hydrogen element are Protium (have one proton), Deuterium (have one neutron and one proton) and Tritium (have two neutrons and one proton).

Atomic Structure of Hydrogen

A hydrogen atom is an electrically neutral atom that contains a single positively charged proton and a single negatively charged electron attached to the nucleus of the atom by some force. The atomic number of a hydrogen atom is \(1.\)

Atomic Structure of Carbon

The atomic number of a carbon atom is \(6.\) Carbon has two electronic shells; the first shell holds two electrons, and the second shell holds four electrons out of a possible eight spaces. When atoms tend to bond, they share electrons present in their outermost shell. Since carbon has four empty spaces in its outermost shell, that enables it to bond to four other atoms. Thus, carbon is tetravalent.

Summary

Thus, we can conclude that atoms are the smallest and ultimate building blocks of all matter in the universe. An atom consists of negatively charged electrons around a central nucleus that contains a mass of positively charged protons and electrically neutral neutrons. The entire history of the development of the structure of an atom is very interesting right from the beginning when John Dalton considered an atom as a solid sphere in \(1808.\) After around a hundred years, another scientist J.J. Thompson discovered electrons and proposed the Plum Pudding Model, where electrons resembled “plum” embedded in the positively charged surface of the atom “pudding”. Later, Rutherford’s Gold foil experiment discovered that an atom has a positively charged nucleus and electrons are moving around it. This could not explain the electrons orbiting around the nucleus instead of simply spiralling into the nucleus.

Niels Bohr brought a solution to this problem. He postulated the quantum theory that proposed the existence of energy levels or shells of electrons in an atom. However, Bohr’s proposal of stable energy levels in an atom resolved the problem of electrons spiralling into the nucleus to a certain extent, but not entirely. In \(1926\) Schrödinger proposed the wave nature of electrons that stated, instead of electrons moving in fixed orbits or shells, they behave as waves. Thus, we can infer that an atom has a positively charged nucleus at the centre surrounded by negatively charged electron clouds where electrons move in definite energy shells.

FAQs on Discovery of the Structure of an Atom

Q.1. Who proposed the structure of an atom first?
Ans: The structure of an atom was proposed by John Dalton in \(1803,\) who developed a more scientific definition of an atom. He considered atoms as small spheres that are indivisible and that atoms of an element are identical to each other. He came up with a theory popularly known as “Dalton’s Atomic Theory”.

Q.2. How did Rutherford discover the structure of an atom?
Ans: In \(1909,\) Rutherford discovered protons as positively charged particles through his famous alpha scattering gold foil experiment. In this experimental set-up, a gold foil was triggered with positive alpha particles and noticed that most of these particles penetrated through the atom, and some scattered in different directions, and some of the alpha particles near to the centre of the atom, even completely deflected back towards the source. This proved that positively charged particles are present at the centre of the atom. This gave the confirmation of the Planetary model of an atom where a nucleus is present at the centre and electrons move around it in definite orbits.

Q.3. What is Dalton’s Atomic Theory?
Ans: According to the postulates of Dalton’s Atomic Theory:
(i) Elements consist of indivisible small particles known as atoms.
(ii) Atoms of the same elements are identical, but different elements have different types of atoms.
(iii) Atoms can neither be created nor be destroyed.

Q.4. What was Thomson’s Atomic Model?
Ans: After the discovery of cathode rays “electrons”, J.J. Thomson in \(1904\) proposed the atomic model that is popularly known as the Plum Pudding Model of an atom where he assumed that the electrons were embedded in the sea of positively charged matter resembling ‘plum’ as electrons scattered in positively charged ‘pudding’. This discovery of electrons made him win the Nobel Prize in \(1906.\)

Q.5. What were the limitations of the Rutherford Atomic Model?
Ans: The main limitation of the Rutherford Atomic Model is that it could not explain the stability of an atom. According to Maxwell’s electromagnetism theory, the revolution of charged electrons around the atom in circular paths should experience acceleration that would compel it to lose energy continuously in the form of electromagnetic radiation and then eventually fall inside the nucleus by a spiral path.

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