0Models of atom
Definition: A model is a simplified representation of anything under consideration, to see how it should look like. In a similar manner, to comprehend the structure of an atom, the scientists put forward various representations of atoms to explain the atomic behaviour. These representations are known as the ‘models of the atom’.
Overview
The Greeks put the idea of the atoms forward in ancient times and they proposed that everything was composed of atoms. The word ‘atom’ was derived from the Greek language and it means something, which cannot be divided further. The atoms were supposed to be indivisible spheres of different shapes and sizes so that they made up into different states of matter. The scholars of those times supposed that all the elements were of different type of atoms due to which they have different behaviour. For eg. It was thought that iron was solid because its atoms were firm and hard and water was a liquid because its atoms were soft and slippery. Although these ideas were trivial they laid a foundation on which scientists of the 19th start working and we got a number of models explaining atomic structure. The primary of them are- Dalton’s Billiard Ball Model, J. J. Thomson's "plum pudding" model, Rutherford's model, Bohr's Atomic model, Mechanics Model.
- Dalton’s model of an atom:
The model proposed by Dalton was identical to the one that was given by the Greeks. He said that all the material things are made up of very small spherical balls called atoms. He added that the atoms of one type of element are identical in mass and show similar kind of properties. The compounds of different materials are formed when different atoms combine with each other. He also suggested that in a chemical reaction, there is a rearrangement of atoms, which results in the formation of new compounds. However, Dalton could not explain the existence of isotopes. At that time, the electrons and nucleus were not discovered but the model of Dalton laid a foundation for the atomic structure.
- J. Thomson’s model of an atom:
The discovery of the electron by J. J. Thomson in 1897 made the scientist wonder that an atom may not be the fundamental entity, but they may constitute smaller particles. While working on his experiments of cathode rays, he found out that the rays were deflecting towards the positively charged electrodes signifying that the rays carry a negative charge. When he determined the properties of these particles in the rays, he concluded that they were approximately two thousand times lighter than hydrogen atoms. From this observation, he put forward that the atom is no longer indivisible, but it is composed of positively and negatively charged particles, which make it electrically neutral. He suggested that the atom is a sea of positive charge in which the electrons were embedded as plums in the pudding. Sometimes, his atomic model is said to be analogous to watermelon, where the electrons are the seeds in the watermelon.
- Rutherford's model:
The efforts of J. J. Thomson provided a pathway to many scientists to work on atomic structure. Ernest Rutherford worked on the radioactive substances and conducted an experiment in which he fired alpha particles emitted from a radioactive element on thin gold foils and the scattered alpha particles were detected. In his experiment, he observed that most of the alpha particles passed without being deviated through the gold foil. A very small fraction of the alpha particles deviated at larger angles which suggested that the positive charge may not be spread throughout the atom but was concentrated at the centre of the atom. This tiny and concentrated centre of the atom, where its positive charge and mass were present is called the nucleus and the rest of the atom was empty. The negatively charged electrons reside outside the nucleus and revolve around in orbits.
- Bohr's Atomic model:
The model suggested by Rutherford has a major drawback that if the negatively charged electrons are orbiting the positively charged nucleus, why do not they eventually fall back into the nucleus due to attraction. To explain this, Bohr postulated the concept of energy quantization. That the structure of an atom is not simply a classical concept but Bohr tried to explain this using the Quantum mechanical approach. He postulated that electrons revolve around the nucleus, not in any arbitrary orbits but very specific orbits. He proposed that only those orbits were possible whose angular momentum is conserved i.e. L=mvr=nh/2π. To change the orbit, electron either radiates or absorbs energy.
- Quantum Mechanics Model:
Bohr’s atomic model described the structure of an atom very adequately still that model required refinements. This model was applicable only for Hydrogen or hydrogen-like atoms. Quantum theory suggests that the position and momentum of the particle cannot be measured simultaneously (Heisenberg’s uncertainty principle). This discarded the idea of atomic orbits and proposed that the nucleus was surrounded by an electron cloud and the electron has the probability to be present within the cloud but we cannot locate their exact position. This is the present-day accepted model of atoms.
Key Takeaways:
- Dalton proposed that the atom is a spherical ball.
- J. Thomson proposed that the atom consists of a sea of positive charge in which electron is embedded like a plum in the pudding.
- Rutherford refined Thomson’s model and observed that the whole of the mass and positive charge of the atom is concentrated at a very tiny spot inside the atom called nucleus and rest of the space of the atom is empty. Electrons revolve around the nucleus.
- Bohr postulated that the electrons revolve in only those orbits whose angular momentum is conserved.
- Quantum theory claims that the exact position and momentum of an electron cannot be measured simultaneously, so the nucleus is surrounded by an electron cloud which represents the probability of the presence of electron in that region.
