## What is Atomic Structure?

The basic structure of an atom is defined as the component-level of atomic structure of an atom. Precisely speaking an atom consists of three major subatomic particles which are protons, neutrons, and electrons. Many theories have been stated for explaining the structure of an atom.

## Atoms

In our day-to-day life, we come across many chemical substances made up of different physical and chemical properties. Each of them is unique in its own way. For example, if we consider one apple and one orange, both of them come under the class of fruits. However, they differ in their color, shape, smell, taste, and so on. This difference originates from the very basic building units of these fruits. They have different building blocks; thus, they are different in nature. These smallest building blocks of matter are called atoms.

## History of Atoms

The word ‘atom’ was coined by the great Greek philosopher Democritus. He derived it from the Greek word ‘atom’s’, which means indivisible. Before that an Indian philosopher, Maharishi Kanadan had mentioned the indivisible building blocks of matter. He named it ‘paramanu’. Both the concepts were similar and what they were concerned about was the indivisible building blocks of matter. Later in 1808, an English chemist, John Dalton had proposed an atomic theory that had put forward the major concepts about atoms.  Some of his concepts were proved wrong later. For example, he stated that an atom is the smallest particle of matter and it is indivisible. However, there exists even smaller particles than atoms which means they can be further broken down into component materials.

## Structure of an Atom

### Subatomic particles

As we have seen, atoms are composed of very small particles. There are three constituent particles, they are also known as subatomic particles. They are:

• Proton
• Neutron
• Electron

Protons are positively charged species and electrons are negatively charged species. Neutrons, on the other hand, do not have any charge, they are neutral. Protons along with neutrons exist together within a place known as a nucleus. Electrons are present outside the nucleus. The size of the nucleus is really small compared to the size of the atom. Consider a football placed at the center of a large football ground. Here, football can be considered as the nucleus, and football ground can be considered as the entire atom.

## The Basic Structure

The structure of an atom can be compared to the solar system. In a solar system, the sun stays at the centre and all other planets revolve around the sun. In an atom, the nucleus is placed at the centre just like the sun, and the electrons revolve around the nucleus just like the planets. The positively charged protons, and neutrons with no charge are placed in the nucleus, for which the charge of the nucleus is positive. The number of positively charged protons and the number of negatively charged electrons are found to be the same in an atom. This makes the total charge of an atom neutral.

The electrons are kept outside of the nucleus. The electrons are positioned in certain paths called orbits or shells. The electrons revolve in these shells around the nucleus in these shells. Each shell contains a certain number of electrons. The loss or gain of electrons leads to the formation of ions with a positive charge and negative charge respectively. Positive ions are called cations and negative ions are called anions.

The number of protons represents the atomic number of the element. The sum of protons and neutrons represents the atomic mass of the element. The elements are arranged on the basis of atomic number and mass in the periodic table.

## Molecules

Molecules are formed by the combination of two or more atoms. The combining atoms can be the same or different. For example, consider the beads in a chain as atoms and chains as molecules. We can make a chain using the same beads and different kinds of beads. Just like that, a molecule can be made with the same atoms or different atoms.

## Dalton's Nuclear Hypothesis

The English logical master John Dalton suggested that all matters include components, which are tough and indestructible. He further stated that all the components of a segment are exactly the same, and at any rate, the bits of various segments fluctuate in size and mass.

## Dalton's Hypotheses

• Atoms are present in all matters.
• They are unchanging.
• Just one kind of atom persists in a particular segment.
• Each of them has a specific mass that varies from part to part.
• During a compound formation, they undergo modification.
• They can neither be created nor be destroyed.

The laws of compound reactions, the Law of conservation of mass, Law of consistent properties, Law of difference, and Law of comparing degrees can be explained with the help of Dalton's atomic theory.

## Flaws in Dalton's Atomic Theory

• This speculation could not clarify the presence of isotopes.
• Nothing about the plan of molecules was reasonably clarified.
• Subsequently, the specialists found the components inside the atom that showed, the particles are particular.

The discovery of atomic species which is smaller than the atoms pawed way to a better understanding of characteristics of matter and different substances.

## Thomson Atomic Model

The model put forward by Thomson is the 'Plum pudding model'. He used the cathode ray experiment to discover the negatively charged species of the atom and electrons. According to his plum pudding model, electrons with a negative charge were located in a positively charged "soup". Here, the pudding is positively charged and the plums are negatively charged electrons.

## Perceptions from Cathode Ray Experiment

• When the power supply is on, fluorescent spots are generally found on the zinc sulphide screen that is used. This formed because of the radiations emerging from the cathode which travels towards anode.  These radiations are known as cathode rays.
• In the presence of an external magnetic field, the path of the cathode rays is found to be away from the positive terminal.
• Rotor Blades tend to move when they are placed in the way of cathode ray emission. This demonstrates that the cathode transmits particles of a particular mass, indicating that they have energy.
• Thus, Thompson concluded that cathode emanates are made of conversely charged particles called "electrons".
• Thomson discovered the charge to a mass degree (e/m) of electrons through applying an electric and alluring field to the cathode transmits (electrons). It has a value of for electron.
• Using this value, Mullikan found the charge of the electron using an experiment named oil drop experiment. [Charge of and Mass of ].

## Rutherford Atomic Speculation

Alpha Ray Scattering Experiment

Construction:

• Alpha shafts (doubly charged Helium${\text{He}}^{2+}$) were used to strike an extremely thin gold foil that was $1000$ particles thick.

Behind the gold foil was a zinc sulphide mirror.

Discernments:

• The $\text{ZnS}$screen experienced a shine (awe-inspiring spots) which was made by the gold foil.
• As a result of hitting the gold foil, a couple of columns were reflected.
• One out of $1000$ columns got reflected by an angle of $180°$(reflected) and was hit back on the gold foil itself.

## Closures

Since most bars were experienced, Rutherford surmised that the majority of the space inside the molecule is vacant.

• ${\frac{1}{1000}}^{\text{th}}$of the shafts moved relentlessly avoiding a solid positive charge which was present in the point of convergence of the molecule. He called this solid positive charge a "center”.
• He stated that the nucleus retains a large portion of the atom's charge and mass.

## Atomic Structure of Isotopes

Nucleons are the subatomic particles present inside the nucleus. They can be either protons or neutrons. Each of the atoms possess a specific number of protons which leads to the formation of a unique atomic number of the atom.  However, the number of nucleons of the same atom may differ in some cases.

These atoms having another nucleon number (regardless called the mass number) are referred to as isotopes of the atom. Subsequently, the isotopes of a segment have an equivalent number of protons yet they contrast with the number of neutrons.

## Common Mistakes

• Attempting to make all particles of the molecule meet the octet rule. A couple of segments can be exclusions, for example - aluminum will have only six valence electrons in a Lewis development of something like aluminum triiodide.
• Utilizing pretty much valence electrons than what the formula considers.

## Practice Problem

Which particles occupy the orbitals surrounding the nucleus?

1. Protons
2. Neutrons
3. Positrons
4. Electrons

Solution

(d) Electrons: Negatively charged particles that form an orbit around the nucleus are called electrons.

## Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

Bachelor of Science Chemistry

Master of Science Physics

Bachelor of Science Physics

Master of Science Chemistry

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