We are made up of atoms. Atoms are small particles that build the entire universe. They are the tiniest unit of matter due to which elements exhibit chemical properties. Think of the wall in your house. The wall is not entirely a single piece but hundreds of smaller bricks. Similarly, everything in nature is made up of atoms. Everything is made up of atoms, from massive stars to the chair in your room.
“Humans are the representatives of atomic evolution over billions of years.”
What are Atoms?
Atoms are the smallest unit of any substance which cannot be further divided into parts. Or, you can say that the smallest part of any substance is known as an atom. Atoms combine to form molecules; then molecules attach to form compounds. The compounds then interact to form solids, liquids, or gases. For instance, water is a compound chemically known as H2O. This means that water comprises two hydrogens and one oxygen molecule.
Composition of an Atom
The origins of atomic structure can be traced back to Democritus, who was the first to argue that matter is made up of atoms. In the 1800s, the English Chemist John Dalton was the first to propose the atomic structure scientifically. We shall study all the atomic models and their advantages and limitations in this article. But first, let us learn the composition of an atom.
Every atom is made up of three main particles known as subatomic particles. They are as follows:
Protons: The subatomic particle with a positive charge is a proton.
Electrons: The particles at the subatomic level having a negative charge are electrons.
Neutrons: Neutrons are particles that have no charge. They are neutral subatomic particles.
Some important notes on subatomic particles:
- The charge on a proton is +1, on an electron is -1, and on a neutron is negligible or 0. There is no contribution of neutrons in charge of an atom. The number of protons and electrons in an atom is equal, making the net charge zero.
- Sometimes an atom may have some charge due to the lack or excess of electrons. These are known as ions. For example, Iron exists in 0, +2, and +3 charged states.
- The mass of a proton and neutrons is said to be one amu. Scientists have standardised the value of 1 amu or 1 Dalton as 1.67 x 10-24 grams. This is the weight of one proton or neutron.
- Neutron contributes to the overall mass of the atom along with protons.
- The mass of an electron is 9.11 x 10-28 grams, which is almost 1/1800 times the mass of a proton. This is so small in value that it is considered negligible.
- The overall mass of the atom is the sum of the mass of all the protons and neutrons.
Atomic Number Versus Mass Number
Scholars have often been confused in distinguishing between the atomic number and mass number of atoms. The number of protons (+1) present inside the nucleus gives us the value of the atomic number, while the sum of the mass of all the protons and neutrons inside the nucleus provides us with the mass number.
In an electrically neutral atom, the number of electrons and protons is the same. This number gives us the atomic number of any element. The atomic number is important to distinguish between the elements. For instance, the atomic number of Oxygen is eight because it has 8 protons in its nucleus. The atomic number is always represented by the uppercase letter ‘Z’.
Curiously Enough: The atoms of the same elements with an equal number of protons but varying electrons are known as isotopes of that element. The best isotopes are carbon 12, carbon 13, and carbon 14, having an atomic weight of 12, 13, and 14, respectively.
The mass number of an element is the total number of subatomic particles in the nucleus. The atomic mass (A) = Number of protons + Number of electrons. This value can determine the number of neutrons inside the nucleus. Subtract the value of the number of protons from the mass number. The mass number is represented by the uppercase letter ‘A’.
Example 1: Find the number of neutrons in carbon 14. The atomic number of carbon is six, and the atomic mass is 14 amu.
Solution: The atomic number of carbon 14 (Z) = 6
The mass number (A) = 14.
We know that A = Z + number of neutrons.
Therefore, number of neutrons → A – Z = 14 – 6 = 8.
Structure of an Atom
The structure of atoms is based on these three subatomic particles. In general, the structure of the atom is given as:
- At the centre of every atom, the nucleus is present, which is the space where the protons and neutrons reside.
- Around the nucleus, there are electron shells that consist of electrons. The electron revolves around these shells.
The atomic structure looks similar to our solar system, where the sun is in the centre and planets revolve around it in their respective orbits. See the figure below:
The arrangement and number of these subatomic particles decide the distinct properties of every atom. For example, let us take the simplest element: Hydrogen. Hydrogen’s atomic structure contains a single proton and electron. No neutron is present in Hydrogen. The number of subatomic particles can be decided using atomic and mass numbers.
Many scientists attempted to describe the structure of the atom using atomic models from the early 18th century to the late 19th century. Each of these models had its own set of advantages and limitations. But, all the models are responsible for creating the present atomic model. This section dives deep into their theories about the structure of the atom.
The English chemist John Dalton is regarded as the pioneer for describing the structure of an atom. He was the first to suggest that the building constituents of all matter are atoms. He described these atoms as indestructible and indivisible. According to Dalton, the atoms all over the universe are the same; only the size of the elements differ in weight and size.
The atomic structure provided by Dalton has the following postulates:
- Everything comprises atoms.
- Each atom has a fixed mass varying from element to element.
- During a chemical reaction, these atoms rearrange themselves.
- Atoms can neither be created nor be destroyed; they can only be transferred from one place to another.
- Specific elements have the same type of atoms.
- The theory was incapable of explaining the occurrence of isotopes.
- There was no proper explanation of the structure of atoms.
- In the later years, after discovering protons and electrons, it was found that atoms are divisible.
- The Thomson atomic model replaced Dalton’s atomic model.
Sir JJ Thomson, a renowned English Chemist, gave atoms a much more detailed structure in the 1900s. He prepared an experiment known as the cathode ray experiment, which helped him receive the Nobel Prize in Chemistry. JJ Thomson discovered the presence of electrons in atoms.
The Thomson model is also known as the ‘Plum Pudding Model’ or the ‘Watermelon Model’. He said that the atom is a positively charged sphere, and electrons are placed randomly inside this shell. Imagine a watermelon cut in half. The red flesh of the watermelon is the positively charged shell, and the seeds of the watermelon are the electrons. That is why this is called the ‘Watermelon Model’.
The model says that the positive and negative charge inside the shell is the equal magnitude, making the atom electrically neutral.
- This atomic model failed to explain the stability of the atoms.
- Protons and Neutrons were discovered after this model was proposed so that this model couldn’t fit protons and neutrons inside the atoms.
The Rutherford atomic model replaced Thomson’s atomic model to cover these limitations.
Rutherford Atomic Theory
Rutherford was a scholar under the guidance of JJ Thomson. He worked and studied the atomic model proposed by Thompson and remodelled it to discover the nucleus. The Rutherford atomic model is based on the experiment known as alpha ray scattering.
Rutherford postulated that:
- Atoms are spherical in structure.
- The centre of every atom is termed the nucleus. The maximum mass and charge of the atom are concentrated on the nucleus.
- The electrons orbit around the nucleus, similar to the way Earth orbits the Sun.
- The Rutherford model failed to define the stability of the atom. The model was challenged that if electrons move around the nucleus, they have to exert a force greater than the force of attraction between the nucleus and electron. The electrons must lose energy while revolving and eventually fall inside the nucleus.
- A continuous spectrum must be depicted by the electrons moving in a continuous circular motion. But instead, the spectrum formed was a line spectrum.
This model was discarded after Bohr proposed his model. With a slight change in Rutherford’s model, he published the model that is widely used to date.
Bohr’s Atomic Theory
Neils Bohr, in 1915, utilised the quantisation theory of Planck’s and remodelled the Rutherford model to explain the stability of atoms. He postulated that:
- The electrons revolve inside the atom in discrete orbits known as ‘stationary orbits’. Quantum numbers give the energy level of these orbits, and the energy of all the stationary orbits is quantised.
- When the electron is revolving in its designated orbit, it will not lose or gain energy. An electron can only shift to higher orbits on absorbing energy and move to lower orbit on emitting energy.
- Bohr’s model only works for atoms having a single electron like Hydrogen.
Frequently Asked Questions on Structure of an Atom
1) Explain subatomic particles?
Ans. The particles which build up the atoms are termed subatomic particles. They are categorised as protons (having positive charge), electrons (having negative charge), and neutrons (the uncharged particle).
2) How are isotopes of an element different from each other?
Ans. We can differentiate between the isotopes of an element by looking at the number of neutrons present in the nucleus. Since the number of protons and electrons are the same in isotopes, the only way to differentiate is using the number of neutrons.
3) Why doesn't a flawless model for atomic structure exist?
Ans. Since the atoms are too small to even view from a microscope, no one can tell what the shape of an atom is. The theories mentioned above and models have been proposed after years of research and are true to date.
4) Why is Bohr's atomic model not perfect?
Ans. Bohr's atomic model cannot explain the spectral predictions of atoms having more than one electron. This model is only perfect for the Hydrogen spectrum. It also gives poor insight into Zeeman's effect.
5) Is the structure of an atom asked in examinations?
Ans. Indeed! The structure of atoms is asked multiple times on the board and competitive examinations like JEE and NEET. Bohr's atomic model and Hydrogen spectrum are even taught in the first year of many IITs and NITs.