Discover the tiny bits that make up everything in CBSE Class 9 Science Notes, Chapter 4 Structure of the Atom. Learn about protons, neutrons, and electrons—the building blocks of atoms. Explore how these parts fit together like a puzzle. From old ideas to new ones, find out how scientists understand the tiniest pieces of everything around us.
Atom
An atom is a tiny particle that makes up matter and defines a chemical element.
Charged Particles in Matters
Those particles in matter that have an electric charge are called charged particles.
Discovery of Electron
J.J. Thomson discovered that electrons exist in an atom in 1897.
Cathode Ray Experiment
In 1897, J.J. Thomson conducted the cathode ray experiment by passing high-voltage electricity through a gas at very low pressure in a discharge tube. Tiny particles came out of the cathode (negative electrode). These particles are called cathode rays because they come from the cathode. Cathode rays are made of small, negatively charged particles called electrons.
This showed that all atoms have negatively charged particles called electrons.
Characteristics of an Electron
Mass of an electron = 9 × 10-31 kg
Charge on electron = 1.6 x 10-19 C (-ve charge)
Relative charge on electron = -1
Discovery of Proton
Since atoms have negatively charged particles called electrons, they must also have an equal number of positively charged particles. In 1886, when Goldstein passed high-voltage electricity through a gas at very low pressure in a discharge tube, heavy particles came out of the anode (positive electrode). These streams of particles are called anode rays or canal rays. Anode rays are made of positively charged particles.
Characteristics of a Proton
Mass of Proton = 1.6 x 10-27 kg
Charge on Proton = 1.6 x 10-19 C
Relative charge on Proton = + 1
Discovery of Neutron
The neutron was discovered by James Chadwick in 1932. He found that atoms contain neutral particles, with no charge, in addition to protons and electrons. Neutrons have a similar mass to protons and help explain the atomic mass and stability of atoms, completing the understanding of atomic structure.
Characteristics of a Neutron
Mass of Neutron = 1.6 x 10-27 kg
Charge on Neutron = Neutron has no charge.
It is electrically neutral.
Structure of the atom
Dalton’s atomic theory suggested that the atom was indivisible – which could not be broken down into smaller particles. However the discovery of subatomic particles inside the atom disproved this postulate of Dalton’s atomic theory.
Various models were given to explain the structure of the atom, some of which are as follows:
(i) Thomson Model
In 1904, J.J.Thomson gave the watermelon model related to the structure of the atom, according to which, the atom is a very small, spherical and electrically neutral particle.
Thomson proposed that:
- An atom consists of a positively charged sphere and the electrons are embedded in it.
- The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model.
(ii) Rutherford Model
In 1919, Ernest Rutherford found the proton in the nucleus (centre) of the atom and correctly explained the position of the proton and electron in the atom – through an alpha particle scattering experiment.
Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. Rutherford designed an experiment for this that was GOLD FOIL EXPERIMENT / alpha-PARTICLE SCATTERING EXPERIMENT.
GOLD FOIL EXPERIMENT / alpha-PARTICLE SCATTERING EXPERIMENT
Rutherford concluded from the a-particle scattering experiment that:
- Most of the atom is empty space because most particles went through the gold foil without changing direction.
- Only a few particles were deflected, which means the positive charge in the atom takes up very little space.
- A very small fraction of a-particles were deflected by 180°, which means all the positive charge and mass of the atom are concentrated in a very small volume within the atom.
Nucleus
- The nucleus of an atom is positively charged.
- The nucleus of an atom is very dense and hard.
- The nucleus of an atom is very small compared to the whole atom.
- The radius of the nucleus is about 100,000 times smaller than the radius of the atom.
Drawbacks of Rutherford’s model of the atom
Any particle in a circular orbit would undergo acceleration. During accelerations, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom would be highly Unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable.
(iii) Bohr’s Model of the Atom
To address the issues with Rutherford’s model, Niels Bohr presented the first successful model of the modern atom in 1913 using Max Planck’s quantum theory.
Postulates of Bohr’s Model
- Only specific orbits, known as discrete orbits of electrons, are allowed inside the atom.
- Electrons in these discrete orbits do not emit energy while they revolve.
- Electrons move quickly around the nucleus in fixed circular paths called energy levels or shells. These levels are represented by numbers (1, 2, 3, 4, 5, 6) or letters (K, L, M, N, O, P).
- The levels are counted from the center outward.
- Each energy level has a fixed amount of energy. The closest shell to the nucleus has the least energy, and the farthest shell has the most energy.
Distribution of electrons in different orbitals
The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury.
It is a well-known fact that a system is most stable when it has the minimum energy. So the electrons occupy the low energy levels first (this will make the atoms more stable). Now K shell is at the lowest energy level, so first of all the electrons fill K shell, then L shell, M shell, N shell, and so on.
The following rules are followed for filling up of electrons in various shells or energy levels.
(i) The maximum number of electrons present in a shell is given by the formula 2n², where ‘n’ is the orbit number of energy level, 1, 2, 3, … Hence the maximum number of electrons in different shells are as follows:
(ii) Electrons are not accommodated in a given shell unless the inner shells are filled. That is the shells are filled in a step-wise manner.
This means that the electron shells in an atom are filled in a step-wise manner. First of all the electrons fill K shell, then L shell, M shell, N shell, and so on.
(iii) The maximum number of electrons that can be accommodated in the outermost orbit is 8. (If however the outermost shell of an atom is the first shell or K shell, then it cannot accommodate more than 2 electrons)
Reason:- This is due to the fact that “having 8 electrons in the outermost shell” makes the atoms very stable.
Electronic Configuration
The arrangement of electrons in the various shells (or energy levels) of an atom of the element is known as the electronic configuration of the element. In other words, electronic configuration is the distribution of electron in various shells (or energy levels) of an atom such as K shell, L shell, M shell, etc.
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Valency
The valency of an element is the number of hydrogen atoms that combine with an atom of that element.
Note: The electrons present in the outermost shell of an element are called valence electrons and the outermost shell is called the valence shell.
Significance of Valence Electrons
- The valence electrons of an atom are responsible for chemical properties.
- The valence electrons determine the combining capacity or the valency of the atom.
- Elements having the same number of valence electrons in their atoms possess similar chemical properties. For example, all alkali metals (group-1 elements) have one valence electron in their atoms.
| Name of Element | Symbol | Atomic Number | Number of Protons | Number of Neutrons | Number of Electrons | K | L | M | N | Valency |
|---|---|---|---|---|---|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | – | 1 | 1 | – | – | – | 1 |
| Helium | He | 2 | 2 | 2 | 2 | 2 | – | – | – | 0 |
| Lithium | Li | 3 | 3 | 4 | 3 | 2 | 1 | – | – | 1 |
| Beryllium | Be | 4 | 4 | 5 | 4 | 2 | 2 | – | – | 2 |
| Boron | B | 5 | 5 | 6 | 5 | 2 | 3 | – | – | 3 |
| Carbon | C | 6 | 6 | 6 | 6 | 2 | 4 | – | – | 4 |
| Nitrogen | N | 7 | 7 | 7 | 7 | 2 | 5 | – | – | 3 |
| Oxygen | O | 8 | 8 | 8 | 8 | 2 | 6 | – | – | 2 |
| Fluorine | F | 9 | 9 | 10 | 9 | 2 | 7 | – | – | 1 |
| Neon | Ne | 10 | 10 | 10 | 10 | 2 | 8 | – | – | 0 |
| Sodium | Na | 11 | 11 | 12 | 11 | 2 | 8 | 1 | – | 1 |
| Magnesium | Mg | 12 | 12 | 12 | 12 | 2 | 8 | 2 | – | 2 |
| Aluminium | Al | 13 | 13 | 14 | 13 | 2 | 8 | 3 | – | 3 |
| Silicon | Si | 14 | 14 | 14 | 14 | 2 | 8 | 4 | – | 4 |
| Phosphorus | P | 15 | 15 | 15 | 15 | 2 | 8 | 5 | – | 3,5 |
| Sulphur | S | 16 | 16 | 16 | 16 | 2 | 8 | 6 | – | 2 |
| Chlorine | Cl | 17 | 17 | 18 | 17 | 2 | 8 | 7 | – | 1 |
| Argon | Ar | 18 | 18 | 22 | 18 | 2 | 8 | 8 | – | 0 |
Atomic Number
The total number of protons present in the nucleus of an element is called the atomic number of that element.
Mass number
The sum of the number of protons and neutrons present in the nucleus of an atom is called the mass number of that atom.
Mass Number = No. of protons + No. of neutrons
Isotopes
Atoms of the same atomic number and different mass numbers are called isotopes.
Isobar
Atoms of the same mass number and different atomic numbers are called isobaric.