Subatomic particles are those which make up the atom. All matter in this universe is composed of atoms. The word ‘atom’ came from the Greek word ‘Atomos’ which means non-divisible. The atomic theory of matter was first proposed by John Dalton in 1808.
There was a time when it was believed that atoms were the fundamental particles of matter and that they could not be further divided. But further research showed that there are electrons spinning around the nucleus of each atom and the nucleus is composed of protons and neutrons.
Read More:- Class 11 Chemistry Chapter 2 Structure of Atom
| Table of Contents |
Key Terms: Matter, Atoms, Electrons, Subatomic particles, Dalton’s atomic theory, Isotopes.
What are Subatomic Particles?
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According to Dalton’s atomic theory,
- Matter is comprised of indivisible, tiny particles called atoms.
- It is impossible to create or destroy an atom.
- Atoms cannot be broken down into smaller particles.
Dalton’s Atomic Theory Video Explanation
Dalton’s atomic model was capable of explaining the law of conservation of mass, the law of constant composition, and the law of multiple proportions very effectively. Despite this, there were a lot of drawbacks. Further studies and research revealed that an atom is composed of subatomic particles, which are mainly the proton, electron, and neutron.
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| Concepts Related to Subatomic Particles of an Atom | ||
|---|---|---|
| Electron Spin | Energies of Orbitals | Hydrogen Spectrum |
| Discovery of Electron | Charge to mass ratio of Electron | Mass Energy and Nuclear Binding Energy |
Electron and its features
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Atoms contain subatomic particles known as electrons that revolve around the nucleus. The process of chemical reactions involves an atom losing or gaining electrons, which results in the formation of ions. Atoms with different electron configurations tend to bond together to form chemical bonds. Some of the characteristics of electrons are as follows:
- Electrons are negatively charged particles.
- British Physicist J.J. Thomson first measured the ratio of electrical charge (e) to the mass of the electron (m) in 1897.
e/m = 1.758820x1011 Ckg-1
Electron
- The charge of an electron is equal to -1.602 x 10-19 Coulombs.
- The mass of the electron is 9.1094 x 10-31 kg. The mass of an electron has been found to be (1/1837) times that of a proton.
- Atoms of all elements contain the same number of electrons and protons.
Nucleus
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Ernest Rutherford, a physicist discovered the nucleus of the atom in 1911. Rutherford introduced the term proton in 1920 to describe positively charged particles in atoms. He stated that there was also a neutral particle within the nucleus.
Almost all of the atom's mass is contained within its nucleus. The protons and neutrons have approximately the same mass (the proton is slightly lighter) and have the same angular momentum or spin. Together, protons and neutrons constitute the nucleus of an atom, so they are called nucleons.
The protons and neutrons are held together by nuclear forces thus overcoming a repulsive electrical force that would otherwise cause them to separate according to the rules of electricity. Depending on the nucleus' size, atomic nuclei may be unstable because of differences in the binding force.
Read More:- Difference between Elements and Compounds
Proton and its features
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Protons were discovered by Ernest Rutherford. Protons are the positively charged particles in the atoms. The exist in the nucleus along with neutrons. The number of protons is equal to the number of electrons present in an atom. Some of the characteristics of protons are as follows:
- As hydrogen has only one electron, the removal of the electron would produce a proton.
- The mass of the proton is 1.672 x 10-24 grams.
- The charge of a proton is +1.602 x 10-19 Coulombs, which is exactly the same amount of opposite charge of an electron.
Neutron and its features
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James Chadwick discovered the neutron in 1932. These particles were discovered during an experiment in which a thin sheet of beryllium was bombarded with alpha particles.
Discovery of Neutron
The characteristics of neutrons are as follows:
- Neutrons are subatomic particles that are neutrally charged.
- In addition to protons, neutrons are also present in the nucleus.
- The difference in the number of neutrons in the same element results in the formation of isotopes.
- Neutrons have a mass of 1.676 x 10-24 grams.
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Things to Remember
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- Atoms are stable when electrons with a negative charge and protons with a positive charge are equal in number. Therefore these two opposite charges cancel out each other resulting in an electrically neutral atom.
- Chemical reactions occur when atoms of two or more substances exchange or share electrons with each other. Atoms and molecules produced by the reaction have different electron configurations.
- Neutrons are accountable for most of the atomic mass. Neutrons are 1839 times heavier than electrons.
- Atomic Number: the number of protons present in an atom is called the atomic number of that atom.
- Mass Number: the total number of nucleons (protons and neutrons) present in an atom is called the mass number.
- Isotope: Isotopes of an element have the same number of protons in their nucleus but the number of neutrons differs, which results in different atomic masses. This results in changes in their chemical and physical properties.
Here, the discovery of the three primary subatomic particles is covered, as well as their general properties. However, advances in quantum mechanics and atomic structure have led to the discovery of other new particles also.
Sample questions
Ques. Name the three subatomic particles. What are the differences between them? (2 marks)
Ans. The three subatomic particles are- Electrons, Protons, and Neutrons. Electrons are negatively charged particles. Protons are positively charged and neutrons are electrically neutral, they do not carry any charge.
Ques. Why is an Atom electrically neutral? (2 marks)
Ans. Every atom has the same number of Electrons and Protons. Like we already know, Electrons are negatively charged and protons are positively charged and neutrons do not carry charges. The amount of charge carried by electrons and protons is also the same. Therefore these two equal but opposite charges cancel out each other, leaving the atom in an electrically neutral state.
Ques. What are atomic numbers and mass numbers? (2 marks)
Ans. The number of protons present in an atom is called the atomic number of that atom. The total number of nucleons i.e. protons and neutrons present in an atom is called the mass number.
Ques. What is the number of protons, electrons and neutrons present in atom of carbon? (2 marks)
Ans. Atomic number= 6 Mass number=12
Therefore,
Number of Protons=6
Number of Electrons=6 (same as protons)
Number of neutrons= (12-6)=6
Ques. Why do Isotopes of an atom have different atomic masses? (2 marks)
Ans. Isotopes of an element have the same number of protons in their nucleus but the number of neutrons differs. The Atomic number of two isotopes are same, but their mass number is different. As neutrons contribute most of the mass of an atom, therefore, atomic masses differ with the different number of neutrons present.
Ques. What are the limitations of Rutherford’s model of the atom? (2 marks)
Ans. According to Rutherford’s model of an atom, the electrons are revolving in a circular orbit around the nucleus. Any such particle that revolves would undergo acceleration and radiate energy. The revolving electron would lose its energy and finally fall into the nucleus, the atom would be highly unstable. But we know that atoms are quite stable.
Ques. Describe Bohr’s model of the atom. (4 marks)
Ans. Bohr’s model of the atom:
(1) Atom has a nucleus in the centre.
(2) Electrons revolve around the nucleus.
(3) Certain special orbits known as discrete orbits of electrons are allowed inside the atom.
(4) While revolving in discrete orbits the electrons do not radiate energy.
(5) These orbits or shells are called energy level.
(6) These orbits or shells are represented by the letters K, L, M, N or the numbers n = 1, 2, 3, 4
Ques. Define valency by taking examples of silicon and oxygen. (3 marks)
Ans. Valency is the combining capacity of an atom.
Atomic number of oxygen = 8 Atomic number of silicon = 14 K L M
Electronic configuration of oxygen = 2 6 –
Electronic configuration of silicon =2 8 4
In the atoms of oxygen, the valence electrons are 6 (i.e., electrons in the outermost shell). To fill the orbit, 2 electrons are required. In the atom of silicon, the valence electrons are 4. To fill this orbit 4 electrons are required.
Hence, the combining capacity of oxygen is 2 and silicon is 4.
i.e., Valency of oxygen = 2
Valency of silicon = 4
Ques. Na+ has completely filled K and L shells. Explain. (3 marks)
Ans. Sodium atom (Na), has atomic number =11
Number of protons =11
Number of electrons = 11
Electronic configuration of Na = K L M – 2 8 1
Sodium atom (Na) loses 1 electron to become stable and form Na+ ion. Hence it has completely filled K and L shells.
Ques. If Z = 3, what would be the valency of the element? Also, name the element. (2 marks)
Ans. Z = 3, (i.e, atomic number —> z)
∴ Electronic configuration = 2, 1
Valency = 1
Name of the element is lithium.
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