Structure of Atoms Short Notes | Class 9 | Chapter 4
Introduction
John Dalton believed that atoms were indivisible, i.e., they cannot be divided into smaller parts. However, by the late 19th century, scientists conducted experiments that showed that this belief was not correct. They discovered that atoms consist of smaller charged particles called electrons and protons, and neutral particles called neutrons. These smaller particles are known as subatomic particles, which showed that atoms can be divided into smaller parts.
Discovery Of Electron - Cathode Rays (By J.J. Thomson)
The electron was discovered by physicist J.J. Thomson in 1897 when he was experimenting with cathode rays. He found that these rays were made up of negatively charged particles, which he called electrons, and proved that atoms are not indivisible.
Discovery of Protons - Anode Rays/Canal Rays (By E.Goldstein)
Protons were discovered by Ernest Rutherford in 1919 when he was experimenting with alpha particle bombardment of nitrogen gas.
Discovery of Neutrons (By J. Chadwick)
The neutron was discovered by James Chadwick in 1932. In his experiment, Chadwick bombarded beryllium with alpha particles and found that a new type of neutral particle was emitted, which he called the neutron.
Atomic Models
The atomic model is a diagram that describes the structure of an atom.
Some Atomic models
(i) Thomson's Model of Atom
(ii) Rutherford's Model of Atom
(iii) Bohr's Model of Atom
(i) Thomson's Model of Atom
- Plum Pudding Model: In 1897, Thomson suggested that an atom is a uniform sphere of positive charge having negatively charged electrons inside it, like "plums" in a "pudding." This model showed that atoms are not indivisible and consist of smaller, charged particles (electrons).
- This models also referred to as Water Melon Model
(ii) Rutherford's Model of Atom
Ernest Rutherford's model, which was created in 1911, stated that an atom has a small, dense, positively charged nucleus surrounded by electrons orbiting at a distance. This was established on the gold foil experiment, which indicated that most alpha particles passed through the foil while some were deflected, showing that there is a concentrated center. This model provided the idea of the nucleus and proved that atoms are largely empty space
<< Conclusion
- Nucleus Presence: Atoms have a small, dense, positively charged nucleus that holds most of the mass of the atom.
- Electrons in Space: Electrons move around the nucleus in the surrounding empty space, showing that atoms are largely empty.
- Atomic Structure: The alpha particles' deflection showed that the positive charge is concentrated in the nucleus, resulting in the understanding that atoms are made of a nucleus and orbiting electrons.
<<Drawback of Rutherford's Model
- Nucleus Presence: Atoms have a small, dense, positively charged nucleus that holds most of the mass of the atom.
- Electrons in Space: Electrons move around the nucleus in the surrounding empty space, showing that atoms are largely empty.
- Atomic Structure: The alpha particles' deflection showed that the positive charge is concentrated in the nucleus, resulting in the understanding that atoms are made of a nucleus and orbiting electrons.
(iii)Bohr's Model of Atom
To eliminate the objections against Rutherford's model of the atom, Neils Bohr introduced the following postulates regarding the model of an atom:
(i) Only certain special orbits referred to as discrete orbits of electrons, are permitted within the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
Atomic Number
Definition: The number of protons in the nucleus of an atom is called the atomic number. It is denoted by the symbol "Z." The atomic number decides the identity of an element and its position in the periodic table. Atoms of various elements have different atomic numbers, which differentiate them from each other. For instance, hydrogen has an atomic number of 1, and carbon has an atomic number of 6.
Mass Number
Definition: The sum of the number of protons and neutrons in the nucleus of an atom is called the mass number.
Symbol: It is denoted by the symbol "A."
Calculation: Mass number = Number of Protons (Z) + Number of Neutrons.
Valency
- Definition: Valency is the quantitative measure of an atom's capacity to combine with other atoms based on the number of electrons an atom can gain, lose, or share to attain a complete outer shell.
- Electrons Involved: It mostly involves the outermost electrons, or valence electrons, which take part in the chemical bonding.
- Common Elements: Elements belonging to the same group of the periodic table have a tendency to possess the same valency because of a similar electron configuration.
- Positive and Negative Valency: Atoms may possess positive valency (loss of electrons) or negative valency (gain of electrons). For instance, sodium possesses a valency of +1, whereas chlorine possesses a valency of -1.
- Ionic and Covalent Bonds: Valency is a deciding factor in the way atoms combine, either with ionic bonds (exchange of electrons) or covalent bonds (sharing of electrons).
Isotopes
Definition: Isotopes are atoms of the same element having different mass numbers but the same atomic numbers. For instance:
- Hydrogen: Protium (1H1), Deuterium (1H2), and Tritium (1H3).
- Chlorine: 37Cl and 35Cl.
- Carbon: 14C, 12C, and 13C.
- Oxygen: 16O, 17O, and 18O.
Chemical Properties: Isotopes of the same element have the same chemical properties due to the same number of protons and electrons.
Physical Properties: Isotopes have different masses due to a different number of neutrons. Therefore, their physical properties are different.
Applications:
- Uranium Isotope: Used as a fuel in nuclear reactors.
- Cobalt Isotope: Used in the treatment of cancer.
- Iodine Isotope: Used in the treatment of goitre.
Applications
(i) An isotope of uranium is utilized as a fuel in nuclear reactors.
(ii) An isotope of cobalt is utilized in treating cancer.
(iii) An isotope of iodine is utilized in the treatment of goitre
Isobars
Isobar are elements which have a different chemical property but the same physical property.