Class 9 Science · Chapter 8 NotesJourney Inside the Atom

This chapter explores the structure of atoms, the basic building blocks of matter. You'll learn about the evolution of atomic models, the discovery of subatomic particles, and how atoms combine to form molecules. The chapter covers key concepts like the nuclear model of the atom, electron orbits, and the discovery of protons and neutrons. You'll also explore isotopes, isobars, and how atoms combine to form molecules. By the end of this chapter, you'll have a solid foundation in atomic structure and understand why atoms are the building blocks of everything around us.

The journey into the atom begins with the early atomic models proposed by scientists. In the late 19th century, atoms were thought to be the smallest, indivisible units of matter. However, scientists discovered that certain elements emit invisible energy and particles called radiation, a phenomenon known as radioactivity. This showed that atoms must be composed of smaller particles, proving that they were not indivisible as previously believed. In 1897, J.J. Thomson studied the conduction of electric current through gases at a very low pressure. He used a glass tube with two electrodes and applied a high voltage. He observed rays moving from the cathode (negative electrode) to the anode (positive electrode). These were called cathode rays. By studying these cathode rays in electric and magnetic fields, he concluded that they are streams of negatively charged particles, with a much smaller mass than atoms. These particles, later called electrons, were emitted from atoms, indicating that atoms are composed of smaller subatomic components.

• Early atomic models proposed atoms as indivisible units
• Discovery of radioactivity showed atoms are composed of smaller particles
• J.J. Thomson discovered electrons, proving atoms have subatomic components

In 1911, Geiger and Marsden, working under Ernest Rutherford, tested Thomson's model of the atom through what became famous as the gold foil experiment. They aimed a narrow beam of alpha particles at an extremely thin sheet of gold foil. Alpha particles are tiny, positively charged particles emitted from certain radioactive elements. According to Thomson's model, the positive charge in the atom was spread out evenly. So they expected the alpha particles to pass straight through the gold foil or be deflected only slightly. But to their surprise, while most particles passed through undeflected, some were sharply deflected, and a few even bounced back. This deflection from the straight path is called scattering. Hence, the gold foil experiment is also called an alpha-ray scattering experiment. Thomson's model failed to explain the results of the gold foil experiment, particularly the deflection of some alpha-particles through large angles and that most of the alpha-particles passed undeflected.

• Gold foil experiment involved aiming alpha particles at a thin gold foil
• Most alpha particles passed through undeflected, but some were sharply deflected or bounced back
• Thomson's model failed to explain the results of the gold foil experiment

To explain why atoms are stable, Niels Bohr proposed a new model of the atom in 1913. According to Bohr, electrons do not move randomly around the nucleus but follow fixed circular paths called stationary states, orbits, or shells. In each shell, an electron has a definite amount of energy, so these shells are also called energy levels. These shells are represented by the letters K, L, M, N, or by the numbers n = 1, 2, 3, 4. The first energy level K (n = 1) is the one closest to the nucleus and has the least energy. The energy of these levels increases as we move away from the nucleus. That is, the energy of an electron in the L-shell (n = 2) is more than that of an electron in the K-shell (n = 1). The farther away a shell is from the nucleus, the higher is its energy. An electron can move to another shell by absorbing or releasing a fixed amount of energy equal to the difference between the energies of the two levels. Each shell can hold only a certain number of electrons. Bohr's model could explain many experimental observations and marked a major step in understanding the atomic structure.

• Bohr's model proposes electrons move in fixed circular paths called orbits or shells
• Each shell has a definite amount of energy, called energy levels
• Electrons can move to another shell by absorbing or releasing a fixed amount of energy
• Each shell can hold only a certain number of electrons

Rutherford's model showed that most of the mass of an atom is concentrated in its nucleus. Electrons that revolve around the nucleus are so light that their mass can be ignored. However, something puzzling appeared early in the 20th century. For instance, a hydrogen atom has one proton, whereas a helium atom has two protons, yet the mass of a helium atom is about four times that of a hydrogen atom, not double. This led scientists to wonder whether besides protons, is there something else in the nucleus adding mass without affecting its charge. In 1932, James Chadwick, a student of Ernest Rutherford, discovered a new subatomic particle with a mass nearly equal to that of a proton but no electrical charge. This neutral particle was named as neutron and is usually represented by the symbol 'n'. The neutrons are found in the nucleus of all atoms except hydrogen. Thus, the mass of an atom comes mainly from its protons and neutrons packed tightly in the nucleus. This also explains why atoms are heavier than the mass of their total number of protons.

• Protons and neutrons are located in the nucleus of an atom
• Electrons move around the nucleus and have negligible mass
• Neutrons were discovered by James Chadwick in 1932
• The mass of an atom comes mainly from its protons and neutrons

Dalton proposed that all atoms of an element are identical and have the same mass. But scientists later discovered that there were atoms of the same element that could have the same number of protons (atomic number, Z) yet could have different numbers of neutrons, and thus, different mass numbers (A = p+ + n0). These 'twin atoms' with the same atomic number but different mass numbers are called isotopes. Let us take hydrogen as an example. Naturally occurring hydrogen is a mixture of three different isotopes: 1H (protium, ~99.98%), 2H (deuterium, ~0.015%), and 3H (tritium, in traces). All of these contain one proton each, whereas deuterium contains one neutron, and tritium contains two neutrons. The chemical properties of isotopes are similar. It is so because they have the same number of electrons and the same electronic configuration. As you have learnt, chemical properties depend mainly on the number of valence electrons. All the isotopes will have the same chemical properties although they differ in their physical properties. Isobars are atoms of different elements with the same mass number but different atomic numbers. For example, calcium (atomic number 20), potassium (atomic number 19), and argon (atomic number 18) have the same mass number of 40.

• Isotopes are atoms of the same element with the same atomic number but different mass numbers
• Isobars are atoms of different elements with the same mass number but different atomic numbers
• Isotopes have similar chemical properties but different physical properties

The number of atoms of hydrogen or chlorine with which one atom of an element can combine to form a compound is called its combining capacity. It is expressed in terms of hydrogen and chlorine because both possess a combining capacity of one. For example, in H2O (water), oxygen combines with two hydrogen atoms, so the combining capacity of oxygen is two. The outermost shell containing electrons of an atom is known as its valence shell. The electrons present in it are known as valence electrons. If the outermost shell of an atom has 8 electrons, it is called an octet. It has been observed that elements with complete octet of electrons (8 electrons), or 2 electrons in the case of helium in their valence shell are largely unreactive and more stable. On the other hand, atoms with incomplete valence shells are usually more reactive. Such elements lose, gain, or share electrons to complete their octet. The number of electrons gained, lost, or shared to complete the octet is called the valency of the element. Generally, if the element has fewer than four electrons in its valence shell, it tends to lose electrons to complete its octet and become stable. On the other hand, if the number of valence electrons is more than four, it tends to gain electrons to complete its octet.

• Valency is the combining capacity of an atom
• Valency is determined by the number of electrons gained, lost, or shared to complete the octet
• Elements with complete octet of electrons are largely unreactive and more stable