Grade 8 Chapter 5: Inside the Atom

Introduction

Everything around us, from the air we breathe to the solid ground we walk on, is made up of tiny particles called atoms. For centuries, atoms were considered the smallest, indivisible units of matter. However, scientific discoveries have revealed that atoms themselves have a complex internal structure. This chapter will take us on a journey to explore the historical development of atomic models and delve into the fascinating world of subatomic particles, their arrangement, and how they determine the properties of elements.

Early Atomic Theories

1. Dalton's Atomic Theory (1803)

John Dalton proposed the first modern atomic theory based on experimental observations. His main postulates were:

• Matter is made up of extremely small, indivisible particles called atoms.
• Atoms of the same element are identical in all respects (mass, size, chemical properties).
• Atoms of different elements are different in all respects.
• Atoms cannot be created or destroyed.
• Atoms combine in simple whole-number ratios to form compounds.

2. Thomson's Atomic Model (Plum Pudding Model, 1904)

J.J. Thomson, after discovering the electron, proposed that atoms are positively charged spheres with negatively charged electrons embedded in them, much like plums in a pudding or seeds in a watermelon.

Main features:

• An atom consists of a positively charged sphere.
• Electrons (negative charges) are embedded in this positive sphere.
• The positive and negative charges are equal in magnitude, making the atom electrically neutral.

3. Rutherford's Atomic Model (Nuclear Model, 1911)

Ernest Rutherford conducted the famous alpha-particle scattering experiment, which led to the discovery of the nucleus and proposed the nuclear model of the atom.

Alpha-particle Scattering Experiment:

• Rutherford bombarded a very thin gold foil with fast-moving alpha particles (positively charged helium nuclei).
• Observations:
  • Most alpha particles passed straight through the foil without deflection.
  • Some alpha particles were deflected by small angles.
  • A very few alpha particles (about 1 in 20,000) were deflected back by large angles or even bounced back.

Conclusions from the experiment:

• Most of the space inside the atom is empty (as most particles passed straight).
• There is a small, dense, positively charged center in the atom called the nucleus (responsible for deflecting and reflecting alpha particles).
• The size of the nucleus is very small compared to the size of the atom.
• Electrons revolve around the nucleus in well-defined orbits.

Main features of Rutherford's Model:

• The atom has a positively charged center called the nucleus, which contains almost all the mass of the atom.
• Electrons revolve around the nucleus in circular paths.
• The size of the nucleus is very small compared to the size of the atom.

4. Bohr's Atomic Model (1913)

Niels Bohr proposed a more stable model of the atom based on quantum theory, overcoming the limitations of Rutherford's model.

Main postulates of Bohr's Model:

• Electrons revolve around the nucleus in specific, fixed orbits called shells or energy levels.
• While revolving in these discrete orbits, electrons do not radiate energy.
• Each shell has a fixed amount of energy. The orbits are designated as K, L, M, N... shells or 1, 2, 3, 4... energy levels.
• Electrons can jump from a lower energy level to a higher energy level by absorbing energy, and from a higher energy level to a lower energy level by emitting energy.

Structure of the Atom

Based on these models, the atom is now understood to consist of a central nucleus and electrons revolving around it.

Subatomic Particles

• Nucleus: The central part of an atom, containing protons and neutrons. It is positively charged and contains almost all the mass of the atom.
• Electrons: Negatively charged particles that revolve around the nucleus in specific orbits or shells.
• In a neutral atom, the number of protons is equal to the number of electrons.

Atomic Number (Z) and Mass Number (A)

• Atomic Number (Z): The number of protons present in the nucleus of an atom. It defines the identity of an element. For a neutral atom, Atomic Number = Number of Protons = Number of Electrons.
• Mass Number (A): The total number of protons and neutrons in the nucleus of an atom. It is the sum of protons and neutrons.

  Mass Number (A) = Number of Protons (Z) + Number of Neutrons (n)

  Therefore, Number of Neutrons (n) = Mass Number (A) - Atomic Number (Z).
• An element is represented as: ^A_ZX, where X is the symbol of the element, A is the mass number, and Z is the atomic number.

Isotopes, Isobars, and Isotones

• Isotopes: Atoms of the same element that have the same atomic number (Z) but different mass numbers (A). This means they have the same number of protons but different numbers of neutrons.
  • Examples: Hydrogen has three isotopes: Protium (¹H), Deuterium (²H), Tritium (³H).
• Isobars: Atoms of different elements that have the same mass number (A) but different atomic numbers (Z). This means they have different numbers of protons but the same total number of nucleons (protons + neutrons).
  • Examples: Argon (⁴⁰₁₈Ar) and Calcium (⁴⁰₂₀Ca) both have a mass number of 40.
• Isotones: Atoms of different elements that have the same number of neutrons but different atomic numbers and mass numbers.
  • Example: Carbon-13 (¹³₆C) has 7 neutrons (13-6=7) and Nitrogen-14 (¹⁴₇N) has 7 neutrons (14-7=7).

Atomic Mass and Valency

• Atomic Mass: The mass of an atom is primarily determined by the sum of the masses of its protons and neutrons in the nucleus. It is expressed in atomic mass units (amu).
• Valency: The combining capacity of an element. It is the number of electrons an atom gains, loses, or shares to achieve a stable outermost electron shell (octet or duplet for hydrogen/helium).
  • If an atom has 1, 2, or 3 electrons in its outermost shell, it tends to lose them, and its valency is 1, 2, or 3 respectively.
  • If an atom has 5, 6, or 7 electrons in its outermost shell, it tends to gain electrons to complete an octet, and its valency is (8 - number of outermost electrons).
  • If an atom has 4 electrons in its outermost shell, it tends to share them, and its valency is 4.
  • If an atom has 8 electrons in its outermost shell (or 2 for hydrogen/helium), it is stable (inert) and has a valency of 0.

Electronic Configuration

Electronic configuration is the distribution of electrons in different shells or energy levels around the nucleus of an atom. The maximum number of electrons that can be accommodated in a shell is given by the formula 2n², where 'n' is the shell number (1 for K, 2 for L, 3 for M, etc.).

• K-shell (n=1): Maximum 2 electrons (2 x 1² = 2)
• L-shell (n=2): Maximum 8 electrons (2 x 2² = 8)
• M-shell (n=3): Maximum 18 electrons (2 x 3² = 18)
• N-shell (n=4): Maximum 32 electrons (2 x 4² = 32)

Rules for filling electrons:

• Electrons first fill the innermost shells before occupying outer shells.
• The outermost shell cannot have more than 8 electrons (except for the first shell, which can hold a maximum of 2).

Chemical Symbols of Elements

Elements are represented by chemical symbols, which are usually one or two letters derived from their English or Latin names. The first letter is always capitalized, and the second letter (if any) is lowercase.

• Examples: Hydrogen (H), Helium (He), Oxygen (O), Sodium (Na - from Natrium), Iron (Fe - from Ferrum).

Summary

• Atoms are the fundamental building blocks of matter, composed of subatomic particles: protons (positive), neutrons (neutral) in the nucleus, and electrons (negative) orbiting the nucleus.
• Historical atomic models include Dalton's (indivisible atoms), Thomson's (plum pudding), Rutherford's (nuclear model from scattering experiment), and Bohr's (electrons in fixed energy shells).
• Atomic number (Z) is the number of protons; Mass number (A) is the sum of protons and neutrons.
• Isotopes have the same Z but different A (different neutrons). Isobars have the same A but different Z. Isotones have the same number of neutrons.
• Valency is the combining capacity, determined by electrons gained, lost, or shared to achieve a stable outer shell (octet rule).
• Electronic configuration describes electron distribution in shells (K, L, M, N...) following the 2n² rule.
• Elements are represented by unique chemical symbols.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 5: Inside the Atom.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.