structure of an atom

Chapter 3: Structure of the Atom

Overview

• Atoms and molecules are the fundamental building blocks of matter.

• Different matter is constituted by varying types of atoms.

• Key questions:

  • What differentiates atoms of different elements?

  • Are atoms indivisible as proposed by Dalton, or do they contain smaller components?

• This chapter will explore subatomic particles and atomic models.

4.1 Charged Particles in Matter

• Experiments on static electricity provide insight into the charged nature of atoms.

• Activities:

  • A: Comb dry hair and attract small paper pieces.

  • B: Rub glass rod with silk and observe its effect on a balloon.

• Findings from earlier experiments highlight:

  • The existence of charged particles in atoms.

• Key Discoveries in Atomic Structure:

  • J.J. Thomson (1900): Discovered the electron (e–), indicating atoms are divisible.

  • E. Goldstein (1886): Discovered canal rays (positively charged particles), leading to the identification of the proton (p+).

• Charge balance:

  • Atoms consist of protons (positive charge) and electrons (negative charge) that balance each other.

  • Protons are located in the atom's interior, making electrons easier to remove.

4.2 The Structure of an Atom

• Dalton’s atomic theory was challenged by the discovery of protons and electrons, prompting new models of atomic structure.

4.2.1 Thomson’s Model of an Atom

• The atom is visualized as a positively charged sphere with electrons embedded, akin to currants in a Christmas pudding or seeds in a watermelon.

• Key aspects:

  • The sphere contains positively charged matter, with electrons embedded.

  • The total charge of the atom is neutral due to equal positive and negative charge magnitudes.

4.2.2 Rutherford’s Model of an Atom

• Conducted an experiment using alpha (α) particles on gold foil.

  • Most α-particles passed straight through, indicating most atomic space is empty.

  • Few α-particles were deflected, and some rebounded, revealing:

    • Mass and positive charge are concentrated in a small nucleus.

    • Proposed a nuclear model where:

      • Electrons revolve around a dense, positively charged nucleus.

• Findings:

  • Nucleus is tiny compared to the overall size of the atom.

Drawbacks of Rutherford’s Model

• Electrons in circular motion would radiate energy and spiral into the nucleus, leading to instability.

• Atoms remain stable in reality.

4.2.3 Bohr’s Model of Atom

• Proposed resolution for inaccuracies in Rutherford's model.

• Postulates:

  • Only certain orbits (discrete) for electrons.

  • Electrons don’t radiate energy while in these special orbits.

4.2.4 Neutrons

• Discovered by J. Chadwick in 1932: neutral particles with a mass similar to that of protons.

• Neutrons exist in all atomic nuclei except in hydrogen.

• Typically represented as ‘n’.

4.3 Distribution of Electrons

• Proposed rules for electron distribution in shells:

  • Maximum electrons in a shell given by 2n², where n = shell number.

  • K shell can hold 2, L shell can hold 8, M shell can hold 18, N shell can hold 32.

  • Outer shell accommodates up to 8 electrons and must be filled sequentially.

4.4 Valency

• Outer shell electrons determine an element's combining capacity (valency).

• Generally, elements with complete outer shells are less reactive (valency=0).

• Example: Reactivity is based on