1. Charged Particles in Matter
Atoms are made of charged particles: electrons (negatively charged, discovered by J.J. Thomson) and protons (positively charged, discovered via canal rays by E. Goldstein). Electrons have negligible mass, while protons have a mass about 2000 times greater. Rubbing objects, like a comb on hair, creates static electricity, showing atoms contain charged particles.
Activity: Rub a comb on dry hair and see if it attracts paper bits!
2. Structure of the Atom
Atoms are not indivisible, as Dalton thought. They contain sub-atomic particles (electrons, protons, neutrons) arranged in specific ways, explained by different models.
2.1 Thomson’s Model
J.J. Thomson proposed atoms as a sphere of positive charge with electrons embedded like seeds in a watermelon. The positive and negative charges balance, making the atom neutral.
2.2 Rutherford’s Model
Rutherford’s alpha-particle scattering experiment showed:
- Most alpha particles passed through a gold foil, indicating atoms are mostly empty space.
- Some were deflected, showing a small, positive nucleus.
- Few rebounded, proving the nucleus holds most mass.
2.3 Bohr’s Model
Bohr improved Rutherford’s model, suggesting electrons move in fixed orbits (K, L, M shells) without losing energy, making atoms stable.
2.4 Neutrons
J. Chadwick discovered neutrons, neutral particles in the nucleus with a mass similar to protons. Most atoms have neutrons, except hydrogen-1 (protium).
Fun Fact: Rutherford’s experiment was like firing bullets at tissue paper, and some bounced back!
3. Electron Distribution in Shells
Electrons are arranged in shells (K, L, M, N) following Bohr-Bury rules:
- Maximum electrons in a shell = 2n² (n = shell number). E.g., K (n=1): 2 electrons, L (n=2): 8 electrons.
- Outermost shell holds up to 8 electrons.
- Inner shells fill before outer ones.
Activity: Calculate the electron distribution for carbon (6 electrons)!
4. Valency
Valency is an atom’s combining capacity, based on electrons in the outermost shell. Atoms aim for 8 electrons (octet) in the outer shell by gaining, losing, or sharing electrons. For example:
- Sodium (1 outer electron) loses 1 electron, valency = 1.
- Oxygen (6 outer electrons) gains 2 electrons, valency = 2.
Try This: Find the valency of chlorine (7 outer electrons)!
5. Atomic and Mass Number
Atomic Number (Z): Number of protons in the nucleus, defining the element. E.g., carbon has Z=6 (6 protons).
Mass Number (A): Sum of protons and neutrons. E.g., oxygen (8 protons, 8 neutrons) has A=16. Notation: ZAX (e.g., 816O).
Activity: Find the mass number of helium (2 protons, 2 neutrons)!
6. Isotopes and Isobars
Isotopes: Atoms of the same element with the same atomic number but different mass numbers due to varying neutrons. E.g., hydrogen: protium (11H), deuterium (12H), tritium (13H). Isotopes have similar chemical properties but different physical properties.
Isobars: Atoms of different elements with the same mass number but different atomic numbers. E.g., calcium (2040Ca) and argon (1840Ar).
Uses of isotopes include uranium in nuclear reactors and iodine in treating goitre.
Fun Fact: Chlorine’s average atomic mass is 35.5 u due to its isotopes!
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