the development of the atomic theory. dalton’s atomic theory founder of the atomic theory atoms in...
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The Development of the Atomic Theory
Dalton’s Atomic Theoryfounder of the atomic theory
atoms in Greek means indivisible, indestructible
1. All matter is composed of atoms.2. Atoms cannot be divided.3. Atoms cannot be created or destroyed (LCM)4. Atoms of the same element are identical in
mass, size & other properties.5. Properties of one element differ from other
elements.6. Atoms combine in small, whole-number ratios
forming compounds.
James Chadwick
• Discovered a neutral particle in the nucleus• Called it a neutron
J.J. Thomson
• Discovered the electron (negatively charged particle)
• Suggested atom consisted of positive & negative charges
• Came up with “raison bun” or “plum pudding” model of the atom
• Suggested atom consisted of a positive charge cloud with electrons dispersed through it
Ernest Rutherford
• Suggested that instead of a positive charge cloud, a nucleus existed in the atom where the positive charges existed
• The nucleus is very small & very positively charged
• Electron cloud surrounds the nucleus• These ideas came from the GOLD FOIL
experiment
GOLD FOIL EXPERIMENT
• If Thomson was right, the alpha particles would pass through the positive charge cloud without being deflected
• But, Rutherford found that some alpha particles were deflected and some came right back
• So, the positive charge is small in size (since most alpha particles passed straight through
• And, when the alpha particles came close to this positive charge, they were deflected a great deal, so the positive charge is very dense
protons
neutrons
Rutherford’s Model of the Atom
The Bohr Atom (Niels Bohr)
• Proposed that electrons in atoms were restricted to certain energy levels
• When electrons moved from one energy level to another, they either absorbed or gave off a specific amount of energy (“what goes up must come down”)
• Continuous spectrum (ROYGBIV)
• Using spectroscopy, the pattern of coloured lines separated by black spaces is called a line spectrum
• Each element has its own unique line spectrum
Energy Levels
n=1
n=7
• The atom is in its ground state when the electrons in the atom are in the lowest possible energy level.
• The atom is in its excited state if an electron moves to a higher energy level by absorbing energy.
• The maximum number of electrons in any given energy level is 2n2, where n is the number of energy level.
• Hence, we get the “Bohr-Rutherford “ Diagrams.
Bohr Atom Video
Limitations to Bohr’s Theory
• Bohr’s theory is only able to explain the line spectrum of atomic hydrogen (1 p+ & 1 e-); however, it cannot explain the observed line spectra of other elements.
• Quantum mechanics or wave mechanics is the theory used to explain the line spectra of elements other than hydrogen.
• Nevertheless, Bohr’s theory is still the basic concept of energy levels in atoms.
Today’s theory of the atom
• The atom consists of a nucleus surrounded by regions of space where electrons are likely to be found.
• These regions are referred to as orbitals.