HSEB Notes on Three Atomic Model - Dalton, Thomson, Rutherford
Chemistry
Dalton’s Atomic Theory
a) Elements are made of extremely small particles called “atoms”. Atoms of a particular element are identical; different types of atoms have different properties. Each chemical compound is unique and is made of specific atoms put in a specific way. Chemical reactions are the shuffling of atoms; new compounds are made from the same atoms of the original compound.
b) Experimented mostly with gases. Noted that water exists as gas in air. Since two particles cannot occupy the same space at the same time, he hypothesized that evaporation is the mixing of water and air particles (atoms), which are particles of different sizes.
c) Atomic Theory does not describe the structure of an atom, nor the subatomic particles.
d) His hypotheses are the basis of 3 fundamental laws:
- Law of Definite Proportions (Law of Constant Composition): pure samples of a particular compound always have same proportion by mass of elements in the compound.
- Law of Multiple Proportions: when different masses of two elements combine, the mass ratios of one element are small whole number ratios.
- Law of Conservation of Mass: mass of reactants equals the mass of products.
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| Dalton's Atomic Model |
Dalton also showed how to calculate atomic masses of atoms in a reaction and how to find the number of each type of atom in molecules (origins of stoichiometry)
J.J. Thomson’s Model of the Atom
a) Atom is a spherical ball of positive charge with negative charges (corpuscles). His model was nicknamed “the plum pudding model” in which raisins are electrons.
b) 3 experiments:
- Built a cathode ray tube in which an electrometer measured negative charge. Bending the rays with a magnetic field resulted in no negative charge measured, thus the negative charge cannot be separated from the rays.
- A charged particle normally curves through an electric field unless surrounded by a conductor. Thomson thought that gas in the cathode ray tube was being ionized and turned into a conductor by cathode rays. So he removed all the gas and found that the cathode rays bent in an electric field.
- Measured the degree to which rays were bent by a magnetic field to calculate the ratio m/e. Discovered that different gases have different ratios.
Sir Ernest Rutherford
a) Atom consists of positively charged nucleus surrounded by a cloud of negatively charged electrons. Nucleus is composed of neutrons and protons and contains most of the atomic mass. To make the atom electrically neutral, a number of electrons equal to the number of protons surrounds the nucleus.
b) Bombarded a piece of thin gold foil with alpha particles, most of which passed through, showing empty space. Some alpha particles bounced back, showing the existence of a tiny region of concentrated mass in the atom, i.e. the nucleus.
c) His “planetary” model of electron behaviour meant that the movement of electrons around the positive nucleus would cause the electrons to radiate energy while passing electric & magnetic fields surrounding nucleus, and eventually collapse into the nucleus. However, neither the radiation nor the collapse occurred, meaning that the planetary model was wrong.
d) Discovered the atomic nucleus. Discovered alpha, beta, and gamma particles and the nature of radioactivity. Predicted the existence of the neutron.
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Chemistry