THE ATOM
The scientific name for materials is matter. Matter is any substance that has mass and takes up space.
The idea that materials are made up of small particles is often referred to as the particulate nature of matter.
History of the Atom
Greek philosophers
(400 BC) were the first to propose that matter was composed of small particles. They believed that the tiny particles were so small that they could not be broken down into even smaller particles (indivisible - atomos atom).
John Dalton
(1808) was an english chemist, he was the first to propose an atomic theory based on experiments
1. All matter is made up of tiny particles called atoms
2.Atoms are indivisible. They cannot be broken down into simpler particles.
3.Atoms cannot be created or destroyed.
4.The atoms of a a given. element are identical.
5.A compound contains atoms of 2 or more elements..
Discovery of the Electron
William Crookes
(1875) was an english chemist. Crookes found that rays were coming from the negative electrode (cathode) and that these rays cast a shadow of a Maltese cross at the far end of the discharge tube. He had discovered cathode rays.
Cathode rays themselves are invisible but when they strike the glass they cause it to glow. The shadow is caused when these cathode rays are stopped by the Maltese cross. He placed a paddle wheel in the discharge tube which spun went the rays hit it, hence there must be particles.
1.Cathode rays travel in straight lines
2.Cathode rays cause glass to fluoresce when they stake it.
3.Cathode rays passes enough energy to move a paddle wheel.
JJ. Thomson
(1897) another English scientist, devised an experiment to investigate cathode rays were charged.He passed the rays between 2 parallel metal plates. This caused the fluorescent screen to glow. He found that if there was no charge on the parallel plates, the beam of cathode rays struck the fluorescent screen straight on. On placing a positive charge on the top plate, the position of the spot moved upwards. He concluded that they were negatively charged particles.
They were called electrons, whose name was proposed by George Stoney.
JJ Thompson performed a second experiment, to calculate the ratio of the charge of the electron to its mass- E/m of the electron.
Measuring the Charge on the Electron
Robert Millikan
(1909) an American physicist, set up an experiment to determine the size of the charge on the electron. It was called the oil drop experiment (1.6×10-9 Coulomb).
Thomson's Plum Pudding Model of the Atom
In 1898 JJ Thompson proposed that since atoms were neutral there must be a positively charged part of the atom to balance the negative charges of the elections.
Sphere of positive charge with negative electrons embedded at random.
Discovery of the Nucleus
Ernest Rutherford
(1909) a scientist from New Zealand was studying how alpha particles were scattered by a piece of gold foil. It was expected that most alpha particles should pass straight through the thin foil and some should suffer a slight deflection. However some alpha particles were deflected at large angles and some were deflected back along their own path. He concluded that the positive charge and the mass of the atoms of the metal foil were concentrated in a small dense core called the nucleus of the atom.
1.Most alpha particles pass straight through gold foil space --Most of the atom is empty
2.Some alpha particles are deflected back at large angles-- The alpha particles are repelled when they pass near the small positive nucleus
3 A small number of alpha particles are reflected back along their own path-- A small number of Alpha particles collide head on with the nucleus
Discovery of the Proton
Rutherford continued bombarding various elements with alpha particles, and they all gave off the same positively charged particles, which Rutherford named the proton.
Discovery of Neutron
In 1932 James Chadwick bombarded a sample of Beryllium atoms and a particle with no charge was knocked out. He called them neutrons. The neutrons helped to cement the positively charged protons together.
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