Chapter 5
Chapter 5
Democritus – thought atoms were indivisible & indestructible Lacked experimental support
4th century B.C.
John Dalton – (1766-1844)Dalton’s Atomic Theory
1. All elements are composed of tiny, indivisible atoms
2. Atoms of the same element are identical. Atoms of 1 element are different from another element
Dalton’s Atomic Theory (cont)3. Atoms of different
elements can either physically mix or chemically combine in simple, whole number ratios to form compounds
Dalton’s Atomic Theory (cont)4. Chemical Reactions occur
when atoms are separated, joined or rearranged. Atoms of 1 element cannot change into another element by a chemical reaction.
Dalton’s Theory is mostly accepted todayExcept that we now know atoms CAN be divided - into subatomic particles
JJ Thompson (1856-1940)discovered electrons using a device called a cathode ray tube
•He sealed gases in a tube fitted at both ends with electrodes (metal disks)
•Connected to a source of high voltage electricity
A glowing beam formed between the 2 electrodes
He called it the cathode ray
Cathode Ray Tube
The cathode ray is attracted to metal plates that have a positive charge
It is repelled by negatively charged plates
The ray must be composed of negatively charged particles moving at a high speed
He called these particles electrons
JJ Thomson’s model of the atom is called the plum-pudding model
He thought the electrons were randomly placed throughout the atom, like the currants in a plum pudding
Ernest Rutherford discovered the nucleusHe used a thin sheet of gold foil and bombarded it with alpha particles (helium nuclei)
Most particles passed through the foil as he expected
However, some were deflected slightly and came through at an angle
Some particles bounced back at him
He hypothesized that there must be a dense positive part of the atom, but most of the atom is empty space
Rutherford’s Gold Foil Experiment
Bohr – proposed that electrons have a fixed energy and move in energy levels around the nucleus – which is why they don’t fall into the nucleus
The energy levels are like the rungs of a ladder – electrons cannot be in between levels, and need a specific amount of energy to move from one to another
Dalton – thought atoms were solid and indivisible
JJ Thomson – discovered the electron, & made the plum-pudding model
Rutherford – discovered the nucleus
Bohr – proposed electrons in orbitals around nucleus