Intro to Atoms and Atomic Theory

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