Atomic TheoryChapter 11, Section 1

Ancient Greeks

Aristotle Democritus (440 BC)

Break things apart forever and keep their identity

All things were composed of 4 ‘elements’ (Earth, Air, Fire, Water)

Keep breaking in half - eventually get to something that can’t be broken down (indivisible particle)

“THE ATOM”

Democritus was a PHILOSOPHER, not a SCIENTIST.He used reasoning, not experiments to come up with his idea.

What are all things made of? The debate about the nature of matter!

Ancient Greeks

Aristotle Democritus (440 BC)

Break things apart forever and keep their identity

All things were composed of 4 ‘elements’ (Earth, Air, Fire, Water)

Keep breaking in half Eventually get to something

that can’t be broken down (indivisible particle)

“THE ATOM”

• Atoms are small, hard particles made of a single material and formed into different shapes and sizes.

• Atoms are always moving.• Atoms form different materials

by joining together.

What are all things made of? The debate about the nature of matter!

• Although Democritus is closer to being correct, Aristotle's ideas were more widely accepted.

• 1800’s experimental data began to support Democritus

Dalton’s Atomic Theory (1803)

• 1st to provide experimental evidence of atoms.

• Experimented with combining elements

• Noticed that elements combined in specific proportions

Dalton’s Atomic Theory (1803)

DALTON SUMMARIZED1. Atoms are the smallest particles

2. Can’t Be Divided (nothing inside atom)

3. Atoms of the same type make ELEMENTS

4. ELEMENTS join to make new substances (COMPOUNDS)– 2H + O H2O

(Water is made of two types of atoms)

Thomson’s Experiment (1897)

• Used a cathode-ray tube.– Invisible beam would “bend” towards positive side

beam made of negative particles – Discovered electrons

Thomson’s Model (1897)

• Discovered atoms are made of smaller parts.

• Since atoms are neutral, they must also have positive particles (to balance the negative ones he discovered).

• Proposed Plum-Pudding model or ‘choc chip ice-cream’ model

• Ice cream = Positive charges (protons)• Chocolate Chips = Negative charges

(electrons)

Rutherford (1911)• Gold foil experiment – aimed beam of positively charged

particles at thin sheet of gold foil– If atoms were “soft blobs” the particles should pass straight through

Rutherford (1911)

• Only a few particles bounced back

Atoms have dense positively

charged centers (nucleus)

• Most particles passed straight through the foil

Atoms have lots of empty space.

Bohr Model (1913)• Used crazy math tricks to build on Rutherford’s model• Electrons travel in paths (around nucleus) and are

found in specific energy levels• Atoms can jump between levels from path to path

We will use the Bohr model to represent atoms in here

AKA “Solar System model”

Modern Theory – Twentieth Century Scientists

• Electrons move unpredictably but spend more time in certain regions than others.

• Electron clouds – where electrons are most likely to be found