Unit 2 – Atoms, Elements, & Compounds

This unit will cover portions of chapters 14, 15, & 16

Atomic Time Line

The next series of slides gives you a brief history of we have come to understand the structure of the atom.

Like the lab yesterday, we cannot see what is inside.

Leucippus & Democritus – 400 BC

First definition of the atom. The smallest part of a thing, that still is that

thing. Examples.

John Dalton – Early 1800s

Famous for his atomic theory. 1. Each element is made of atoms. 2. All atoms of an element are identical. 3. Atoms of different elements have different

properties such as mass and density. 4. Atoms cannot change during a reaction. 5. Compounds are formed from two or more

atoms. 6. Compounds are defined by proportions.

John Dalton – Early 1800s

Dalton’s picture of the atom? Very small spheres that were solid.

J.J. Thomason - 1897

Discovered the electron by accident. We can conduct this experiment today. We have to review a bit of atomic interactions.

+ and + - and – + and –

J.J. Thomason - 1897

Thomason’s picture of the atom? Plum pudding model or today?

Hantaro Nagaoka - 1904

Figured out that electrons are moving around the center of the atom, like planets do around the sun.

Hantaro Nagaoka - 1904

Planetary model

Ernst Rutherford - 1911

Gold Foil Experiment

Ernst Rutherford - 1911

Gold Foil Experiment Results

Ernst Rutherford - 1911

Conclusion Atom is mostly empty space. What is at the middle, must be positive. Discovered the proton and named the nucleus.

Neils Bohr - 1913

Electrons move in energy levels. When an electron moves between energy

levels, it gives off some energy, a.k.a., light. Each element has a unique display of colors.

Neils Bohr - 1913

James Chadwick - 1932

Saw that atoms were twice as heavy as they should be.

p’s e’s n’s

charge

mass

James Chadwick - 1932

Atomic Models – always 3D

Modern Picture of the Atom

The atomic cloud