GREEK MODEL

Greek philosopher

Idea of ‘democracy’

Idea of ‘atomos’

Atomos = ‘indivisible’

‘Atom’ is derived

No experiments to

support idea

Democritus’s model of atom

No protons, electrons, or neutrons

Solid and INDESTRUCTABLE

Democritus“To understand the very large,

we must understand the very

small.”

DALTON’S ATOMIC THEORY OF MATTER

His atomic theory of matter contains four hypotheses:

Elements are made of tiny particles called atoms.

All atoms of a given element are identical and different from those of any other element

Atoms of one element can combine chemically with atoms of another element to form compounds. A given compound always has the same relative number and types of atoms.

Atoms are indivisible in chemical processes. Atoms are not created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together.

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

THOMSON MODEL OF THE ATOM

Thomson proposed that atoms consist

of small, negative electrons

embedded in a massive, positive

sphere.

The electrons were like raisins in a

plum pudding, called the ‘plum

pudding’ model of the atom.

J.J. Thomson discovered the

electron and knew that electrons

could be emitted from matter

(1897).

CATHODE RAY EXPERIMENT

Thompson’s 1897 Experiment

Using a cathode ray tube,

Thomson was able to deflect

cathode rays with an electrical

field.

The rays bent towards the

positive pole, indicating that

they are negatively charged.

J.J. THOMPSON’S

A CATHODE RAY TUBE

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

Rutherford Model

In 1911, Ernest Rutherford conducted his gold foil experiment which helped improve our understanding of atomic structure.

He directed a narrow beam of alpha particles at a very thin sheet of gold foil.

Alpha particles (a) are He atomsthat have been stripped of their electrons

RUTHERFORD’S APPARATUS

beam of alpha particles

radioactive

substance

gold foil

circular ZnS - coatedfluorescent screen

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

What he expected…

Because he thought the mass was

evenly distributed in the atom.

--

-

-

-

What he got…richocheting

alpha particles

RESULTS OF GOLD-LEAF EXPERIMENT

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

n +

DENSITY AND THE ATOM

Since most of the particles went through, the atom was mostly empty.

Because the alpha rays were deflected so much, the positive pieces it was striking were heavy.

Small volume and big mass = big density

This small dense positive area is the nucleus

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THE PLANETARY MODEL

+

Nucleus

Electron

Orbit

The Bohr

model of the

atom, was

built upon the

incorrect idea

that electrons

orbit the

nucleus like

planets

around the

sun.

WAVE/CLOUD MODEL OF THE ATOM

In Schrödinger’s wave model of the atom, the

electron behaves as an energy wave as well as a

matter particle.

Einstein had predicted that energy and matter were

related in his equation E = mc2.

If we could see an electron it

might look like this.

THE COMPLETE ATOM

An atom is mostly empty

space with a nucleus

containing protons and

neutrons which contain most

of the mass of an atom.

Electrons are in energy

levels around the nucleus.

Electrons jump between

levels, emitting and

absorbing energy as they

jump.

electron e- -1 1/1840Outside of

nucleus

proton p+ +1 1Inside

nucleus

neutron n0 0 1Inside

nucleus

Structure of the Atom

STRUCTURE OF THE ATOM

There are two regions1. Nucleus

Contains protons and neutrons

Has a positive charge

Almost all the mass

2. Electron Cloud Where electrons are found

Has a negative charge

Almost all the volume

Single Atom

Water Molecule

SIZE OF AN ATOM

Atoms are incredibly tiny.

Measured in picometers (10-12 meters)

Hydrogen atom, 32 pm radius

Nucleus tiny compared to atom

Radius of the nucleus near 10-15 m.

Density near 1014 g/cm3

IF the atom was the size of a stadium,

the nucleus would be the size of a

marble.California WEB