What is matter?

Matter is anything that has mass and

takes up space

What is matter made of??

Atoms.

All matter is made of atoms.

Atoms are the building blocks of Matter

Remember????

• The Cell theory - 3 basic principles:

• All organisms are made of one or more

cells

• The cell is the basic unit of organization

in organisms

• All cells come from cells

There is also an Atomic Theory

Different discoveries made by different

scientists let to today’s atomic model:

-Greeks

-Dalton

-Crookes

-JJ Thomson

-Rutherford

-Niels Bohr and the Atomic Cloud Model

-Schrodinger ( not in the book)

First Thoughts

• Greeks- believed matter is made of tiny

particles.

• Geeks named the tiny particles atom

• Atom = “uncuttable” in Greek

• now we know that the atom can be

divided into smaller particles by nuclear

reactions

**Science today and in the past

• Past – no means to perform experiments

• Discoveries were based in discussions,

observations and reasoning

• There were philosophers instead of

scientists

**Science and Technology today

• Technology –allows us to perform

experiments and to develop and prove

theories

• That is why knowledge in constantly

evolving

• Technology gives us the tools to do

Science

Dalton’s Concept – early 19th century

• Matter is made up of atoms - correct

• Atoms can’t be divided into smaller parts -

wrong

• All the atoms of an element are exactly alike –

almost correct – isotopes are different

• Different elements are made of different atoms –

correct – Element gold has only atoms of gold

• His model: Dalton pictured an atom as a hard

sphere that was the same throughout, like a

marble.

(textbook fig. 3)

Crooke - brought some evidence of Dalton’s ideas.

Late 19th century

BatteryCRT – Cathode ray tube with two metal electrodes

connected to a battery

An electrode is a piece of metal that can conduct

electricity.

When connected to the battery – generates cathode rays

Cathode Rays are a stream of particlesthat traveled from the cathode to the anode.

The cross was blocking the way of the particles and created a shadow

Since Crookes removed all the air from the CRT, he did not know what the particles were.

Thomson experiment(1897)

used the CRT to understand what were those

particles – the particles were attracted by a

positive plate

Thomson conclusions: Cathode Rays

Light is energy in form of waves, not particles and does not have charge, therefore cannot be bent by a magnet.

The cathode rays couldn’t be light.

Thomson conclusions: Cathode Rays

Thomson concluded that the beam must be made up of charged particles of matter that came from the cathode.

The charges were attracted by the + plate, so the

particles had a negative charge.

Thomson and The Electron

Thomson repeated the experiment with other metals, with the same results.

He concluded that these particles were present in all materials.

These negatively charged particles are now called electrons.

Thomson’s Atomic

Model

If atoms contain negatively charged particles, then all matter, should be negatively charged as well.

But all matter isn’t (–) charged.

Matter and atoms are neutral

*So the atom must have + and - regions

Rutherford’s Experiments ( 1906)

• Scientists needed more evidence to support Thompson’s model

• Rutherford fired fast-moving, positively charged bits of matter, called alpha particles, at a thin film of gold.

Rutherford’s Experiment – most particles passed

through the foil, some were deflected and some did not

pass at all.

Results: He discovered the nucleus ( positive charge)

The deflected particles passed close to a + area

The repelled ones hit a positive area :+ and + repel

The ones that passed ( majority) hit a - area

Results: He discovered the nucleus

The deflected particles passed close to a + area: nucleus

The repelled ones hit a positive area: nucleus

The ones that passed ( majority) hit a – area: electrons

Rutherford’s Model

The Nucleus

• He hypothesized that almost all the mass of the atom and all of its

positive charge are

crammed into an

incredibly small region at the center of the atom called the nucleus.

The Proton

• Rutherford’s experiment was done in 1906 and in 1920 scientists called the positive charges in the nucleus protons.

• A proton is a positively charged particle present in the nucleus of all atoms.

The discovery of the neutron and the

revised atomic model

The discovery of the neutron and the

revised atomic model

Rutherford described the atom as a massive positive center surrounded by an empty space occupied by electrons, that have almost no mass.

How was the Neutron discovered?

Scientists knew at that time that:

1) Atoms have electrons and protons

2) Electrons have very little mass, so…..

3) The mass of an atom should have been approximately equal to the mass of its protons, but…..

4) The mass of most atoms is at least twice as much as the mass of its protons.

WHERE is the rest of the mass coming from?

• There must be something else in the

nucleus to account for the extra mass.

• This third particle, which was later call

the neutron would have the same mass

as a proton and be electrically neutral.

The model of the atom was revised :

A central nucleus with

protons and neutrons and

electrons surrounding

the nucleus moving in orbits.

Niels Bohr (1913) and the Atomic Cloud Model

Soon after Niels Bohr model, scientists soon learned that electrons are in constant, unpredictable motion and can’t be described by moving in an orbit.

*Erwin Schrodinger: The Electron Cloud

Model ( 1926) – current model

• Electrons travel in a region surrounding the nucleus, which is called the electron cloud.

• The electrons can be anywhere, most likely close to the nucleus, attracted by the + forces

Important to know:

The model of the atom as we know today

The particles name, where they are and

the charges

In a neutral atom the number of protons

and electrons are the same

The importance of technology in the

development of Science