What is matter?

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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

CRT – Cathode ray tube with two metal electrodes Battery 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 particles that 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