HW for Next Class:

Read pages 28-29Worksheet: 5.2 (Structure of Nuclear Atom)Quiz: Next time

CHAPTER 4

ATOMIC STRUCTURE

and the PERIODIC TABLE

J. J. ThomsonEnglishPhysicist1897 – discovered electrons:

negatively charged subatomic particles

Experiments using flow of electric current through gases

J. J. Thomson (1856-1940)

CATHODE RAY TUBE

CATHODE RAY TUBE

Thomson found that cathode rays were attracted to metal plates with a positive electrical charge.

Plates with a negative charge repel the ray.Knowing that like charges repel and

opposite charges attract, he determined that the ray was made of negatively charged particle – electrons.

Deflection of Cathode Ray

CATHODE RAY

Thomson showed that the production of the cathode ray was not dependent on the type of gas in the tube, or the type of metal used for the electrodes.

He conclude that these particles were part of every atom.

Determined the mass of the electron as 1/2000 the mass of a Hydrogen atom.

HW for Next Class:

Chapter 2: Read pgs. 26-28Worksheet: 5.1 AtomsQuiz: Mon. Tues. Wed. (Nov. 1, 2 or 3)Retest – sign up sheet (below 80) –

Thursday October 28 or Friday October 29

ATOMIC STRUCTURE

Democritus – teacher, 4th Century BCAtomist school of thoughtMatter is composed of tiny particles

called atomsThese atoms are invisible,

indestructible fundamental units of matter

Democritus

John Dalton (1766-1844)

English school teacherStudied the ratios in which

elements combine in chemical reactions

formulated Dalton’s Atomic Theory

John Dalton (1766-1844)

Dalton’s Atomic Theory

1. All elements are composed of submicroscopic, indivisible particles called atoms

2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

Dalton’s Atomic Theory3. Atoms of different elements can physically

mix together, or can chemically combine with one another in simple WHOLE number ratios to form compounds.

4. Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

THE ATOMSmallest particle of an element that

retains the properties of that element

How small are they?– If you placed 100,000,000 Cu atoms

side by side they would form a line only 1 cm long.

Atomic Theory

Much of Dalton’s theory still holds today

Revision in Dalton’s theory: atoms are NOT indivisible

HW

Read Chapter 2 pages: 29-31Worksheet: Atomic number and mass

numberTest: November 17, 18 and 19 (Wed. Thur.

Friday)

Consider these four ideas about matter and electric charge…

Take away the electrons and what is left?

2. Electric charges are carried by particles of matter

1. Atoms have no net electric charge.

4. When a number of negative particles combine with an equal number of positively charged particles, the resulting particle has no net charge.

3. Electric charges always exist in whole number multiples of a single basic unit.

Protons

A particle with one positive charge must be present to balance each negatively charge electron.– Evidence of these particles provided by E.

Goldstein in 1886.– He observed a ray traveling in the opposite

direction of the cathode ray – canal ray.– Composed of positive particles – Protons.

Is that all?

In 1932 James Chadwick (English physicist 1891-1974) confirmed the existence of another sub-atomic particle – the neutron.

Neutrons have mass similar to protons, but no charge

The Atomic Nucleus

How is the atom put together?

– Difficult to study because of small size.

– Originally it was believed that the positive and negative charges were evenly distributed throughout the atom.

Ernest Rutherford (1871-1937)

University of Manchester, EnglandTested theory of atomic structureBombarded thin gold foil with a beam of

‘alpha’ particles. If the positive charge was evenly spread

out, the beam should have easily passed through.

Ernest Rutherford (1871-1937)

Rutherford's ExperimentRutherford and coworkers aimed a beam of alpha particles at a sheet of gold foil surrounded by a florescent screen.

Rutherford's Experiment

Most particles passed through with no deflection, while some were highly deflectedRutherford concluded that most particles passed through because the atom is mostly empty space.

Rutherford’s Experiment

Based on his results, Rutherford concluded that all of the positive charge, and most of the mass of an atom are concentrated in a small core.

This region would have a positive charge capable of causing the deflection observed.

This region is called the nucleus.