1. the atom
Chapter 4

2. The Development of Atomic Theory
Chapter 4- Section 1
Covers:
Atomic Theory
Elecron
Nucleus
Different Models

3. “Atom” “Atomos” – Greek for indivisible or unable to be cut or divided
They determine the properties of matter
Matter is made up of particles called atoms

4. Early Models
Democritus – thought atoms were indivisible & indestructible
Lacked experimental support
4th century B.C.

5. Early Models John Dalton – (1766-1844) Dalton’s Atomic Theory
All elements are composed of tiny, indivisible atoms
Atoms of the same element are identical. Atoms of 1 element are different from another element

6. Early Models Dalton’s Atomic Theory (cont)
Atoms of different elements can either physically mix or chemically combine in simple, whole number ratios to form compounds

7. Early Models Dalton’s Atomic Theory (cont)
Chemical Reactions occur when atoms are separated, joined or rearranged. Atoms of 1 element cannot change into another element by a chemical reaction.

8. Atomic Structure
Dalton’s Theory is mostly accepted today
Except that we now know atoms CAN be divided - into subatomic particles (i.e. electrons, protons, neutrons)

9. Atomic Structure-Electrons
JJ Thompson ( )
discovered electrons using a device called a cathode ray tube

10. Cathode Ray Tube
He sealed gases in a tube fitted at both ends with electrodes (metal disks)
Connected to a source of high voltage electricity

11. Cathode Ray Tube
A glowing beam formed between the 2 electrodes
He called it the cathode ray
CATHODE has a negative charge

12. Cathode Ray Tube
Cathode Ray Tube

13. Cathode Ray Tube
The cathode ray is attracted to metal plates that have a positive charge
It is repelled by negatively charged plates
From Catode(-) to Anode(+)

14. Cathode Ray Tube
The ray must be composed of negatively charged particles moving at a high speed
He called these particles electrons
ANODE has a positive charge

15. Atomic Structure-Electrons
JJ Thomson’s model of the atom is called the plum-pudding model
He thought the electrons were randomly placed throughout the atom, like the currants in a plum pudding

16. The Atomic Nucleus Ernest Rutherford discovered the nucleus
He used a thin sheet of gold foil and bombarded it with alpha particles (helium nuclei)

17. The Atomic Nucleus
Most particles passed through the foil as he expected
However, some were deflected slightly and came through at an angle
Some particles bounced back at him

18. The Atomic Nucleus
He hypothesized that there must be a dense positive part of the atom, but most of the atom is empty space
Rutherford’s Gold Foil Experiment

19. Bohr’s Model
Bohr – proposed that electrons have a fixed energy and move in energy levels around the nucleus – which is why they don’t fall into the nucleus

20. Bohr’s Model
The energy levels are like the rungs of a ladder – electrons cannot be in between levels, and need a specific amount of energy to move from one to another

21. Review Dalton – thought atoms were solid and indivisible
JJ Thomson – discovered the electron, & made the plum-pudding model

22. Review Rutherford – discovered the nucleus
Bohr – proposed electrons in orbitals around nucleus

23. STRUCTURE of ATOMS!!! What do you already know?
What are the subatomic particles?
How can you distinguish between the subatomic particles?