1. 5.1 Evolution of the Atomic Model

2. Learning Goals
I can describe the components of atoms and their locations within atoms.
I can describe how the work of Dalton, Thomson, Rutherford and Bohr contributed to the development of the atomic model.

3. Atomic Theory
John Dalton’s atomic theory described elements in terms of atoms which he believed to be small, indivisible particles that make-up all matter.
Atomic Theory
He stated that all the atoms of the same element are identical in mass and size, but different elements are made up of different atoms.

4. Joseph John Thomson determined that atoms contain negatively charged particles, which are now called electrons.
He developed a model of the atom that shows electrons inserted throughout a mass of positively charged material.
Thomson model

5. Ernest Rutherford updated the model of the atom as mostly empty space, with a small, dense, positively charged nucleus at the centre.
His continued work, as well as that of others, eventually led to the identification of the proton and neutron in the nucleus.
Rutherford model
Neils Bohr revised Rutherford’s model of the atom by stating that electrons are stable in specific energy levels around the nucleus.
Bohr Model

6. 1. What is an atom? What are the three subatomic particles of an atom?
An atom is the smallest particle of an element that has the same properties of the element.
Protons – positively charged
found in the nucleus
1 amu (atomic mass unit)
Neutron – neutral (no charge)
Electron – negatively charged
found in energy levels surrounding the nucleus
0 amu (atomic mass unit)

7. 2. What is Dalton’s description of an atom? List Dalton’s Atomic theory.
Elements are made of tiny particles called atoms.
Atoms cannot be created nor destroyed
All atoms of a given element are identical.
The atoms of a given element are different from those of any other element.
Atoms of one element can combine with atoms of other elements to form chemical compounds.

8. Thomson’s Discovery of electrons:
3. Thomson’s experimental evidence indicated that all atoms contain the same negatively charged particles. Describe Thomson’s experimental evidence.
When a ray was emitted from the negatively charged cathode it travelled to the positively charged anode.
Thomson discovered that the same kind of negative ray was always emitted for all elements.
These particles are now called electrons and are found in all atoms.

9. 4. What is the difference between Thomson’s model of the atom and Dalton’s model of the atom?
Dalton stated that different elements are composed of different atoms and that atoms cannot be divided into smaller particles.
Thomson built on Dalton’s model by stating that the atom is a lump of positively charged materials with negative electrons inserted throughout.

10. 5. Rutherford's experiment video

11. 6. What are alpha particles
6. What are alpha particles? What is the source of these alpha particles?
Alpha particles are positively charged particles
The source is a very small amount of radioactive element called radium.

12. 7. Describe Marie Curie’s contribution to science.
Marie Curie was a polish born physicist who received two Nobel Prizes (one for physics and one for chemistry)
She discovered and isolated the radioactive element radium.

13. 8. What is the result of Rutherford’s gold foil experiment?
Some of the particles were repelled backward instead of passing through the foil.
Since the alpha particles are positively charged, Rutherford proposed that all the positively charged materials in an atom formed a small dense centre and that the electrons would have to be separated from it.
He named this centre the nucleus

14. 9. What did Bohr discover about the movement of electrons?
Bohr proposed that electrons could only move within fixed regions (energy levels), rather than being able to move anywhere around the nucleus.

15. 10. How can an electron move from one energy level to a higher energy level?
For an electron to move from one energy level to a higher one, it must absorb a specific amount of energy (quantum).

16. 11. Describe the nucleus of an atom. What is it made of?
The nucleus is composed of two types of particles: the positively charged proton and a neutral neutron.

18. 12. Draw the four models of an atom using diagrams.
John Dalton
Joseph John Thomson

19. 12. Draw the four models of an atom using diagrams. (continued)
Ernest Rutherford
Niels Bohr

20. Practice
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