1. Section 4.1 & 4.2 Defining the Atom & Structure of the Nuclear Atom
Chapter 4
Section 4.1 & 4.2
Defining the Atom
&
Structure of the Nuclear Atom

2. Inquiry Activity Pre-read the Inquiry Activity on Page 100
Write your hypothesis to the following questions in your lab notebook:
What will happen when you bring the 2 pieces of “like” charged tape toward each other?
What will happen after you “clean” the tape between your fingers and then try to bring the 2 pieces of tape toward each other?
Perform the experiment
Record your results in your lab notebook
Answer the Think About It questions in your lab notebook

3. Early Models of Atoms
Democritus believed that atoms were indivisible and indestructible.
There was no scientific evidence to support this claim.
Dalton’s Atomic Theory was based on Democritus’s ideas on atoms but he was able to use experimental methods to change the ideas into scientific theory.

4. Dalton’s Atomic Theory
All elements are composed of tiny indivisible particles called atoms.
Atoms of the same element are identical. The atoms of any one element are different from those of any other element.
Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.
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.

5. Sizing up Atoms
Think about a penny…it’s made of Cu (copper)…it contains 2.4 x 1022 atoms.
Compare that to the population of the entire planet Earth…6 x 109 people.
That is
24,000,000,000,000,000,000,000 Cu atoms
or 6,000,000,000 Earthlings
Atoms are extremely small particles.
It would seem impossible to see something so small but there are some special instruments like the Scanning Tunneling Microscope that allow scientists to observe atoms of different elements.

6. Structure of the Nuclear Atom
Subatomic particles – 3 types of particles found inside an atom
Electrons – atomic particles with a negative charge
Protons – atomic particles with a positive charge
Neutrons – atomic particles with no electric charge

7. Positive and Negative Charges
In 1808 John Dalton thought atoms were the smallest particles.
In 1890 JJ Thomson showed that atoms could be broken down into smaller particles with charges
Positive attracts (pulls toward) negative (+ to -)
Negative attracts (pulls toward) positive (- to +)
Positive repels (pushes away) positive (+ to +)
Negative repels (pushes away) negative (- to -)

8. Cathode Ray Experiments
JJ Thomson passed electric through gases at low pressure in his experiments that led to the discovery of the electron.
Cathode Ray Tube

9. Experimental Findings
A cathode ray (glowing beam) traveled from the cathode (- charge) to the anode (+ charge) in the tube when electric current was added.
The cathode ray could be deflected using a magnet or electrically charged metal plates.
Using what was known about attracting and repelling, Thomson concluded that the particles in the cathode ray must be negatively charged. He originally called them ‘corpuscles’ but the name was later changed to electrons.

10. Electrons
Small negatively charged particles in the atoms of every element
The charge of an electron is minus one (-1)
The mass of an electron is 1/1840 the mass of a hydrogen atom.

11. Relative Mass (assuming the mass of a proton = 1) Actual Mass (g)
Particle
Symbol
Relative Charge
Relative Mass (assuming the mass of a proton = 1)
Actual Mass (g)
Electron e- 1-
1/1840
9.11 x 10-28
Proton p+ 1+ 1
1.67 x 10-24
Neutron n0
These are the
commonly used
values for mass.

12. Protons and Neutrons
Four simple ideas about matter helped identify protons and neutrons…
Atoms are neutral (they have no charge)…we know this is true since we don’t get shocked when we touch everything.
Electric charges are carried by particles of matter.
Electric charges exist in whole number multiples…there are no fractions in electric charges.
To become neutral…an equal number of positive particles must be present to balance the number of negative particles present.

13. Canal Rays The Raisin Pudding Model of the Atom (Eugen Goldstein)
In 1886 Eugen Goldstein noted that cathode-ray tubes with a perforated cathode emit a glow from the end of the tube near the cathode. Goldstein concluded that in addition to the electrons, or cathode rays, that travel from the negatively charged cathode toward the positively charged anode, there is another ray that travels in the opposite direction, from the anode toward the cathode. Because these rays pass through the holes, or channels, in the cathode, Goldstein called them canal rays.

14. Protons
Small positively charged particles in the atoms of every element
The charge of a proton is plus one (+1)
Found in the nucleus of the atom
Although the charge of a proton is equal to but opposite of the electron…the mass of a proton is 1840 times larger than an electron
Protons are
the red, +
particles.

15. Neutrons
James Chadwick confirmed the existence of the third subatomic particle, the neutron, in 1932.

16. Neutrons
Small particles in the atoms of every element that have no electric charge
Found in the nucleus of the atom
Neutrons have a neutral charge
Similar in size and mass to protons

17. Plum pudding model of the atom
The Atomic Nucleus
Plum pudding model of the atom
An early model about the structure of the atom was called the “plum pudding
model”. The atom was imagined to be a sphere of positive charge with
negatively charged electrons dotted around inside it like plums in a pudding.
Scientific models can be tested to see if they are wrong by doing
experiments. An experiment carried out in 1905 showed that the plum
pudding model could not be correct.

18. The Atomic Nucleus Nuclear Model of the Atom
A scientist called Rutherford designed an experiment to test the plum
pudding model. A beam of alpha particles was aimed at very thin gold foil
and their passage through the foil was detected. The scientists expected the
alpha particles to pass straight through the foil, but something else also happened.
Some of the alpha particles emerged from the foil at different angles and some
even came straight back. The scientists realized that the positively charged
alpha particles were being repelled and deflected by a tiny concentration of positive
charge in the atom. As a result of this experiment, the plum pudding model was
replaced by the nuclear model of the atom.

19. The Atomic Nucleus
Rutherford proposed, based on his scientific results, a new theory for atoms.
He proposed that the atom is mostly empty space, which explained the alpha particles NOT deflecting.
He concluded that all of the positive charge and almost all of the mass must be concentrated in a small region.
He called this region the nucleus.
The nucleus is the tiny central core of an atom that is composed of protons and neutrons.

20. The Atom The Nuclear Atom
The protons and neutrons are located in the nucleus.
The electrons are distributed around the nucleus and occupy almost all of the volume of the atom.
Suppose an atom were the size of a football stadium…the nucleus would be about the size of a marble in comparison.
The Atom
The Nucleus

21. Atomic Models
These models were not able to hold up through time and experimentation but they were the foundation from which later scientists were able to begin new research.
We will learn more about other models in Chapter 5.

22. Class Work
Book pg 103 (1-7)
Book pg 108 (8-14)
WS 4.1
WS 4.2