Chapter 4Section 4.1 & 4.2

Defining the Atom &

Structure of the Nuclear Atom

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

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.

Dalton’s Atomic Theory

1. All elements are composed of tiny 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.

3. Atoms of different elements can physically mix together or can chemically combine 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.

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.

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

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 -)

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

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.

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.

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 0 1 1.67 x 10-24

These are thecommonly usedvalues for mass.

Protons and Neutrons

• Four simple ideas about matter helped identify protons and neutrons…

1. Atoms are neutral (they have no charge)…we know this is true since we don’t get shocked when we touch everything.

2. Electric charges are carried by particles of matter.3. Electric charges exist in whole number multiples…

there are no fractions in electric charges.4. To become neutral…an equal number of positive

particles must be present to balance the number of negative particles present.

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.

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 arethe red, + particles.

Neutrons

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

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

The Atomic Nucleus

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.

Plum pudding model of the atom

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.

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.

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

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.

Class Work

• Book pg 103 (1-7)• Book pg 108 (8-14)• WS 4.1• WS 4.2