Atomic Structure and the Periodic Table - Early Models of the Atom
Defining the Atom
Atom -
Democritus’s Atomic Philosophy
How did Democritus describe atoms?
Democritus:
(Draw a picture)
Model
Evidence/Experiment: he did not have any experimental evidence to support his beliefs
Model: Atoms are indivisible and indestructible and are the same shape and texture as the object
(Fuzzy object = Fuzzy atoms)
Dalton’s Atomic Theory
How did John Dalton further Democritus’s ideas on atoms?
Dalton:
(Draw a picture)
Model
Evidence/Experiment:
Model: Atoms are hard solid SPHEREs (indivisible)
(Different atoms = Spheres with different masses)
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The result was Dalton’s atomic theory. (Very important!!)
1.
2.
The atoms of any one element are different from those of any other element.
(All oxygen atoms have a mass of 12 AMU, but all nitrogen atoms have a mass of 14 AMU)
3. Atoms of different elements can:
a)
b)
4.
Atoms of one element are never changed into atoms of another element in a chemical reaction.
Structure of the Nuclear Atom – The discovery of subatomic particles
Subatomic Particles
What are three kinds of subatomic particles?
Electrons – were discovered
 In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron.
 Electrons are negatively charged subatomic particles.
 Thomson performed experiments that involved passing electric current through gases at low pressure. The
result was a glowing beam, or cathode ray, that traveled from the cathode to the anode
J.J. Thomson’s discovered the electron (e-)
What did Thomson conclude from his experiments with the Cathode Ray?
Thomson:
(Draw a picture)
Model
Evidence/Experiment:
Model:
Protons and Neutrons
In 1886, Eugen Goldstein (1850–1930) observed a cathode-ray tube and found rays traveling in the direction opposite
to that of the cathode rays. He concluded that they were composed of positive particles.
PROTONS – subatomic particles with a POSITIVE charge
In 1932, the English physicist James Chadwick (1891–1974) confirmed the existence of yet another subatomic particle:
the neutron.
NEUTRONS- subatomic particles with NO charge, but with a mass nearly equal to that of a proton. (Same mass as proton)
In 1911, Rutherford and his coworkers at the University of Manchester, England, directed a narrow beam of alpha
particles at a very thin sheet of gold foil. – Rutherford’s Gold-Foil Experiment
Ernest Rutherford discovers the NUCLEUS of an atom
What did Rutherford conclude from his experiments?
Thomson:
Evidence/Experiment:
Model:
(Draw a picture)
Model
The NUCLEUS is the (positively charged) tiny central core of an atom and is composed of protons and neutrons.
In the nuclear atom – (What we have learned so far)
The protons (P+) and neutrons (N0) are located in the nucleus. The protons and neutrons have almost all of the MASS of the atom.
The electrons (e-) are distributed around the nucleus (outside of the nucleus) and occupy almost all the VOLUME of the atom.
Fill in the table below:
Charge
Location
AMU
Actual Mass (g)
Proton
Neutron
Electron
AMU = Atomic Mass Units, where both protons and neutrons have a mass of 1 AMU
Sizing up the Atom
What instruments are used to observe individual atoms?
Quiz: Defining the Atom
1. The ancient Greek philosopher credited with suggesting all matter is made of indivisible atoms is
a) Plato.
B) Aristotle.
C) Democritus.
D) Socrates.
2. Dalton's atomic theory improved earlier atomic theory by
a)
b)
c)
d)
teaching that all matter is composed of tiny particles called atoms.
theorizing that all atoms of the same element are identical.
using experimental methods to establish a scientific theory.
not relating atoms to chemical change.
3. Individual atoms are observable with
a) the naked eye.
B) magnifying glass.
C) a light microscope.
D) a scanning tunneling microscope.
Quiz: Structure of the Nuclear Atom
1. Which of the following is NOT an example of a subatomic particle?
a) proton
B) molecule
C) electron
D) neutron
2. The nucleus of an atom consists of
a) electrons only.
B) protons only.
C) protons and neutrons.
D) electrons and neutrons.
3. Most of the volume of the atom is occupied by the
a) electrons.
B) neutrons.
C) protons and neutrons.
D) protons.
The Atom – How our Understanding of the Atom has Changed.
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