Download File

Chapter 4
The Structure of an Atom
Early Theories of Matter - Democritus
• Thought that matter is composed of the empty space
through which atoms move
• Atoms are solid, homogenous, indestructible and indivisible
• Different kinds of atoms have different shapes
• The properties of matter are due to the different sizes,
shapes and movements of atoms
• Changes in matter result from the different groupings of
atoms – NOT from changing atomic properties
Early Theories of Matter - Aristotle
• One of the influential philosophers
• Denied the existence of atoms
Early Theories of Matter – John Dalton
• All matter is composed of extremely small particles called
atoms
• All atoms of a given element are identical (same size, mass
and chemical properties)
• Atoms cannot be created, divided and/or destroyed
• Different atoms combine in whole-number ratios to form
compounds
• In a chemical reaction, atoms are separated, combined
and/or rearranged.
What is an atom?
• An atom is the smallest particle of an element that retains
its physical properties
Discovering the Electron
• Sir William Crookes (English Physicist) discovered the
electron while working in a cathode ray lab
• Cathode Rays were streams of charged particles that carried a
negative charge
• He changed both the electrode and type of gas in the cathode ray, but
neither affected the cathode ray produced.
• Therefore, conclusion was negatively charged particles were found in
all forms of matter
• These are called electrons
Discovering the Electron, Cont.
• Robert Millikan, an American Physicist, determined the
charge of an electron (-1)
• Mass of an electrons:
• 9.1 x 10-28 = 1/1840 Mass of an hydrogen atom
The Nuclear Atom
• Rutherford conducted an experiment with gold foil that
created the Rutherford Model of an Atom. He concluded
that there was a tiny centrally located dense region of an
atom called a nucleus, which contains most of the atoms
mass.
Atoms
The Structure of an Atom
Completing the Atom
• The Atom is made up of three things:
• Proton: Found in the nucleus and is positively charged
• Electron: Found in the space surrounding the nucleus and negatively
charged
• Neutron: Found in the nucleus and is neutrally charged
Particle
Symbol
Location
Electron
e-
Proton
p+
Neutron
No
Space
surrounding the
nucleus
Relative
Electrical
Charge
Relative Mass
Actual Mass
1-
1/1840
9.11 x 10-28
Nucleus
1+
1
1.673 x 10-24
Nucleus
0
1
1.675 x 10-24
How Atoms Differ
• Atomic Number = Number of Protons = Number of Electrons
Chemical Name
Chemical Symbol
Atomic Number
Average Atomic
Mass
Isotopes and Mass Number
• Atoms with the same number of protons but differing
amounts of neutrons are isotopes
• Different isotopes have different mass numbers, which are
added after the element, to denote the different amount of
neutrons
• For example, Carbon-15 has 6 Protons and 9 Neutrons
How Do You Find the Number of
Neutrons?
Number of Neutrons = Mass Number – Atomic Number
Mass of Individual Atoms
• Atomic Mass Unit is defined as 1/12 of a carbon atom
• Atomic Mass of an element is the weighted average mass of
the isotopes
Particle
Mass (AMU)
Proton
1.007276
Electron
Neutron
0.000549
1.008665
Atoms and Their Interactions
Elements
• A substance that can’t be broken down
into simpler chemical substances
• Everything is made up of elements