3.2
Introduction to the Atom
The Nucleus
Atomic History
• The term atom coined by the Ancient Greeks.
– Greeks had no evidence for the atom. It was more
of a philosophical idea.
• John Dalton – early 1800s – witnessed
elements reacting in “set ratios.” Assumed
there must be smaller parts combining in
specific ways.
– “Atoms are indivisible!”
Atomic History
• JJ Thompson – 1897 – showed cathode rays
had mass but very low amounts. Showed
these particles were negatively charged and
he believed them to be evenly spaced
throughout the atom. Nobel Prize – 1906
• Ernest Rutherford – 1909 – bombarded gold
foil with alpha (positive) particles to see if
they would pass straight through. Some
particles were deflected leading to the idea
that their was a tiny positive nucleus in the
atom.
History of the Atom
• Niels Bohr – 1913 – proposed that electrons
orbited the atom in fixed positions
• Many others were involved in the
development of atomic theory: Shrodinger, de
Broglie, Heisenberg, Chadwick, Einstein, Max
Planck
Earth’s Matter
Everything in the universe is composed of matter.
Matter: anything that has volume and mass.
Mass: the quantity of matter an object has.
Elements and Atoms
 Elements are pure substances that cannot be broken
down into simpler forms of matter.
 All matter is composed of elements.
 Each element is made up of tiny particles called
atoms.
 Atom: the smallest part of an element that has all the
properties of that element.
Atomic Structure: The Nucleus
 The central core of the atom.
 Consists of two kinds of particles:
 Protons: have a positive electrical charge.
 Neutrons: have a neutral charge.
 Both have almost exactly equal masses of 1 amu
(atomic mass unit).
 Most of the mass in an atom is held in the nucleus.
Atomic Structure: The Nucleus
How do we know how many protons?
BY USING THE PERIODIC
TABLE!!!!!
Atomic Number:
Number of protons in an
atom; usually appears
directly above the chemical
symbol.
How do we know how many neutrons?
BY USING THE PERIODIC TABLE!
Atomic Mass (Weight):
Average masses of all the
isotopes
Mass (Weight) Number:
Number of protons plus
neutrons in an atom;
usually appears directly
below the chemical
symbol.
Atomic Structure
How many protons?
How many neutrons?
What atom is this?
What is the Mass Number?
Atom Equations
Atomic Number =
# of Protons
Mass Number = # of Protons + # of Neutrons
# of Neutrons = Mass Number - # of Protons
# of Protons = Mass Number - # of Neutrons
Example
• Copper – 65
– How many protons?
– How many neutrons?
Isotopes
Isotopes:
Have the same atomic # but different
mass numbers because of a different
number of neutrons.