Atomic Structure Notes
I.) Historical Development
A. Aristotle (350 BCE)
1. Greek Philosopher that said that the 4 core elements (earth, fire, wind, and water) can
be transformed into one another
a. Meaning that matter is divisible or able to be divided
B. Democritus (380 BCE)
1. Greek philosopher who suggested that the universe was made of invisible unit called
atoms
a. “Atomos” is Greek word meaning “indivisible” or “unable to divide”
C. John Dalton (1766-1844)
1. In 1808, proposed a theory that said atoms could not be divided and that all atoms of
the same element are identical
2. Also proposed that atoms of different elements could join to form compounds
3. Theory considered the foundation for modern atomic
theory
D. John “J.J.” Thomson (1856-1940)
1. Discovered the electron in 1897
2. Proposed the “Plum Pudding” model of the atom in 1904
a. Solid positively-charged sphere with negatively-charged electrons embedded
in the sphere
E. Ernest Rutherford (1871-1937)
1. Discovered the nucleus of an
atom in Gold Foil Experiment
in 1911
a. Directed alpha particles (positively-charged Helium atoms) at a thin piece of
gold foil
b. Some of the alpha particle went straight through, some deflected at 90o
angles, and still other bounced right back at the source of the alpha particles
c. Rutherford then concluded that there must be a small positively-charged
nucleus orbited by electrons
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d. Predicted a third subatomic particle, neutron, but that was not discovered until
1932 by James Chadwick
F. Neils Bohr (1885-1962)
1. Proposed the idea that electrons travel in set paths, or orbits, around the nucleus in
1913
a. Each electron has a certain energy that is determined by its path around the
nucleus
b. Only way to move level is to gain energy to go up or loose energy to go down
1. Evidence for this is visible light is produced
when electrons move from one level to next
G. Erwin Schrödinger
1. In 1926 he proposed that electrons did not behave as Bohr said, but rather the
electrons act more like waves or particles on a vibrating string
a. Formed the Electron Cloud Model that predicts the location of electrons
around the nucleus (darker the shading of “cloud”, more likely the location)
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II.) The Structure of an Atom
A. Properties of subatomic particles (See chart on page 106)
1. Proton: a positively charged subatomic particle that’s is found in the nucleus of an
atom
a. Nucleus is the center of the atom
b. Each proton has a +1 charge
2. Neutrons: a neutral subatomic particle that is found in the nucleus of an atom
a. Each neutron has a charge of 0
3. Electrons: a negatively charged subatomic particle that is found in the space around
the nucleus
a. Each electron has a -1 charge
B. Comparing Subatomic Particles
1. Protons, neutrons, and electrons can be distinguished by mass, charge, and location in
an atom (See chart on page 106)
D. Atomic Number and Mass Number
1. Atomic Number: a unique number for each element that equals the number of
protons of that element
a. Atoms of the same element always has the same number of protons
1. Example: There is 1 proton in nucleus of every hydrogen atom
b. Each positive proton is balanced by a negative charge because atoms are
neutral. So the atomic number also equals the number of electrons*
1. Example: Hydrogen has 1 electron; Sulfur has 16 electrons
2. Mass Number: The sum of the protons and neutrons in a nucleus of that atom.
a. Example: Atom of aluminum with 13 protons and 13 neutrons has a mass
number of 26. If you know the atomic number and the mass number of an atom,
you can find the number of neutrons by subtracting
Mass Number = # Protons + # Neutrons
# Neutrons = Mass Number – # Protons
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E. Isotopes
1. Every atom of an element does have the same number of protons and electrons*, but
every atom of element does not have the same number of neutrons.
2. Isotopes: atoms of the same element that have different numbers of neutrons and
different mass numbers
a. Example: Every atom of oxygen has 8 protons
 Some oxygen atoms have 8 neutrons and therefore have a mass
number of 16 (8 protons + 8 neutrons)
 Some oxygen atoms have 9 neutrons and therefore have a mass
number of 17 (8 protons + 9 neutrons)
 Some oxygen atoms have 10 neutrons and therefore have a mass
number of 18 (8 protons + 10 neutrons)
3. When you want to distinguish between one isotope and another, the isotopes are
referred to by their mass numbers (i.e. oxygen-16, oxygen-17, oxygen-18)
4. The atomic mass of an element (on periodic table) is determined by taking the
average masses of all the elements isotopes
a. Why you see atomic masses on periodic table listed as 15.99 rather than 16
III.) Bohr Model of the Atom
1. See notes on Bohr in section I above
2. Energy Levels: According to the model, there are a limited number of electrons allowed in
each energy level. The electrons can move from level to level if gain/loose energy, but cannot
exist in between levels
Energy
Maximum
Number of
Level
Number of e-
Orbitals
1
2
1
2
8
4
3
18
9
4
32
16
3. Electrons in outermost energy level are referred to as valence electrons. These determine the
element’s chemical properties and its ability to form bonds.
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