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4.1 – Studying Atoms
Science Depends on Evidence
 Remember, we know what we know based on the
EVIDENCE that we have collected over many, many
years.
 As new evidence comes along, our knowledge changes.
 Old ideas are thrown out
 New ideas are tested, and if the evidence is there,
accepted
 Science is not really a quest to be “right”, but to get less
and less “wrong”.
Ancient Greek Models of Atoms
 Democritus (460-370 B.C.E.)
 Idea: Matter is made up of tiny individual particles called
atomos.

Atomos are:
 Solid, homogeneous, indestructible, and indivisible.
 Different kinds of atomos have different sizes and shapes.
 Size, shape, and movement of atomos determine the properties of
matter.
 Evidence: None
 Aristotle (384-322 B.C.E.)
 Idea: Matter is made up of earth, fire, air, and water (the “four
elements”).
 Evidence: None
Dalton’s Atomic Theory (19th
Century, 1800s)
 Idea: Dalton’s Atomic Theory
 All elements are composed of atoms.
 All atoms of the same element have the same mass, and
atoms of different elements have different masses.
 Compounds contain atoms of more than one element.
 In a particular compound, atoms of different elements
always combine in the same way.
 Atoms are pictured as solid, homogeneous,
indestructible spheres.
 Evidence:
 Evidence provided by other scientists from all over the
world (ex: Lavoisier)



A given compound always forms from the same mass ratio of
components.
 Ex: 1000 g of magnesium oxide always forms from 397 g of
oxygen and 603 g magnesium
 Ex: 1000 g of water always forms from 889 g of oxygen and 111 g
of hydrogen
Matter is never created or destroyed in a chemical reaction.
 Ex: Combine 100 g of stuff, and you will get 100 g of new stuff
All atoms of an element appeared to have the same mass.
Thomson’s Model of the Atom
(Late 19th-Early 20th Century, 1850)
 Evidence:
 Used a cathode ray tube to experiment on matter (different metals)
and electricity.
 Noted that the tube had a glowing beam.



Charged plates and magnets outside the tube would cause the beam to
bend (repelled by negative plate, attracted by positive plate).
The same charged particles came from every type of tested metal plates.
Beam must be negatively charged particles that were much lighter than
the lightest atom known (hydrogen).
 Ideas:
 Atoms are NOT solid, indestructible spheres.
 Atoms are a smeared-out positive charge with negatively charged
particles imbedded inside.
 Atom is like a plum pudding or chocolate chip cookie
Rutherford’s Atomic Theory (1911)
 Evidence:
 Used alpha particles to probe the inside of the atom
 Expected almost all alpha particles to burrow through
smeared out positive charge with some small deflection.
 Most particles passed through, but there were more
deflections than expected


Some of the deflections were over 90o.
This was like shooting bullets at a piece of toilet paper and
having some of the bullets bounce off.
 Ideas:
 Atom is NOT a smeared out positive charge.
 Atom contains a super-small, super-dense positively
charged nucleus (high mass, low volume).
 Negative charges are somewhere outside the nucleus.
 Atom is over 99% empty space
4.2 – The Structure of the Atom
Properties of Subatomic Particles
 Protons
 1+ charge
 Found in nucleus of atom
 2nd particle discovered
 Electrons
 1- charge; very light
 Found outside the nucleus
 1st particle discovered
 Neutron
 No charge; slightly heavier than proton
 Found in nucleus
 3rd particle discovered
Isotopes
 Discovery: Atoms of the same element (same number
of protons) that have different number of neutrons
and different mass numbers.
 Examples:



Hydrogen (1 proton): Hydrogen-1, Hydrogen-2, Hydrogen-3
Carbon (6 protons): Carbon-12, Carbon-14
Oxygen (8 protons): Oxygen -16, Oxygen-17, Oxygen-18
 Isotopes disproved that atoms of the same element
always had the same mass.
Atomic and Mass Number
 Number of protons in the nucleus = atomic number
 Atomic number identifies element
 Number of electrons in atom = number of protons (neutral
atom only)
 Number of protons + neutrons = mass number
 Mass number based on mass of carbon-12 isotope
 Number of neutrons = mass number – atomic number
 Example: Lithium has an atomic number 0f 3 and an atomic
mass of 7…



3 protons
3 electrons
7 – 3 = 4 neutrons
4.3 – Modern Atomic Theory
Bohr’s Model of the Atom
 Ideas:
 Like Rutherford’s model, except electrons:


Exist on specific energy levels
 Energy levels can be imagined like orbits
Electrons can jump from orbit to orbit by either absorbing or
emitting a specific energy photon
 This photon would have a specific energy and specific color, if
visible.
 Evidence:
 Excited elements would give off specific colors of lights.
 The spectrum of the element would only contain specific
colors that would correspond to the energy levels in the atom.
Sample Evidence
Electron Cloud Model (Atom Gets
Weird)
 Heisenburg showed that precise electron orbits are
impossible to describe.
 Electrons actually move in a less predictably precise way.
 Schrodinger treated electrons as waves and assigned
them probabilities.
 Electron cloud model describes possible/probable
locations of the electron.
 Orbits replaced by clouds that contain orbitals.
 Can be visualized as a spinning fan.
Atomic Orbitals
 Orbitals are regions of space where an electron is likely to be
found.
 Each orbital can hold 2 electrons.
 Each energy level has at least one sublevel and orbital.
 An electron cloud is the combination of all orbitals in an atom.
Energy Level
Sublevels
Present
Sublevel
# of orbitals
s
1
1
s
p
3
2
s, p
d
5
3
s, p, d
f
7
4 and higher
s, p, d, f
Probability Models of Orbitals