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Development of the
Atomic Theory
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ATOMIC MODELS
• In ancient Greece Democritus argued:
– Atoms were the smallest particles of matter.
– “Atom”: (from
Greek atomos,
meaning “not able to be divided”
Toga!
Toga!
John Dalton’s model:
**An atom was a hard sphere that was the
same throughout
• Matter is made up of
atoms
• Atoms cannot be divided
into smaller pieces
• All atoms of an element
are exactly alike
• Different elements are
made of different atoms
William Crookes’ experiment
• Cathode ray tube
• Electrode = metal that
conducts electricity
• Anode (+) Cathode (-)
• Battery connected,
greenish glow appeared in
tube
• Shadow of cross fell at
other end of tube
• Proof that something was
traveling in a straight line
through the tube and was
deflected by the cross
JJ Thomson Model
• Discovered electrons!!
• Thomson thought the
electrons were evenly
mixed throughout the
atom
The CRT particles were negatively
charged.
These particles – electrons – are part
of every kind of atom
PLUM
PUDDING
MODEL
Ernest Rutherford
• Rutherford discovered that atoms contain a
small, dense, positively (+) charged center
called the NUCLEUS.
**1920 – He
identified
protons!!
Neils Bohr:
Improvements on Rutherford’s model
Electrons move around
nucleus in different
energy levels.
Protons
Neutrons
+
Electrons
Bohr Model
Electrons (negative charge) move around nucleus in
energy levels in the electron cloud.
Electrons
Valence electrons are in
outermost level. Give
atom its chemical
properties/reactivity
Nucleus
Electrons
Current Atomic Theory
• 1932 - neutrons were
discovered because of
inconsistencies in the
mass of atoms (Chadwick)
Matter
What is Matter?
• Matter is anything that has mass and takes up
space.
• The basic building blocks of all matter are
atoms.
• An atom is the smallest particle into which an
element can be divided and still be the same
substance.
Parts of the Atom
• Proton: a positively (+) charged particle of the
nucleus.
• Neutron: a neutral particle of the nucleus.
• Electron: a negatively (-) charged particle of
the nucleus.
Electron (-)
Nucleus
(98% of the
mass of atom)
Neutron (neutral)
(no charge)
Electron Cloud
(levels/orbits)
Proton (+)
• Atomic Mass Unit (amu): the SI unit for the
masses of particles in atoms.
• Atomic Number: the number of protons in the
nucleus of an atom. (P+)
• Mass Number: number of protons and neutrons
in an atom. (P+ + No)
• Atomic Mass: the weighted average of the
masses of all the naturally
occurring isotopes of an element.
Molecules
• A particle made of two or more atoms bonded
together forms a molecule.
• For example, two hydrogen atoms join with
one oxygen atom to form a water molecule.
Elements
• An element is the simplest form of matter that
cannot be changed into another simpler form
by ordinary means.
• Examples of elements: Oxygen, Helium, Gold,
Silver….
Compounds
• A substance made of two or more elements
chemically combined form a compound.
• Example: NaCl, H2O, CO2
What are atoms made of?
• Protons
18e-e32
– Found in nucleus
– Positive charge (+)
8e- e8 (18)
• Neutrons
– Found in nucleus
– NO charge (0) –
neutral
• Electrons
– Orbits around the
nucleus
– Negative charge (-)
8e8 e-
P+
No
22e-e-