Democritus and Aristotle
◦ Democritus thought all matter consisted of
extremely tiny particles that could not be divided.
(Cut aluminum foil in half)
◦ Also thought matter in liquids was round and
smooth; in solids rough and prickly
◦ Aristotle thought there was no limit to the number
of times matter could be divided.
Developed a theory to explain why the
elements in a compound always behave in the
◦ 1) All elements are composed of atoms.
◦ 2) All atoms of the same element have the same
mass, and atoms of different elements have
◦ 3) Compounds contain atoms of more than one
◦ 4) In a compound, atoms of different elements
always combine in the same way.
Used a cathode ray tube to show evidence for
subatomic, charged particles.
Plum pudding model (chocolate chip ice
Negatively charged particles evenly spaced
throughout a solid mass of positive charge.
A. An atom
1. smallest part of matter
2. Ripping paper example
B. Areas of an atom
1. Nucleus – center of an atom
-holds protons and neutrons
a. Protons (P) – positively charged
particle found in the nucleus.
-has a mass of 1
-has a charge of 1+
b. Neutrons (N) – non-charged particle found
in the nucleus.
-has a mass of 1
-has no charge “0”
2. Electron Cloud – area around the nucleus
which contains electrons
a. Electrons (e) – negatively charged particle
found outside of the nucleus
-has a mass of 0
-has a charge of 1-
-moves around the outside of the
Particle Mass Charge Location of
Particles in an Atom Review
A. Def – the # of protons in an atom’s nucleus.
1. Every carbon atom has 6 protons…
2. Every neon atom has 10 protons…
B. On P.T. – located right above the chemical
*Protons dictate the type of element we have*
*Electrons always equal the # of protons*
A. Def – The sum of protons and neutrons in the
nucleus of an atom.
1. On P.T. – bottom number
2. Ex: What is the mass number of the following
3. Mass # = Protons + Neutrons
4. How many Neutrons on average are found
in the following elements?
Silver Gold 118
A. Def – An atom that has a different number of
1. Does not affect the element
2. Only changes the weight of the element
3. Ex: Boron-10 and Boron-11
p p p p p p p p p p
n n n n n
n n n n n
5 protons 5 protons
5 neutrons 6 neutrons
the mass #
4. How many neutrons do the following isotopes
A. Def – average mass of all isotopes of one
1. Found on P.T. underneath chemical symbol.
2. Ex: Carbon = 12.011
“Carbon has 3 known isotopes.
Carbon-12, 13, and 14. Carbon-12 makes up 99%
of all carbon on earth. Carbon-13 and 14 make
1. Draw a picture of the following atoms with the
correct number of e’s, p’s, and n’s…
a. Na d. He
b. Al e. C
c. K f. S
2. Assign: Read pg. 118 and 119
3. What is the difference between a ground state
and an excited state?