Download Chapter 2: Matter is Made up of Atoms

Greek philosophers (300 BC)
proposed matter was made of 4
elements: earth, air, fire, water.
 Democritus coined the word
“atom” meaning “cannot be
broken.”

 Atom
seen as a solid sphere

John Dalton (1766-1844)
1. All matter is made of atoms
2. Atoms are indestructible and can’t be divided
3. All atoms of one element are exactly alike, but
different from atoms of other elements.
4. Atoms of diff elements combine in simple,
whole-number ratios to make compounds
5. In reactions, atoms are combined, separated, or
rearranged.
 1800s-
scientists still thought
atom was a tiny solid ball.
 THEN…JJ Thomson (1897)
discovers the electron (e-)


Vacuum tube (no air inside) w/ electrode on
each end, attached to a terminal.
He send electricity through the tube and saw
 A bright ray travelling from the negative end
(cathode) to the positive end (anode).
“cathode rays”
 Cathode ray bends toward a + end of a
magnet.
 “ “ bends away from a - end of a magnet.
Television
Computer Monitor
Cathode ray tubes pass electricity
through a gas that is contained at a
very low pressure.
Conclusion:
1. The cathode ray was actually ____ charged
particles.
2. The atom could not be ________ as scientists
had thought, but must contain charged
particles.
1909-scientists now believe the atom is like
chocolate chip cookie dough




In 1910 Thomson discovered that neon atoms
have different masses.
Conclusion: there must be another particle that
has no charge, called a neutron.
(In 1932, James Chadwick confirms existence of
the neutron)
•
•
•
1911-Rutherford’s
Gold Foil Experiment
Shot “alpha Particles”
(helium nuclei) at
gold foil.
Hypothesis: they
would pass through
unaffected.
Expected
results
Actual results
Most of the particles passed right through
 A few particles were deflected
 VERY FEW were greatly deflected
“Like howitzer shells bouncing
off of tissue paper!”

Conclusions:
a) The nucleus is small
b) The nucleus is dense
c) The nucleus is positively
charged
d) Most of the atom is empty
space


In 1920, Bohr proposes that electrons around
the nucleus exist at only distinct distances from
the nucleus
This is known as the Bohr model
Particle
Charge
Mass (g)
Location
Electron
(e-)
-1
9.11 x 10-28
Electron
cloud
Proton
(p+)
+1
1.67 x 10-24
Nucleus
Neutron
(no)
0
1.67 x 10-24
Nucleus



The + charge on a proton is equal to the charge on an electron.
Atoms are neutral (have no overall charge)
Therefore, the # of protons = # electrons in an
atom.
 determines
the identity of the
atom.
 tells us the # of protons in the
atom.
 also tells us the # of electrons (b/c
an atom is neutral in charge.)
 Ex: atomic number of carbon, C
 Question: how many protons?
=6
How many electrons? How many
neutrons?...
Slide 2.2
Atomic number (Z) of an element is
the number of protons in the nucleus
of each atom of that element.
Element
# of protons
Atomic # (Z)
Carbon
6
6
Phosphorus
15
15
Gold
79
79
Mass number is the number of
protons and neutrons in the nucleus
of an isotope: Mass # = p+ + n0
p+
n0
e- Mass #
8
10
8
18
Arsenic - 75
33
42
33
75
Phosphorus - 31
15
16
15
31
Nuclide
Oxygen - 18




(Def) Atoms of the same element w/different
#s of neutrons.
The number of neutrons can vary from atom to
atom in an element.
In order to know how many neutrons in an
atom you must be told.
The mass number tells you how much mass the
atom has.


Since p+ and n0 are the heavy parts,
mass # = # of p+’s + n0’s.
Mass #
11
5
Atomic #
B
Symbol for
element

The atom in the prior slide can be called
“boron-11”
Name of element
Mass #

How many protons?
How many electrons?
How many neutrons?

LET’S PRACTICE!


Whiteboard
 Marker
 Paper towel

Practice Problem #1

If an element has an atomic
number of 34 and a mass
number of 78, what is the:
a) number of protons
b) number of neutrons
c) number of electrons
d) complete symbol
Practice Problem #2
 If an element has 78
electrons and 117 neutrons
what is the
a) Atomic number
b) Mass number
c) number of protons
d) complete symbol


Atoms are weighed in a.m.u.
1 a.m.u. is based on the mass of a Carbon-12
atom.
it has 6 p+ and 6 n0,
 1 a.m.u = 1/12 the mass of a carbon-12 atom.



(definition) Weighted average of all the
isotopes of an element. See p 68 of text.
calculating atomic mass
Located below element symbol on periodic
table.
Isotopes are atoms of the same element having
different masses, due to varying numbers of
neutrons.
Isotope
Protons Electrons
Neutrons
Hydrogen–1
(protium)
1
1
0
Hydrogen-2
(deuterium)
1
1
1
1
1
2
Hydrogen-3
(tritium)
Nucleus
Elements
occur in
nature as
mixtures of
isotopes.
Isotopes are
atoms of the
same element
that differ in
the number of
neutrons.
 Multiply
the atomic mass of
each isotope by it’s
abundance (expressed as a
decimal), then add the
results.

If not told otherwise, the mass of
the isotope is expressed in atomic
mass units (amu)
Atomic mass is the average of all the
naturally occurring isotopes of that element.
Isotope
Symbol
Carbon-12
12C
Carbon-13
13C
Carbon-14
14C
Composition of
the nucleus
6 protons
6 neutrons
6 protons
7 neutrons
6 protons
8 neutrons
Carbon = 12.011
% in nature
98.89%
1.11%
<0.01%
Copper has the following isotopes
ISOTOPE
MASS # % ABUNDANCE
 Copper-63
63
69.15
 Copper-65
65
30.85


Calculate the atomic mass (average mass)
Finding Atomic Mass on Periodic Table
Ch 5 – Chem IH
Ch2.2 & 7 – Chem I
•
•
•
•
Energy can travel in waves.
There are high energy and low energy waves.
The ones we can see are called “the visible
spectrum.” ROY G BIV
Red is the low energy end: violet is the high
energy end.

1. Wavelength: distance between crests of a
wave. Ex: radio waves = 102 m

2. Frequency: number of wave cycles to pass a
point per unit time.
Energy of Electrons
 Why electrons don’t crash into the nucleus:
they have enough energy to keep them away.
 Why e-s (usually) don’t fly off of atoms: they
have enough attraction to the nucleus to keep
them in “orbit.”
(Kind of like planets in orbit around the sun.)
(Don’t write this!)
DISCUSS WITH YOUR NEIGHBOR:
 You are an electron. If you have a lot of
energy, will you stay close to the nucleus or
will you move further from it?
Answer: you may still stay in “orbit” but you will be
able to move further away from the nucleus.

Neils Bohr studied w/Rutherford

His model is also called the planetary model

He discovered that e-s could only exist at
certain distances from the nucleus. (Energy
Levels)

"The opposite of a correct statement is a false
statement. But the opposite of a profound
truth may well be another profound truth."
Neils Bohr


See p 75 of text: electron energy levels are like
rungs of a ladder.
Ladder



To climb to a higher level, you can’t put your foot at
any level,
you must place it on a rung
Electron energy levels

e-s must move to higher or lower e.l.’s in specific
intervals

Interactive Bohr Model
Atoms are arranged in energy levels (e.l.’s), at
different distances from nucleus
 Close to nucleus = low energy
 Far from nucleus = high energy
 e-s in highest occupied level are “valence e-s”
 Only so many e-’s can fit in energy levels
 e-s fill lower e.l.’s before being located in higher
e.l.’s*
(* There are exceptions we will learn later!)


Only so many e-’s can fit in energy levels
Energy Level
1st
2nd
3rd
4th
# of electrons
2
8
18*
32*
VALENCE
ELECTRONS
DETERMINE HOW
ELEMENTS
BEHAVE!!!
Let’s practice drawing some atoms/ions
In your teams, pick up enough of the following
for your team:
1 white board per person
1 marker per person
1 paper towel per team (Please save a tree &
share!)



Electrons aren’t in perfect orbits.
Energy levels are regions of space in which an
e- is likely to be found most of the time.
The area in which they move is like a cloud, an
area of space surrounding the nucleus.

Show # of protons and neutrons in the nucleus
Draw e.l.’s and show each electron in the
proper e.l.

Ex: Bohr Model of BORON-11







Hydrogen-2 (Practice together)
Helium-4
Lithium-6
Beryllium-8
Carbon-12
Magnesium-24
Have 2 parts
1.
Chemical symbol of element
2.
Valence e-s, represented by dots
Are placed in one of four locations







Above
Below
Right
left
Are not paired unless there is 1 e- in each location.
Ex: Oxygen
TEACHER DEMONSTRATION
 Hydrogen
 Helium
 Lithium
STUDENT PRACTICE
 Beryllium
 Boron
 Carbon


Bohr Models
Lewis dot diagrams