Chapter 2: Matter is Made up of
Atoms
Early Ideas About Matter
• Greek philosophers (2500 y.a.) proposed
matter was made of 4 elements: earth, air,
fire, water.
• They coined the word “atom” meaning cannot
be broken.
“Modern” Atomic Theory
• Lavoisier (1743-94) law of conservation of
matter
– Matter is neither created nor destroyed
– Matter never “goes away,” even when an object is
burnt, eaten, etc.
– Atoms are just rearranged into new matter.
• Proust (1799) discovered water was always 11%
hydrogen, 89% oxygen – law of definite
proportions
– Compounds’ components are always in a specific
proportion by mass
Dalton’s Atomic Theory, cont.
• 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.
Hypotheses, Theories, Laws (review)
• Hypothesis: testable prediction to explain an
observation
• Theory: well tested explanation that explains
many observations. May change over time
• Law: fact of nature observed so often it is
accepted as truth. Doesn’t change.
Discovery of Atomic Structure
• 1800s- scientists thought atom was a tiny solid
ball.
• THEN…JJ Thomson (1897) discovers the
electron (e-)
JJ Thomson Cathode Ray Experiment
• 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”
– picture of cathode ray tube
– Cathode ray bends toward a positive end of a
magnet.
– “ “ bends away from a negative end of a magnet.
Cathode Ray Experiment
• Conclusion:
1. The cathode ray was actually ____ charged
particles.
2. The atom could not be ________ as scientists
had thought, but must contain charged
particles.
Discovery of Protons & Neutrons
• Later scientists realized there was a second ray
in the vacuum tube.
– It bent toward the – end of a magnet.
– It bent away from the + end of a magnet.
– Therefore, this ray was made of _____ charged
particles.
Discovery of Protons & Neutrons, cont.
• In 1910 Thomson discovered that neon atoms
have different masses.
• Conclusion: there must be another particle
that has no charge, called a neutron.
Discovery of Nucleus
• 1909-scientists now believe the atom is like
chocolate chip cookie dough (see Fig 2.8 p 63 of
text)
• Rutherford’s Gold Foil Experiment
• Shot “alpha Particles” (helium nuclei) at gold foil.
• Hypothesis: they would pass through unaffected.
• Data: most did pass through
– Some were deflected
– Others bounced straight back!
The Nuclear Model of the Atom
Conclusion:
1. Atoms are nearly all empty space!
2. Atoms have small, densely packed central nucleus
Composition of Atoms
• Protons: positive charge, in nucleus, are heavy,
“p+”
• Neutrons: no charge, in nucleus, are heavy,
“n0”
• Electrons: negative charge, outside nucleus
“electron cloud”, very light (1/1840 of a
proton or neutron), “e-”
Charges in an Atom
• 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.
Atomic number
– determines the identity of the atom.
– It tells us the number of protons in the atom.
– It also tells us the number of electrons (b/c an
atom is neutral in charge.)
– Ex: atomic number of carbon, C = 6
– Question: how many protons? How many
electrons? How many neutrons?
Slide 2.2
Isotopes
• The number of neutrons can vary from atom
to atom in an element.
• Atoms of the same element w/different #s of
neutrons are called ISOTOPES.
• 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.
QUESTION: If the mass number of
a carbon atom is 14,
• How many protons?
• How many electrons?
• How many neutrons?
• LET’S PRACTICE!
– Whiteboard
– Marker
– Paper towel
Atomic Mass Units
• 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.
Atomic Mass
• (definition) Weighted average of all the
isotopes of an element. See p 68 of text.
calculating atomic mass
• Finding Atomic Mass on Periodic Table
• Located below element symbol on periodic
table.
(Section 2):Electrons in Atoms
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.)
Energy of Electrons (cont.)
(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.
Bohr’s Model of Atom
• 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.
Bohr’s Model of Atom
• 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
Electron Cloud Model of Atom
• 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.
Electrons in Energy Levels
• 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!)
Electrons in Energy Levels
• Only so many e-’s can fit in energy levels
Energy Level
1st
2nd
3rd
4th
# of electrons
2
8
18*
32*
KEY CONCEPT!!!
•VALENCE ELECTRONS
DETERMINE HOW
ELEMENTS BEHAVE!!!
Drawing Bohr Models
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!)
Drawing Bohr Models
• 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
Practice
•
•
•
•
•
•
Hydrogen-2 (Practice together)
Helium-4
Lithium-6
Beryllium-8
Carbon-12
Magnesium-24
Lewis-Dot Diagrams
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
Practice Lewis Dot Diagrams
TEACHER DEMONSTRATION
• Hydrogen
• Helium
• Lithium
STUDENT PRACTICE
• Beryllium
• Boron
• Carbon
PRACTICE WORKSHEET
• Bohr Models
• Lewis dot diagrams