Download Atomic Number

Atomic Structure
Chapter 4
What is an atom?
•
Draw a circle map for atoms
• Atom: the smallest particle of an element
that retains its identity in a chemical reaction
The Atomists:
The first atomic theory
• 460 BCE: Greek
Democritus suggested that
matter is “ composed of
minute, invisible,
indestructible particles of
pure matter which move
about eternally in infinite
empty”
http://www.winneconne.k12.wi.us/middle_school/7th%20Grade/LENZ/history_of_atomic_theory.htm
Dalton’s Atomic Theory
• John Dalton (1766-1844), an
English schoolteacher and chemist,
• Studied the theories and the results
of experiments by other scientists.
• He formed a hypothesis,
experimented, and came up with a
theory.
• Dalton proposed his atomic theory
of matter in 1803.
Dalton’s Atomic Theory
(The main points)
1. All matter is made up of indivisible particles
called atoms.
2. All atoms of one element are exactly
alike, but are different from atoms of
other elements.
3. Atoms mix in simple whole number ratios to
form compounds.
4. Chemical reactions happen when atoms
are separated, joined, or rearranged.
* Atoms of one element can NEVER be turned
into atoms of a different element!
Modern Change to Dalton’s Theory…
• Atoms are NOT indivisible!
• Atoms can be broken into three subatomic
particle:
1. Electrons
2. Protons
3. Neutrons
Review
• Dalton’s Atomic Theory (shortened)
1. All matter is made of indivisible atoms
2. All Na atoms are the same but are different from He
atoms
3. Atoms mix in whole number ratios
4. Chemical reactions happen when atoms are separated,
joined, or rearranged
Modern Change to Dalton’s Theory…
• Atoms are NOT indivisible!
• Atoms can be broken into three subatomic particle:
1. Electrons
2. Protons
3. Neutrons
Discovery of the Electron
• Because of Dalton’s atomic theory, most scientists in
the 1800s believed that the atom was like a tiny
solid ball that could not be broken up into parts.
• In 1897, a British
physicist, J.J. Thomson,
discovered that this
solid-ball model was not
accurate.
Cathode-Ray Tube
• Cathode Ray: a stream of electrons produced at
the negative electrode of a tube containing a gas
at low pressure
• Thomson knew that objects with like charges
repel each other, and objects with unlike
charges attract each other
Cathode-Ray Tube
The Electron
• Thomson named the negative particles found
in his experiment electrons.
• Electrons (e-):
– Are the smallest subatomic particles
– Have a charge of -1
• You don’t get shocked when you touch
EVERYTHING
• So … there must be something positively
charged in the atom to balance the electrons.
Rules about Electrically Charged Atoms
1. Atoms have no overall charge (neutral)
2. Electrical charges are carried by matter
3. Electrical charges always exist in whole
numbers
4. # of negatively charged particles = # of
positively charged particles = neutral atom
Protons
• Protons (p+):
– Positively charged subatomic particles (+1)
– Mass 1 proton = mass 1840 electrons
– Very, very heavy when compared to electrons
• At this point, it seemed that atoms were made up
of equal numbers of electrons and protons
• However, in 1910,
Thomson
discovered that
neon consisted of
atoms of two
different masses
Isotopes
• Atoms of an element that are chemically
alike but differ in mass are called isotopes of
the element.
So why the difference in mass?
Neutrons
• Neutrons (n0):
• Neutral charge
• Same mass as a proton
• Because of the discovery of isotopes, scientists hypothesized
that atoms contained still a third type of particle that
explained these differences in mass.
Cookie Dough Model
Rutherford’s Gold Foil Experiment
• In 1909, a team of
scientists led by Ernest
Rutherford in England
carried out the first of
several important
experiments that
revealed an arrangement
far different from the
cookie-dough model of
the atom.
The Gold Foil Experiment
The Nuclear Model of the Atom
• Since most of the particles passed through the foil,
they concluded that the atom is nearly all empty
space.
The Nuclear Model of the Atom
• Nucleus:
• Contains most of the mass of the atom
(protons and neutrons)
• Is very small compared to the rest of the
atom
• the atom has a small, dense, positively
charged central core, called a nucleus.
Draw an atom
Bracing Map of an atom
Review
• Elements are different because they
have different numbers of protons
How do you figure out the # of protons?
• Atomic Number : the number of protons in
the nucleus of an atom of that element
• Atomic number identifies the element
Where to find the atomic
number on the periodic
table…
Atomic Number
# of protons in the
nucleus of the atom.
1
H
1.0013
• Remember elements are electrically
neutral…and protons are positively charged
• So if the atomic number is the number of
protons …..
• Then it must also be the number of electrons
in order to balance the atom
Practice
• What is the atomic number of carbon?
– How many protons?
– How many electrons?
• What is the atomic number of aluminum?
– How many protons?
– How many electrons?
• From looking at the periodic table you can
tell any element’s number of electrons and
protons
You know you want to know how
many of these particles are in an
atom, right?
• Atomic Number : # of protons in the atom
• Mass number: # of protons + # neutrons
How to get the info from the
periodic table
1
Atomic Mass:
Average mass
of all isotopes
of that element.
H
1.0013
Atomic Number:
Number of protons in
the nucleus of the
atom.
Hey, why isn’t this a
whole number?
The atomic mass unit (amu)
• Atoms don’t have much mass! (10-24 g)
• Use carbon-12 isotope as a ref. and make
12 carbon atom= 1 amu
• On the periodic table the value is a weighted
average of all isotopes of the element – that
is why it isn’t a whole number.
What is a weighted average?
• Let’s calculate your grade….
In this chapter you will fill out
lots of tables…
Name
Protons
Electrons
Neutrons
Helium
2
2
4-2 = 2
Carbon-12
6
6
12-6 = 6
Carbon-13
6
6
13-6=7
Carbon-12 and Carbon-13 are isotopes
How many subatomic particles?
• # protons = atomic number (always)
• For a NEUTRAL atom # electrons=# protons
(this will be a bit different in upcoming chapters…)
• # neutrons = mass number - # protons
(always)
THE END 