Ch. 4 Atomic Structure
Early Models of the Atom
• Democritus’s Atomic Philosophy
– He believed atoms were indivisible and
indestructible.
– *Philosophy because there is no experimental
support
Dalton’s Atomic Theory
• John Dalton (1766-1844)
• Theory because he had experimental
support
Dalton’s vs. Modern Atomic Theory
1. Elements are composed
of tiny, indivisible atoms.
2. Atoms of same element
are identical
3. Different elements can
mix or react to make
compounds.
4. When chemical
reactions occur,
elements rearrange but
no new elements.
1. Elements are made of
atoms BUT atoms are
divisible.
2. Atoms are not absolutely
identical. Isotopes –
different mass but same
properties.
3. Same as Dalton’s
4. Same as Dalton’s
Sizing up the atom*
• The radii of most atoms are
5x10-11 to 2x10-10 m
• A pure copper coin the size of a
penny contains about 2.4x1022
atoms
4.2 Structure of the Atom
Location of Particles
• Nucleus (positive charge)
– Protons and Neutrons
• Electron Cloud (negative charge)
– electrons
Electrons
• Electrons were discovered by J.J.
Thomson through the use of a cathode
ray tube.
• One end was positive, the other end
negative.
• The particles that were sent through the
tube we pulled towards the positively
charged plate.
• Because of this, Thomson concluded that
the beam was negatively charged since
opposite charges attract
Plum pudding model
• Thomson called these particles electrons.
He knew they were a part of the atom but
wasn’t sure where they went.
• Decided electrons were mixed in with
the other particles
Video
• https://www.youtube.com/watch?v=IdTxGJ
jA4Jw
Gold Foil Experiment
• In 1909 Ernest Rutherford designed an
experiment to study the parts of the atom
• Rutherford aimed a beam of small
positively charged particles at a thin sheet
of gold foil.
• He put a coating behind the foil that
glowed when hit with particles.The coating
allowed Rutherford to see where the
particles went after going through the foil.
What did this show?
• If the atom had particles all mixed
together, the particles should have gone
straight through
• In 1911 he revised the atomic theory. He
proposed the center of the atom is a
tiny, dense positively charged area
called a nucleus (an atom's central
region, which is made of protons and
neutrons) and electrons were
surrounding it
• Because like charges repel each other,
Rutherford reasoned that positively
charged particles that passed close to the
nucleus were pushed away.
• This is why some of the particles were
deflected.
• He was able to calculate the diameter of a
nucleus to be 100,000 times smaller than
the atom itself.
• Imagine a pinhead in AT&T park
• This means that atoms are mostly empty
space!
~Videos~
• https://youtube.com/watch?v=XBqHkraf8i
E
• https://youtube.com/watch?v=wzALbzTdn
c8
4.3 Distinguishing among atoms
• Atomic number: The number of protons in
the nucleus of an atom. It is the same for
all atoms of that element.
• This determines what the element is.
• Example: Carbon has an atomic number
of 6, and always has 6 protons
• Mass number: This is the combined
number of protons and neutrons in the
nucleus
• May be written as X-##, where X is the
element abbreviation and ## is the mass
• Carbon with 6 protons and 6 neutrons
would be written as C-12
•Atomic Number: # of p (in neutral atom
also # of e•Mass Number: # of p + # of n
•To find # of n = Mass # - Atomic #
• How many p, n, e- does Fe have?
• Isotope: different types of atoms of the
same element. They have the same
number of protons but a different number
of neutrons
• Same number of proton=same element
Difference between isotopes
• Mass number shows the difference
between isotopes (number of neutrons
differs)
• Remember, the number of protons will
always stay the same for each element.
• Carbon-12
– 6 protons
– 6 neutrons
• Uranium-235
– 92 protons
– 143 neutrons
C-14
6 protons
8 neutrons
U-237
92 protons
145 neutrons
Learning check
• How many neutrons are found in Lead206. Lead has 82 protons
– 124 neutrons
• What is the mass number for neon with 10
protons and 10 neutrons?
– Neon-20
Learning check
16.How many protons and electrons are in each atom?
1. fluorine (atomic number 9)
2. calcium (atomic number 20)
3. aluminum (atomic number 13)
17.How many neutrons are in each atom:
1. O-16
2. S-32
3. Ag-108
4. Br-80
5. Pb-207
• Many elements have several isotopes
• To determine an average weight of all the
isotopes, we use the atomic mass.
• This is given in atomic mass units (amu)
– 1 amu=1/12 the mass of a C-12 atom
Atomic mass
•
•
•
•
Copper-63=69%
Copper-65=31%
(63x0.69)+(65x0.31)
Weighted average: 63.55 AMU=atomic
mass
Periodic Table Intro
• The periodic table is a way to organize the
elements by similar or repeating properties
– A quick way to compare different elements
• The elements are arranged in order of
increasing atomic number, as shown
centered above the symbol
• Each horizontal row is called a period.
• 7 periods
• The properties vary as you move down the
periods
• Each column is called a group
• Elements within a group have similar
chemical and physical properties
• Each group is classified with a number
and either A or B