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Section 2
The Structure of an Atom
Key Concepts
• What are three subatomic particles?
• What properties can be used to compare
protons, electrons, and neutrons?
• How are atoms of one element different
from atoms of other elements?
• What is the difference between two
isotopes of the same element?
Properties of Subatomic
Particles
• By 1920, Rutherford had seen evidence
for the existence of two subatomic
particles and had predicted the existence
of a third particle.
• Protons, electrons, and neutrons are
subatomic particles.
Protons
• A proton is a positively charged
subatomic particle that is found in the
nucleus of an atom.
• Each proton is assigned a charge of +1.
Electrons
• An electron is a negatively charged
subatomic particle that is found in the
space outside the nucleus.
• Each electron has a charge of -1.
Neutrons
• In 1932, the English physicist James
Chadwick designed an experiment to
show that neutrons exist
• A neutron is a neutral subatomic particle
that is found in the nucleus of an atom.
• It has a mass almost exactly equal to that
of a proton.
Comparing Subatomic Particles
• Protons, electrons, and neutrons can
be distinguished by mass, charge, and
location in an atom.
• Atomic Number
• The atomic number of an element equals
the number of protons in an atom of that
element.
• Atoms of different elements have
different numbers of protons.
• Each positive charge in an atom is
balanced by a negative charge because
atoms are neutral.
• So the atomic number of an element also
equals the number of electrons in an atom.
Atomic Number and
Mass Number
• What is the atomic number of the following
elements.
• Potassium (K) 19
• Oxygen (O) 8
• Iron (Fe) 26
• Helium (He) 2
Atomic Number and
Mass Number
• Mass Number
• The mass number of an atom is the sum
of the protons and neutrons in the nucleus
of that atom.
• An atom of aluminum with 13 protons and
14 neutrons has a mass number of 27.
• If you know the atomic number and the
mass number of an atom, you can find the
number of neutrons by subtracting.
Number of neutrons = Mass number – Atomic number
Atomic Number and
Mass Number
• What is the Atomic Mass of the following
elements
• Potassium (K) - Neutrons = 20 + 19 = 39
• Oxygen (O) – Neutrons = 8 + 8 = 16
• Iron (Fe) – Neutrons = 30 + 26 = 56
• Helium (He) – Neutrons = 2 + 2 = 4
Atomic Structure
Element
Symbol
Atomic
Number
Mass
Number
# of
Protons
# of
Neutron
# of
Electron
Helium
He
2
4
2
2
2
Magnesium
Mg
12
24
12
12
12
Zinc
Zn
30
65
30
35
30
Bromine
Br
35
80
35
45
35
Aluminum
Al
13
27
13
14
13
Uranium
U
92
238
92
146
92
Sodium
Na
11
23
11
12
11
Krypton
Kr
36
84
36
48
36
Calcium
Ca
20
40
20
20
20
Silver
Ag
47
108
47
61
47
Isotopes
• Isotopes are atoms of the same element
that have different numbers of neutrons
and different mass numbers.
• Isotopes of an element have the same
atomic number but different mass
numbers because they have different
numbers of neutrons.
Isotopes
• Hydrogen atom and it’s 3 isotopes
• Hydrogen isotopes are the only isotopes to
have special names.
Hydrogen
Deuterium
Tritium
Isotopes
Isotopes
Element
Hydrogen
Deuterium
Helium-3
Helium
Carbon
Carbon-13
Nitrogen
Nitrogen-15
Oxygen
Oxygen-17
Oxygen-18
Protons
1
1
2
2
6
6
7
7
8
8
8
Neutrons
0
1
1
2
6
7
7
8
8
9
10
Mass
1
2
3
4
12
13
14
15
16
17
18
Natural %
99.99
0.01
1.3*10-4
100
98.90
1.10
99.64
0.37
99.76
0.04
0.20
Questions
• What is the difference between Oxygen-16
and Oxygen-18?
• Ans. Different numbers of neutrons
• How are they the same?
• Ans. The same number of protons
• What part of the atom do isotopes have to
do with?
• Ans. The nucleus
Reviewing Concepts
• 1. Name three subatomic particles.
• 2. Name three properties you could use to
distinguish a proton from an electron.
• 3. Which characteristic of an atom always varies
among atoms of different elements?
• 4. How are the isotopes of an element different
from one another?
• 5. What do neutrons and protons have in
common? How are they different?
• 6. How can atoms be neutral if they contain
charged particles?
• 7. What is the difference between atoms of
oxygen-16 and oxygen-17?