The Discovery of
Isotopes
• Francis William Aston and Antoine
Becquerel
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The Atom
An atom consists of a
• nucleus
–(of protons and neutrons)
• electrons in space about the nucleus.
Electron cloud
Nucleus
• An _____ is the smallest particle of
an element that has the chemical
properties of the element.
Copper
atoms on
silica
surface.
Distance across = 1.8 nanometer (1.8 x 10-9 m)
ATOM
COMPOSITION
The atom is mostly
empty space
•protons and neutrons in
the nucleus.
•the number of electrons is equal to the
number of protons.
•electrons in space around the nucleus.
•extremely small. One teaspoon of water has
3 times as many atoms as the Atlantic Ocean
has teaspoons of water.
ATOMIC COMPOSITION
• Protons (p+)
–
–
–
+ electrical charge
mass = 1.672623 x 10-24 g
relative mass = 1.007 atomic
(amu) but we can round to 1
mass units
• Electrons (e-)
–
–
negative electrical charge
relative mass = 0.0005 amu
but we can round to 0
• Neutrons (no)
–
–
no electrical charge
mass = 1.009 amu but we can round to 1
Atomic Number, Z
All atoms of the same element
have the same number of
protons in the nucleus, Z
13
Al
26.981
Atomic number
Atom symbol
AVERAGE Atomic Mass
Mass Number, A
• C atom with 6 protons and 6 neutrons
is the mass standard
• = 12 atomic mass units
• Mass Number (A)
= # protons + # neutrons
• NOT on the periodic table…(it is the
AVERAGE atomic mass on the table)
• A boron atom can have
A = 5 p + 5 n = 10 amu
A
10
Z
5
B
Isotopes
• Atoms of the same element (same Z)
but different mass number (A).
• Boron-10 (10B) has 5 p and 5 n
• Boron-11 (11B) has 5 p and 6 n
11B
10B
Isotopes
• The existence of isotopes explains the
apparent contradiction between the mass
number and atomic mass of the elements.
The various isotopes of the elements in
nature occur in different proportions.
Therefore , the atomic mass on the
periodic table is the weighted average of
all the natural isotopes of that element,
Two isotopes of sodium
How we classify
Isotopes
• Stable isotopes are chemical isotopes that are
not radioactive .By this definition, there are 256
known stable isotopes of the 80 elements which
have one or more stable isotopes.
• Commonly analyzed stable isotopes include
oxygen, carbon, nitrogen, hydrogen and
sulfur
Unstable or
Radioactive Isotopes
•
•
•
•
•
Naturally occurring fall into three categories:
primordial
secondary
cosmogenic
Artificially produced radioisotopes can be
produced by particle accelerators or by
nuclear reactions.
What are Radio
Isotopes
• since they have different numbers of
neutrons, isotopes of the same element may
have different radioactivity. An isotope that is
radioactive is called a radioisotope.
What is Half-life of
the Isotopes
• The half-life he amount of time it takes for half
of the atoms in a sample to decay. The halflife for a given isotope is always the same
• Suppose there's an alien species with a halflife of, say, 70 years. You randomly pick out
16 baby aliens and track them to see how
long they live. After 70 years, of course, 8 of
them will still be alive.
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Examples
• The most stable isotope of uranium, U-238, has an
atomic number of 92 (protons) and an atomic weight of
238 (92 protons plus 146 neutrons). The isotope of
uranium of greatest importance in atomic bombs, U235, though, has three fewer neutrons. Thus, it also
has an atomic number of 92 (since the number of
protons has not changed) but an atomic weight of 235
(92 protons plus only 143 neutrons). The chemical
behavior of U-235 is identical to all other forms of
uranium, but its nucleus is less stable, giving it higher
radioactivity and greater susceptibility to the chain
reactions that power both atomic bombs and nuclear
fission reactors.
Examples
• Iodine is one of the earliest elements whose
radioisotopes were used in what is now called nuclear
medicine. The most common, stable form of iodine has
an atomic number of 53 (protons) and an atomic weight
of 127 (53 protons plus 74 neutrons). Because its
nucleus has the "correct" number of neutrons, it is
stable and is not radioactive. A less stable form of
iodine also has 53 protons (this is what makes it
behave chemically as iodine) but four extra neutrons,
for a total atomic weight of 131 (53 protons and 78
neutrons). With "too many" neutrons in its nucleus, it
is unstable and radioactive, with a half-life of eight
days. Because it behaves chemically as iodine, it
travels throughout the body and localizes in the
thyroid gland just like the stable form of iodine. But,
because it is radioactive, its presence can be detected.
Iodine 131 thus became one of the earliest radioactive
tracers.
Isotopes &
Their Uses
Bone scans with
radioactive
technetium-99.
Separation of Isotopes
• Difference in physical properties enables
separation of the isotope to be carried out.
• Evaporation
• Distillation
• Centrifuging
• Laser methods
• Diffusion
Calculating the%
abundance
• Lithium has 2 isotopes. Li-6 and Li-7. If
the atomic mass of Lithium is 6.94,
calculate the proportion ( % of
abundance) of the isotopes.
Atomic Symbols
 Show the name of the element, a hyphen, and
the mass number in hyphen notation
sodium-23
 Show the mass number and atomic number
in nuclear symbol form
mass number
23 Na
atomic number
11
Counting Protons, Neutrons,
and Electrons
• Protons: Atomic Number (from periodic table)
• Neutrons: Mass Number minus the number of protons
(mass number is protons and neutrons because the
mass of electrons is negligible)
• Electrons:
– If it’s an atom, the protons and electrons must be
the SAME so that it is has a net charge of zero
(equal numbers of + and -)
– If it does NOT have an equal number of electrons, it
is not an atom, it is an ION. For each negative
charge, add an extra electron. For each positive
charge, subtract an electron (Don’t add a proton!!!
That changes the element!)
Learning Check – Counting
Naturally occurring carbon consists of three
isotopes, 12C, 13C, and 14C. State the number of
protons, neutrons, and electrons in each of
these carbon atoms.
12C
6
13C
6
14C
6
#p+ _______
_______
_______
#no _______
_______
_______
#e- _______
_______
_______
Answers
12C
6
13C
14C
6
6
#p+ 6
6
6
#no 6
7
8
#e- 6
6
6
Learning Check
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
C. The element is
1) Si
2) Ca
3) Se
D. Another isotope of this element is
1) 34X
2) 34X
3) 36X
16
14
14
Learning Check – Counting
State the number of protons, neutrons, and electrons in
each of these ions.
39
K+
19
16O -2
41Ca +2
8
20
#p+ ______
______
_______
#no ______
______
_______
#e- ______
______
_______
AVERAGE
ATOMIC
MASS
11B
10B
• Because of the existence of isotopes, the
mass of a collection of atoms has an average
value.
• Boron is 20% 10B and 80% 11B. That is, 11B is
80 percent abundant on earth.
• For boron atomic weight
= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu
Isotopes & Average Atomic Mass
• Because of the existence of isotopes, the mass of a
collection of atoms has an average value.
• 6Li = 7.5% abundant and 7Li = 92.5%
– Avg. Atomic mass of Li = ______________
•
28Si
= 92.23%, 29Si = 4.67%, 30Si = 3.10%
– Avg. Atomic mass of Si = ______________