2. Atoms and Elements
chapter 2
Scanning Tunneling Microscope (STM) Image of
gold atoms (Nanotechnology)
Atomic radius of gold is ~ .2 nm (1 nm =10-9m or
one billionth of a metre)
Subatomic Particles (to Chemists)
A proton is a subatomic particle with
a mass of 1 amu
and a charge of +1
An electron is a
subatomic particle
with negligible mass
and a charge of -1
A neutron is a subatomic particle with
a mass of 1 amu and
no electrical charge
The nucleus is the
positively charged
central core of an
atom
Subatomic Particles
grams
amu Location in atom Charge
Neutron 1.68 x 10-24
1 Nucleus
0
1.67 x 10-24
1 Nucleus
1+
Proton
Electron 0.0009 x 10-24 0 Outside the nucleus 1-
An atomic mass unit(amu) is exactly 1/12 the
mass of the most common kind of carbon atom.
This gives the proton a mass of 1.007(~1) amu, the
neutron a mass of 1.009(~1) amu and the electron a
mass of 0.0005(~0) amu.
Element Distribution(by atoms)
Universe
90% Hydrogen
>9% Helium
<0.3% all others
Human Body
63% Hydrogen
25.5% Oxygen
9.5% Carbon
1.5% Nitrogen
0.5% all others
Origin of the elements
• Big bang created the lighter elements-ie H, He, Li
Be and Boron (B) ~15 billion years ago
• Theory suggests that it takes ~700 million years
for H and He to coalesce into stars
• Nuclear reactions inside stars are believed to
produce the heavier elements
• Infrared (IR) data from the Hubble telescope
indicates the presence of heavy elements such as
Iron and Zinc in stars
Origin of the Earth
• Formed ~ 5 billion years ago
• Solidified cloud of dust and gases left over
from creation of the Sun
• Iron and silicates main ingredients, along
with small amounts of other elements
• Resulting heavy iron core produces
magnetic field
• Silicates are major part of Earth’s crust
Water on the Earth
• As Earth cooled, fairly stable “crust” formed
• Depressions in the crust were natural basins in which water
arising from volcanoes and fissures, collected to form
oceans
Origin of life on Earth
• Water required
• Anerobic conditions for first bacteria
• Fossil remains give carbonaceous material
on Earth aged 3.5 billion years ago
• Details still not known!
• Evolution vs Creation!!
What’s in a Name?
• Hydrogen: from Gr. hydro genes (water
forming). Identified as an element by Henry
Cavendish in 1766
• Helium: From Gr. Helios (sun); made by
fusion in the sun-1st identified on Earth by
William Ramsay in 1895
In all atoms of all elements, the number of
electrons surrounding the nucleus exactly
equals the number of protons within the
nucleus.
All atoms of all elements are electrically neutral.
The mass number of an atom(A) is the sum of
the protons and the neutrons in its nucleus.
Because the mass of an electron is negligible
compared with the mass of both a proton and the
neutron, virtually the entire mass (99.95%) of an
atom lies in its nucleus.
The atomic number (Z) is the total number of
protons in the nucleus of an atom
All atoms of the same element have the same
atomic number.
Thus, an element is a substance whose
atoms all have the same atomic number
Mass number (A) - Atomic number (Z) =
the number of neutrons.
19
9
F
# of neutrons:
19-9 =10
Isotopes: Deuterium and Tritium
Isotopes are atoms with the same atomic number
(ie.same element) but with different mass numbers.
Isotopes of a particular element all have the same
number of protons (same atomic number) but carry
different numbers of neutrons in their nuclei
(therefore different mass numbers).
Naturally occurring deuterium is extremely rare;
only about 1 D atom for every 67,000 H atoms
1
1H
Protium
2
1H
Deuterium
3
1H
Tritium
Stable Isotopes of Carbon
•
12 C
6
natural abundance 98.892%, atomic
mass 12.00000
• 136C natural abundance 1.108%, atomic
mass 13.00756 amu
• Thus atomic weight C =(.98892x12.00000)
+(.01108x13.00756) = 12.01 amu
The radioactive isotope of C
•
•
•
•
•
14C
is radioactive with a half life of about 5700 years. It is
very useful in dating archeological artefacts with ages up
to about 10 x the half life of 14C
Living matter has a constant (low) 14C content, due to
14CO in photosynthesis
2
14C has 6 electrons and 8 neutrons
Shroud of Turin (Christ’s burial cloth?) No, 14C dating
indicated it was only about 500 years old.
Spring 2009: 14C dating results questioned –impure
sample?
Shroud of Turin
Isotopes in Nuclear Medicine
• Modern diagnostic techniques use isotopes
with short half lives
• Technetium 99Tc (t1/2= 6hours) is most
common: results from decay of 99Mo
• Crisis at Chalk River (AECL) in Nov-Dec
2007
• Repair schedule caused disruptions in
supply
Production of short lived isotopes
• Need a nuclear reactor
• Start with a stable isotope of Molybdenum
• Neutron activation of 9842Mo
• 9842Mo + 10n  9942Mo (half life 67 hours)
• a balanced “nuclear” equation
Decay of
99
42Mo
• Goes spontaneously to 99Tc (t1/2=6 hours)
• Emits gamma rays (~ same E as X-rays)
• Radiopharmaceuticals combine Tc with tin,
pyrophosphate, monoclonal antibodies: for
mapping circulatory disorders, heart muscle
damage and intestinal cancers, respectively
• Many other applications :over 30
radiopharmaceuticals presently in use
Advantages of Tc imaging
• Low doses of radioactivity used
• Gamma radiation emitted is detected with
an X-ray “camera” more sensitive than Xray absorption
• Short half life (6 hr) means that in 24 hours;
4 half lives ie.5x.5x.5x.5=.0625 Tc remains
• Thus only 6% of the radiation still in the
body after 1 day
Isotopes in the House of Commons
• Rumour has it that PM Harper called up
Bush to discuss the “problem with the
Isotopes”
• “Fallout”: President of AECL resigned
• Safety concerns were trumped by the need
to get the supply up and running again
A Continuing Problem
• AECL reactor at Chalk River is 50 years old
• Supplies ~half of all medical imaging
isotopes used globally and almost all of
those in NA.
• Suppliers outside Canada now being sought
• need Health Canada approval
• Stayed tuned!!
Iodine -131 in Ottawa waste
• Biosolids showed low levels of radioactivity in Feb
2009,due to iodine 131. Commonly used medical isotopeparticularly for thyroid disorders: levels deemed too low
for concern
• Half life~8days; beta and gamma emitter
• 131MIBG (meta-iodobenzylguanidine) used for cancer
treatment
• Formed by fission of 235U; decays giving γ radiation, a beta
particle (-1β0) and 54Xe131.
Chernobyl (1986) accident-large amounts of 131-iodine given
off- thyroid cancers in children observed