Download Masses of Atoms and the Periodic Table

Masses of Atoms and the
Periodic Table
Atomic number
• To identify which element an atom is, we look
at the number of protons.
• The number of protons in an atom is called its
atomic number.
– every atom with 2 protons is a __________ atom
– every atom with 6 protons is a __________ atom
– every atom with 8 protons is a __________ atom
• Elements are organized on the periodic table
by their atomic number
Mass number
• The sum of the number of protons and
neutrons in an atom is the mass number
– A fluoride atom with 9 protons and 10 neutrons
has a mass number of _________
– A sodium atom with 11 protons and 12 neutrons
has a mass number of _________
– A hydrogen atom with 1 proton and 0 neutrons
has a mass number of ________
• If we know the mass number and atomic
number, we can find the number of neutrons.
• Number of neutrons =
mass number – atomic number
– A carbon atom with a mass number of 14 has how
many neutrons?
Isotopes
• Not all atoms of an element have the same
number of neutrons.
• Atoms of the same element that have
different numbers of neutrons are called
isotopes.
• Nearly all elements have an isotopes
• To identify isotopes, we write the element
name, then a dash, then the mass number
• Oxygen (O) with 8 protons and 8 neutrons:
• Oxygen (O)with 8 protons and 10 neutrons:
• Helium (He) with 2 protons and 1 neutron:
• Helium (He) with 2 protons and 2 neutrons:
• Uranium (U) with 92 protons and 143 neutrons:
• Uranium (U) with 92 protons and 146 neutrons:
Radioactive isotopes
• Many isotopes are unstable.
• An unstable atom can emit particles or even
change neutrons into protons to become
more stable
– This is known as radioactivity
• Some radioactive particles are very dangerous
– For example, radon gas
Half-lives
• Radioactivity occurs at a constant rate
• The time it takes for half of an isotope to
decay is called the half-life
• After one half-life, 50% of the original isotope remains
• After two half-lives, 25% of the original isotope remains
• After three half-lives, 12.5% of the original isotope remains
Half-life example
• Carbon-14 is an unstable isotope of carbon
with a half-life of 5700 years.
• After 5700 years, half of the C-14 will have
decayed into another element.
• The ancient Egyptians often mummified cats.
Scientists are able to calculate how much C-14
is present in the cat to find out its age.
Half-life example
• Assume scientists take a reading of the C-14 in a
mummified cat and find that only 25% of the
expected C-14 remains.
• Approximately how long ago was the cat
mummified?
–
–
–
–
Half-life of C-14 is 5700 years.
After 1 half-life, 50% of C-14 has decayed
After 2 half-lives, 25% of C-14 has decayed
2 half-lives = 5700 years + 5700 years = 11,400 years
Atomic Mass
• Because most elements have different
isotopes, each element has an average atomic
mass
• The unit we use for calculating atomic mass is
the atomic mass unit (amu)
• One amu refers to the mass of a proton or
neutron
• The atomic mass of an element is a weighted
average of the masses of its isotopes
Review: Mass number vs. Atomic Mass
• The mass number is unique for every atom. It
is the sum of protons and neutrons.
• Different atoms of the same element may
have different mass numbers. These are
known as isotopes.
• Mass number has no unit
Review: Mass number vs. Atomic Mass
• The atomic mass is the weighted average of
the mass of all the different isotopes of an
element
• Atomic mass is used to describe the mass of
an element, not of an individual atom
• Atomic mass has the unit amu