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Transcript
Distinguishing Between
Atoms
Remember, atoms consist of three
subatomic particles:
Atoms of different elements contain
different numbers of protons
• Hydrogen has only one proton while oxygen
has 8 protons in the nucleus.
• The atomic # on the periodic table describes
the number of protons in the nucleus
Atomic #
So, the question is:
How do atoms of different elements differ
from each other?
O
8
15.9994
Atomic Symbol
Average Atomic
Mass
Remember, protons have a positive (+)
charge. Therefore, for neutral atoms, the number
of electrons, having a negative charge (-) must
equal the number of protons.
So, for neutral atoms the atomic number
describes the number of protons and electrons
in the atom.
Symbols of Elements
Elements are symbolized by one or two
letters.
Atomic Number
All atoms of the same element have the
same number of protons:
The atomic number
1
Atomic Mass
• The mass of an atom in atomic mass units
(amu) is the total number of protons and
neutrons in the atom.
• Called the mass number
Nuclear symbols vs. Hyphen notation
•Nuclear symbols, as shown above, uses the
symbol of the element preceded by a superscript
mass # and subscript atomic number.
•The same atom can be described by hyphen
notation which lists the atom name or symbol
followed only by the mass number hyphenated.
Carbon-12
or
C-12
To find the number of neutrons in an
atom, you subtract the atomic number
(number of protons in the nucleus) from
the mass number (total number of protons
and neutrons in the nucleus)
Number of Neutrons
= mass number – atomic number
238
How many neutrons are in 92 U ?
How many protons are in Sulphur-33?
How many neutrons are in Zn-60?
Isotopes:
• Atoms of the same element with different
masses.
• Isotopes have different numbers of neutrons.
MOVIE
2
Atomic Mass
The Atomic mass is different for each
isotope of an element because each isotope has
a different number of neutrons in the nucleus.
Atomic and
molecular masses
can be measured
with great
accuracy with a
mass
spectrometer.
Since the mass of atoms is so increadibly
small, a unit known as an atomic mass unit
(amu) was defined to make the masses of atoms
easier to work with.
•The amu is defined by the carbon-12 isotope of
carbon.
•1 amu is equal to 1/12 the mass of the carbon-12
isotope. (roughly the mass of a neutron or
proton)
Notice, the mass of electrons is so much less
than that of a neutron or proton that its mass is
negligible.
Now the question
becomes:
Since there
are different
isotopes for each
element, how do
we report the
mass on the
periodic table to
reflect all of the
isotopes of an
element in a
sample of that
element?
Average Atomic Mass
Atomic #
O
8
15.9994
Atomic Symbol
Average Atomic
Mass
• Because in the real world we use large amounts
of atoms and molecules, we use average
masses in calculations.
• Average mass is calculated from the isotopes of
an element weighted by their relative
abundances.
3
Calculating the Average Atomic Mass
To calculate the average atomic mass
of an element, multiply the mass of each
isotope by its natural abundance,
expressed as a decimal, then add the
products.
4