Atoms have a nucleus located in the center of
the atom.
Protons and neutrons are in the nucleus.
Protons
Positively charged particles
# protons defines the element
ex) if it has 8 protons, it’s always oxygen
Mass of a proton = 1.67 x 10-27 kg
How much is 1.67 x 10-27 kg?
One nickel masses about 5 g; divide a nickel into
5 pieces. Each is about a gram.
Divide one of those pieces into a trillion equal
pieces.
Divide one of the trillion into a trillion smaller,
equal pieces. Divide one of those into a thousand
equal pieces. That’s about 1 x 10-27 kg
The nucleus is a VERY TINY part of the atom,
but it has almost all of the mass. It is very
dense.
One cm3 of nucleus would weigh about 22
trillion pounds. That would
be like squashing 7,000,000,000
Toyota Camrys into a cube
the size of one dice.
Compared to the
size of the entire
atom, the nucleus
is like a small ball
sitting in the
middle of a majorleague baseball
stadium. The
electrons are way
out in the
“nosebleed” seats.
The diameter of an atom ranges from
1 x 10-10 m to 5 x 10-10 m.
Divide the thickness of a penny into a million
equal slices: the diameter is between 1 and 5 of
those slices.
Neutrons
No charge
neutral particles
Mass is a little more than a proton.
Number of neutrons can vary… An isotope
is an atom of a particular element (same
number of protons) that can have a
different number of neutrons.
Ex) all oxygen atoms have 8 protons in the
nucleus.
O-16 has 8 protons and 8 neutrons
O-17 has 8 protons and 9 neutrons
O-18 has 8 protons and 10 neutrons.
These are all isotopes of oxygen.
Electrons
Negatively charged particles “orbiting”
the nucleus in discreet levels.
Mass is negligible. Almost all of the atom’s
mass is in the nucleus.
Electrons
In between the nucleus and electrons is
just…empty space.
Atoms are mostly empty space. And since
everything is made of atoms, everything is
mostly empty space.
Strange, don’t you think?
In an electrically neutral atom,
electrons = protons.
Atoms that have gained or lost electrons
(ions) are shown with a charge sign:
Ca2+ has lost 2 electrons
Cl- has gained 1 electron
The atomic number (Z)
is the number of
protons in the atom.
The number of protons uniquely
identifies the element.
The atomic number is a whole number, not a
decimal, usually located at the top of the box.
8
O
15.9994
O
8
15.9994
The mass number (A) is not a mass; it’s the
sum of the number of protons and neutrons
for a particular isotope.
This is not on the periodic table.
Isotopic notation includes the atomic
number and the mass number.
Mass number (total
protons + neutrons)
A
Z
Atomic number
(# protons)
E
17
8
O
This isotope of oxygen has
8 protons (atomic number),
8 electrons and 9 neutrons.
(Recall that mass number is the
total of protons and neutrons. )
Isotopes are also often written as
C-12
C-13
C-14
The number is the mass number.
The atomic mass (often called atomic
weight) is the weighted average of the
masses of the isotopes of an element.
The atomic mass (or weight) is a decimal
number and may be found in different
locations in the box.
8
O
15.9994
8
15.9994
O
Atomic mass is measured in atomic mass units.
1 amu = 1/12 mass of one C-12 atom.
1 amu = 1.66 x 10-27 kg
1 g = 6.022 x 1023 amu
Calculating atomic mass:
(% abundance isotope 1)(mass isotope 1) +
(% abundance isotope 2)(mass isotope 2) +
and so forth.
Example:
isotope % abundance mass of 1 atom (amu)
O-16
99.762
15.99492
O-17
0.038
16.99913
O-18
0.200
17.99916
Atomic mass of Oxygen =
(0.99762)(15.99492)+
(0.00038)(16.99913)+
(0.00200)(17.99916)
= 15.999 amu
Check the periodic table; this is the
atomic mass given.