Atomic Structure
Sub-Atomic Particles
• Nucleus – a dense, positively charged
region at the center of the atom
Proton
p+
Positively charged particle located
in the nucleus
Neutron
no
Neutral particle (no-charge)
located in the nucleus
Electron
e-
Negatively charged particle
“surrounding” the nucleus
Sub-Atomic Particles
• Mass:
Protons and neutrons have roughly the
same mass 1 AMU. Electrons are much,
MUCH smaller (in fact we usually don’t
count them in mass calculations!)
• Charges:
Protons and electrons have equal but
opposite charges. Neutrons are
electrically neutral.
Atomic Number
• Number of protons in
an atom
• This is the WHOLE
number next to the
symbol on the
periodic table.
17
Cl
35.453
Atomic Mass
26
Fe
55.845
• Mass of atom
• Calculated by adding
together protons and
neutrons
• Recorded in amu
(atomic mass units)
• This is the decimal
number found next to
the symbol
Calculating Atomic Mass
• Atomic Mass = #protons + #neutrons
• Most often you will calculate the #neutrons from
the information found on the periodic table
• #neutrons = atomic mass - #protons
• #neutrons = 56 – 26
26
• #neutrons = 30
Fe
• Round the decimal part
55.845
Average Atomic Mass
• Since atoms of any given element can exist in
different forms (isotopes), we average together
the masses of the isotopes.
• From this number we can infer which are the
most common isotopes.
Isotopes
• Isotopes still have the
same atomic number,
they just have a
different mass
number due to the a
different number of
neutrons.
Ions
• Neutral atoms have
equal amounts of
positive and negative
charge.
• When an atom gains
or loses electrons, it
will become charged,
and then we call it an
ion.
Drawing Bohr Models
• First, determine the number of protons,
neutrons, and electrons.
• Draw the nucleus with the protons and
neutrons.
• Next, determine the number of energy
levels.
• Draw the energy levels with the correct
number of electrons on each level