Atomic Structure
Inside an Atom
• Over time, scientists have
come up with many
theories about the inside
of atoms.
• Today we believe that an
atom has a center
nucleus surrounded by
one or more electrons.
• The nucleus is the central
core of the atom.
nucleus
Subatomic Particles
Atoms are made up of 3 different kinds of particles:
1. Protons- positively charged particles located
inside the nucleus of an atom.
2. Neutrons- particles located inside the nucleus
that are neutral (no charge).
3. Electrons- negatively charged particles located
outside the nucleus.
Particle
Proton
Neutron
Electron
Charge
positive
neutral
negative
Location
inside
inside
outside
Subatomic Particle Facts
• Protons are located in the nucleus, have a
positive charge, and a mass of 1 amu.
• Neutrons are located in the nucleus, have no
charge, and a mass of 1 amu.
• Electrons are located outside the nucleus, have a
negative charge and a mass of 1/1836 amu
(almost zero).
• Because electrons have basically no mass, 99% of
the mass of an atom is found in the protons and
neutrons.
The Periodic Table
• Different atoms have different numbers of
protons, neutrons and electrons.
• The periodic table can you find out how many
p, n and e- an atom has.
Atomic Number
• The number of protons in
the nucleus of an atom is
called the atomic number.
• Each element has a
different number of
protons and therefore, a
different atomic number.
• The atomic number (or #
of protons) is different
for all elements.
The atomic number
is the large number
at the top of the
box.
Positives = Negatives
• In atoms, the number of protons is equal to
the number of electrons.
• Therefore, the atomic number tells us the
number of protons AND the number of
electrons.
protons
=
electrons
Example…
If the atomic number of potassium is 19, how
many protons and how many electrons does a
potassium atom have?
Protons= 19
Electrons= 19
Every atom has mass.
• The mass of an atom is equal to the weight of its
nucleus.
• This is because electrons have basically no
weight.
• Therefore the number of protons and neutrons
in the nucleus gives us the mass of the atom.
Mass Number
• An atom’s atomic
weight is listed on
the Periodic Table.
• If you round the
atomic weight to the
nearest whole
number you get the
atom’s mass number.
Round 4.003 to 4
Number of neutrons
Since the mass number is equal to the number of
protons plus neutrons, we can use it to find out
how many neutrons there are.
Round the atomic weight to a whole
number= 20
This means that protons + neutrons
in Neon= 20
The atomic number of neon is 10
which means neon has 10 protons.
Atomic weight of neon is 20.1797
How many neutrons does it have?
20-10= 10
To find the number of neutrons,
you can…
Mass number – atomic number
Try this…
How many protons? 42
How many electrons? 42
How many neutrons? 54
(Remember: 96-42)
Number of neutrons can vary
• All atoms of the same element will have the
same atomic number and therefore the same
number of protons.
• Sometimes, however, an atom of an element
can have a different number of neutrons.
• This would still be the same element, it would
just have a different atomic weight.
Isotopes
• An isotope is an atom with the same number of
protons but a different number of neutrons from
other atoms of the same element.
• The atomic weight of an atom is an average of the
weights of the different isotopes of that element.