There are three subatomic particles which make
up the atom:
Protons
Electrons
Neutrons
 The
proton is a positively charged
particle which is found in the nucleus of
the atom.
• The number of protons of an element is also
equal to the atomic number.
• i.e. Sodium has an atomic
number of 11 so therefore
has 11 protons.
 Neutrons
are uncharged particles which
are also found in the nucleus of the atom.
• Neutrons are uncharged, but they do contain
mass.
• The number of neutrons can be found by
subtracting the atomic number from the atomic
mass.
 i.e. Sodium
23- 11 = 12
contains 12 neutrons.
 Electrons
are negatively charged
particles which are found outside the
nucleus of the atom in energy levels
(orbitals)
 In
a neutral atom (meaning not charged),
the number of protons is equal to the
number of electrons.
 If
an atom is positively charged this
means that there are less electrons. In an
negatively charged atom, there are more
electrons.
 i.e. Na+
has 11 protons, 12 neutrons and
10 electrons.
 The
protons NEVER change, only the
electrons change in charged atoms, AKA
ions.
 Try
to find how many protons, electrons
and neutrons are in the following atoms:
A)
O-2
B)
Al+3
 While
working with neon atoms, J.J.
Thomson discovered two different types
of neon atoms.
 Both were exactly the same chemically,
but their masses were different.
 Atoms
of the same element but with
different masses are known as isotopes.
 Since
their masses differ, they have the
same number of protons, but different
numbers of neutrons.
 For
example, a neutral neon atom has 10
protons and 10 neutrons.
Neon-21 isotope has 10 protons, but 11
neutrons.
Neon-22 has 10 protons, and 12 neutrons
A
particular type of atom containing a
definite number of protons and neutrons
is called a nuclide.
• i.e. Protium , hydrogen-1, is a nuclide of
hydrogen.
 Protons
and neutrons are not truly the
smallest parts of matter.
 The
electron however, as of present, is
known to be an elementary particle.
 The
proton and neutron are composed of
even smaller particles.
 Scientists
have divided sub-atomic
particles into two broad categories:
• Leptons
• Hadrons
 Leptons
are true elementary particles.
The electron is the most well-known
lepton.
 Hadrons
are made of even smaller
particles. Protons and neutrons are
examples are hadrons.
 Hadrons
are subdivided into mesons and
baryons. Protons and neutrons are
baryons.
 Baryons
and mesons are made up of
smaller particles called quarks.
 The
atomic mass of an element is actually
the average of all the isotopes’ masses.
 Not
all the isotopes are present in equal
amounts. Therefore when calculating the
average atomic mass, we use a weighted
average.
 To
find the average atomic mass:
• First multiple each isotope by its abundance.
• The products are then added together.
• The sum is divided by the total abundance
 In
a sample of 100 neon atoms, 90 will be
neon-20 and 10 will be neon-22.
Calculate the average atomic mass of
neon.
 Mass has units of atomic mass units (u)
(90x19.992 u) + (10x21.991u)/100
= 20.192 u