The Atom and
The Periodic
Table of
Elements
We can classify (arrange) elements in
different ways:
• naturally occurring / made by scientists
• solid/liquid/gas
• metal/non-metal
 Elements
are pure substances that
cannot be broken down into any
simpler substance. Elements are the
basic building blocks of all matter.
 Compounds
are any substances that
are formed by the chemical
combination of two or more elements
and acts like a single substance



The Periodic Table is a system of
classifying all of the known elements
and was first developed by Dmitri
Mendeleev.
Metals are found to the left of the
zigzag line of the periodic table.
Nonmetals are found to the right of the
zigzag line of the periodic table.
 Metalloids
are metal or metal-like
elements found on both sides of
the zigzag line. Metalloids have
properties of both metals and non
metals. EXAMPLES: Boron,
Silicon, Arsenic.
A vertical column of elements on the
periodic table is called a
group or families.
The elements in the same group of
the periodic table have similar
chemical properties.
• Periods are the horizontal rows
of elements on the
periodic table. They do not have
similar properties.
Every element is made up of very
small particles called atoms.
Atoms of different elements have a
different number called the atomic
number.
Atoms have a very small, positively
charged nucleus, with negatively
charged electrons outside the
nucleus in energy levels.
+
–
The nucleus of every atom (except
hydrogen) contains two particles:
• Protons (positively charged). Protons
are equal to the atomic number.
• Neutrons (no charge) In order to get
the number of neutrons, subtract the
atomic weight from the atomic
number.
In energy levels outside the
nucleus we find:
•Electrons (negatively charged) The
number of electrons is equal to the
number of protons.
Atoms are neutral because the positive
charge of the nucleus is equal to all of the
negative charges of the electrons added
together.
–
+
• 11 electrons
• 11 negative charges
• 11 protons
• 11 positive charges
Atoms are neutral because the
numbers of protons and electrons are
equal - the opposite charges cancel.
Nuclide notation – how many protons,
neutrons, and electrons are in atoms.
Mass number
(protons + neutrons)
Atomic number
(number of protons)
35
Cl
17
As atoms have no charge, the number of
electrons is the same as the number of
protons. This atom has 17 electrons.
(Electrons = Protons)
 Subatomic
Particles are particles
that are smaller than an atom. The
three main subatomic particles are
protons, neutrons, and electrons.
 Electron
Cloud is the space in which
electrons are likely to be found (think
of a beehive).

1.
The four forces that account for the
behavior of subatomic particles are:
Electromagnetic Force – a force
that can attract or repel the particles
in an atom. This force holds the
electrons around the nucleus.
Example: If there were 2 protons (same
charge) the E.F. is repulsion. If there
were 1 electron and 1 proton (opposite
charge) the E.F. is attraction.
2.
Strong Force – opposes the
electromagnetic force of repulsion
between protons. It binds protons
together to form the nucleus.
3.
Weak Force – responsible for a
process known as radioactive
decay. A neutron in the nucleus
changes into a proton and an
electron.
4.
Gravity – the force of attraction
between all objects in the universe.
Of the four forces, it is the weakest
force. Why? The masses of the
particles in atoms are so small, the
force of gravity within atoms is very
small.
Drawing Atoms
Energy levels can only hold a certain number of
electrons. You must completely fill the energy level
that you are working on before moving on to the
next level.
1st = 2 electrons
2nd = 8 electrons
3rd =18 electrons
4th = 32 electrons
Scientists from our past.
Dmitri Mendeleev – developed the first
periodic table in 1869.
In 430 BC Greek Philosopher
Democritus gave the name to the
smallest piece of matter the atom.
Joseph John “JJ” Thomson discovered
the electron in 1897. His model is
sometimes called the “plum pudding”
model. According to his model, the atom
was made of a pudding-like positively
charged material throughout which
negatively charged electrons were
scattered.
In 1911 English scientist Ernest
Rutherford disproved Thomson’s model.
He reasoned that all of an atom’s
positively charged particles were
contained in the nucleus. The negatively
charged electrons were scattered outside
the nucleus around the atom’s edge.
(credited for splitting the first atom)
– Nucleus – the tiny, extremely dense,
positively charged region in the center of an
atom.
In 1913 Danish Scientist, Niels Bohr,
discovered that electrons move or orbit
around the nucleus. The current atomic
theory states that electron clouds are
where electrons are most likely to be in the
space around the nucleus.