TOPIC 3
Atoms and the Periodic Table
Atoms and the Periodic Table
• All known Atoms are arranged in the Periodic
Table(PT).
• Each type of Atom is known as an Element.
• There are approx 118 known elements.
• Columns down the way in the PT are known as
Groups and Rows across the way are called
Periods.
Names of Groups
Certain groups have specific names
• Group 1 is called
the Alkali Metals.
This is because
they form alkalis
when placed in
water.
Group 7 is called • Group8 or 0 is
the Halogens.
called the Noble
This group is the
gases. The
most reactive of
elements in this
the non-metals
group are
extremely
unreactive
Atomic Mass No.
Atomic No.
x
Atomic Numbers and
Mass Numbers
• The Atomic Number: This tells us the
number of protons in an atom
For an atom
• The Atomic Mass No: This
to be neutral
it must have
tells us the number of protons
and neutrons added together. the same no.
23
eg
11
Na
of Protons
and
Electrons
To calculate the no.of Neutrons in an atom
No.of Neutrons.= Atomic mass – Atomic number
Atoms
• All things are made up of Atoms.
• Atoms are so small and light that a special scale
called Atomic Mass Units had to be made up (amu).
• Atoms are made up of three sub-atomic particles
called, Protons, Neutrons and Electrons
e
The Nucleus
p
n
Electrons orbit
the nucleus
Particle
Mass(amu)
Proton
1
Electron
0
+
-
Neutron
1
0
Protons and neutrons are found in the nucleus
Charge
Chemical Definitions
• Atoms are so light that we find we have to use a special
instrument to measure their mass. It is called a Mass
Spectrometer.
• Using this instrument it has been found that atoms of
the same element have slightly different masses. These
are called Isotopes. In order to determine the mass of
an atom we have to take an average of the masses,
taking into account the isotopes. This is called the
Relative Atomic Mass (r.a.m).
• Isotopes are atoms of the same element with
different Mass Numbers because they have different
numbers of Neutrons.
Ions
• When Atoms join they must collide with each
other. Since the electrons are on the outside of the
atom, it is these that are affected and not the
protons or the neutrons.
• Ions are Charged atoms
– These are formed when atoms either lose or gain
electrons.
• When an Atom loses electrons a Positive Ion is formed
• When an Atom gains electrons a Negative Ion is formed
To calculate the number of protons ,neutrons and
electrons in Atoms and Ions
24
Mg
12
No.of p = 12
24
No.of e = 12
12
Mg2+
No. of e = 12-2 = 10
No. of n = 24-12=12
No.of n = 24-12 =12
16
8
O
No.of p = 8
16
No.of e = 8
8
No.of n = 16-8 = 8
No. of p = 12
No. of p = 8
O2-
No. of e = 8+2 = 10
No. of n = 16-8= 8
Electron Arrangement
• Atoms join together so as to achieve their most
stable electron arrangement(EA).
Electrons are arranged in Energy Levels(or
shells) around the nucleus.
xx
xx
x
x
x
x
xx
xx
xx
x
x
x
x
The first Energy Level can take up to 2 electrons
The second energy level can take up to 8 electrons
The third energy level can take up to 18
electrons(but at this stage we shall consider it can
take up to 8 electrons)
Examples of electron arrangements
• A Ca Atom has 20 electrons
therefore it has an EA of 2,8,8,2
• A Ca2+ Ion has 18 electrons(since it has lost two)
therefore it has an EA of 2,8,8
• A S Atom has 16 electrons
therefore it has an EA of 2,8,6
• A S2- Ion has 18 electrons(since it has gained two)
therefore it has an EA of 2,8,8
• Atoms join so as to be closest to their
nearest most stable Electron Arrangement
ie 2 or 2,8 or 2,8,8 etc.