Atoms and the Periodic Table
• All known Atoms are arranged in the Periodic
• 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
Names of Groups
Certain groups have specific names
• Group 1 is called • Group8 or 0 is Group 7 is called
the The Alkali called the the Halogens.
Metals. This is Noble(or inert) This group is the
because they gases. The most reactive of
elements in this the non-metals
form Alkalis; group are
when placed in extremely
Atomic Mass No.
x Atomic Numbers and
• The Atomic Number: This tells us the
number of protons in an atom
For an atom
• The Atomic Mass No: This to be neutral
tells us the number of protons it must have
and neutrons added together. the same no.
23 of Protons
eg Na and
To calculate the no.of Neutrons in an atom
No.of Neutrons.= At. Mass No. – At. No.
• 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 Particle Mass(amu) Charge
The Nucleus Proton 1 +
Electrons orbit Electron 0 -
n the nucleus
p Neutron 1 0
Protons & Neutrons are found in the Nucleus
• Atoms are so light that we find we have to use a special
instrument to measure their mass. It is called a Mass
• 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
• Isotopes are atoms of the same element with
different Mass Numbers because they have different
numbers of Neutrons.
• When Atoms join they must collide with each
other. Now 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
They are formed when atoms either lose or
• 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 24 No. of p = 12
No.of p = 12
12 12 No. of e = 12-2 = 10
No.of e = 12
No.of n = 24-12 =12 No. of n = 24-12=12
No.of p = 8 No. of p = 8
8 No. of e = 8+2 = 10
8 No.of e = 8
No. of n = 16-8= 8
No.of n = 16-8 = 8
• Atoms join together so as to achieve their most
stable electron arrangement(EA).
xx Electrons are arranged in Energy Levels(or
shells) around the nucleus.
x x x x The first Energy Level can take up to 2 electrons
x x xx x x
xx 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.