Chapter 3 Atomic Structure and Periodic Table.docx.pdf -...

Chapter 3: Atomic Structure And Periodic
Table:
Earlier, we’ve studied that elements are the purest substances of all.
And each element has its own type of atoms. When scientists first
discovered the atom, they believed it was a spherical structure like
marbles. Later on other scientists discovered that there the atom is
made up of even smaller sub-atomic particles.
The Atomic Structure:
It was discovered that an atom is made up of three types of sub-atomic
particles, these are protons , neutrons and electrons . It was also
discovered that in the center of an atom, there is a Nucleus which is
made up of protons and neutrons. Around the nucleus there are energy
shells in which electrons are. Electrons are always orbiting the nucleus in
the energy shells.
Protons and
Neutrons are said to be Nucleons because together they make the
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nucleus. Each type of these sub-atomic particles has its own physical
properties which are explained in this table:
These physical properties tell us several factors:
● The mass of the atom is concentrated in the nucleus, this is because
the mass of an electron is negligible.
● A neutron has no charge, it is neutral
● An atom is also neutral, this is because it always contains as much
positive protons as negative electrons.
The mass of the sub-atomic particles is in atomic mass unit (AMU). This
is because they are so light that they can’t be measured in grams.
There are two numbers given to each type of atoms:
Atomic/Proton Number: it is the number of protons in an atom. And
since an atom has an equal number of protons and electrons, it is the
number of electrons in the atom too.
Mass/Nucleon Number: it is the number of protons + number of
nucleons in the nucleus of an atom. It is called mass number because its
value is also mass of the atom.
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Particle Symbol Mass Charge
Proton p 1 +1
Neutron n 1
Electron ѐ 1/200 (Neglig -1

If the atomic number is the number of protons, and the mass number is
the number of protons and neutrons, then subtracting the proton
number from the mass number will give you the number of neutrons in
the atom.
When we represent the atom of an element, we give it a symbol of one
or two letters where the first letter is always in capitals and the second
one is in lowercase. The mass number goes above the symbol and the
atomic number goes below the symbol.
Example:
4
He
2
This represent the Helium Atom
Its symbol is He
Its mass number is 4
Its atomic number is 2
It has 2 protons, 2 neutrons (4-2=2), and 2 electrons.