Download Atomic Structure

Atomic Structure
What is an atom ?
Element : Aluminium (Al)
An Atom of Aluminium
Atoms of Aluminium
What is an atom ?
•smallest particle of any substance
•average diameter is approx. 10-10 m
(analogy : 1016 atoms needed to cover a full stop)
•each atom is made up of even smaller sub-atomic part
•atoms differ from each other due to the different
combinations of these sub-atomic particles
Atoms of different elements differ in
size and mass.
Trends in
Discovery of structure of atom
Source : CD-ROM Chemistry Concepts and Applications
by Glencoe
Structure of an Atom
nucleus
shell
electrons
protons and
neutrons
Structure of Helium
atom
2 protons
2 neutrons
2 electrons
Source : Intro to Chemistry by Cyber Ed Inc.
Sub-atomic particles
particle
proton
neutron
zero
electron
-1
charge
+1
mass
1
1
1/1850
location
In
nucleus
In
nucleus
In shells
around
nucleus
Mass number; Atomic Number



Mass number (or nucleon number)-the sum of the
number of protons and neutrons in an atom
Nucleon = protons + neutrons
Atomic number (or proton number) - the
number of protons in an atom
The atomic number is fixed for each atom. An atom
is identified by its atomic number. Different
elements have different atomic number.
Proton and Mass Numbers
The data are available in the Periodic Table :
E.g.
Mass
number
23
11
Proton
number
Na
Proton (Atomic) Number
The proton (atomic) number of an element
is the number of protons in its atom.
E.g. Since the proton number of sodium is 11, it
means that
•every sodium atom has 11 protons.
•Conversely, any atom with 11 protons is a
sodium atom.
Mass (Nucleon) Number
The mass (nucleon) number refers to the total number of
protons and neutrons in an atom.
Hence, number of neutrons in an atom
= mass number - proton number
Example
Derive the number of protons, electrons and neutrons
in a sodium atom
23
11
Mass number = 23
Na
Atomic number = 11
Number of protons = 11
Number of electrons = 11
Number of neutrons = 23 - 11
= 12
A short Quiz…….
Malaysium (Ma), an imaginary element contains
111 protons and 141 neutrons. How would you represent
the atom of Malaysium in symbolic form?
Atomic number = Number of protons
= 111
Mass number = Number of protons + Number of neutrons
= 111 + 141 = 252
Hence
252
111
Ma
Isotopes


Definition : Isotopes are atoms of the same
element with the same number of protons but
different number of neutrons;
Isotopes have the same atomic number but
different mass numbers.
Isotopes

Isotopes have the same atomic number but
different mass numbers.
Isotopes
23
11
Na
24
11
Na
number of protons
11
11
number of electrons
11
11
number of neutrons
23 - 11 = 12
24 – 11 = 13
Arrangement of electrons in the
atom


Electrons are arranged in energy levels (called electron
shells) around the nucleus. The nearer the shell to the
nucleus, the lower the energy level.
Each energy level can hold a certain number of
electrons. For the first 20 elements in the Periodic
Table,
1st shell : 2 electrons
2nd shell : 8 electrons
3rd shell : 8 electrons
To work out the arrangement of
electrons
Find out the proton number of the atom or
element ; then for a neutral atom,
the no. of electrons = the proton number
(i)
(i)
Main rule -- electrons always go into the shell
nearest to the nucleus ,if there is room. Once the
shell is filled up, the electrons go into the next
available shell and so on.
Eg. electronic structure of Al
-
-
-
-
-
+
-
-
-
-
Valence electrons
Outermost shell OR Valence shell
- shell outermost from the nucleus
 Valence electrons (Outermost electrons)
 electrons in the outermost shell
 determines the chemical properties of
an element

Electron Arrangement & the
Periodic Table



The position of an element in the Periodic
Table is determined by its proton number.
All elements in the same group have the
same number of valence electrons, which
is the same as the Group number.
All elements in the same period have the
same number of electron shells.
Electron Arrangement & the
Periodic Table

Elements in :
(a)the same Group  same number of
outermost / valency electrons
(b)the same Period  same number of
electron shells
electronic structures and the
Periodic Table
the Periodic Table
VIII
group number
proton number
name of element
symbol
full electronic
structure
simplified
electronic
structure
I
II
III
IV
V
VI
VII