atomic structure. element : aluminium (al) what is an atom ? an atom of aluminium atoms of aluminium
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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 particles
•atoms differ from each other due to the different
combinations of these sub-atomic particles
Structure of Helium atom
Source : Intro to Chemistry by Cyber Ed Inc.
2 protons2 neutrons2 electrons
Sub-atomic particles
particle proton neutron
electron
charge +1 zero -1
mass 1 1 1/1850
location In nucleus
In nucleus
In shellsaroundnucleus
Mass number; Atomic Number
Mass number (or nucleon number)-the sum of the number of protons and neutrons in an atomNucleon = 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.
Na23
11
Massnumber
Protonnumber
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
Derive the number of protons, electrons and neutronsin a sodium atom
Example
11 Na
23
Mass number = 23Atomic number = 11
Number of protons = 11Number 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
Ma252
111
Hence
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.
1111
23 - 11 = 12 24 – 11 = 13
11 11
number of protons
number of neutrons
number of electrons
IsotopesIsotopes Na Na 2424
1111 Na Na 2323
1111
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
(i) Find out the proton number of the atom or element ; then for a neutral atom,
the no. of electrons = the proton number
(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.
Valence electrons
Outermost shell OR Valence shell - shell outermost from the nucleus
Valence electrons (Outermost electrons)
electrons in the outermost shelldetermines 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