Atomic Structure

Element : Aluminium (Al)

What is an atom ?

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 particles

•atoms differ from each other due to the different

combinations of these sub-atomic particles

Atoms of different elements differ insize and mass.

Trends in

Discovery of structure of atom

Source : CD-ROM Chemistry Concepts and Applications by Glencoe

Structure of an Atom

electrons

protons andneutrons

nucleus

shell

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.

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 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

group number

simplified electronic structure

I II III IV V VI VII

VIII

full electronic structure

the Periodic Tablethe Periodic Table

proton numbername of element

symbol

electronic structures and the Periodic Table

electronic structures and the Periodic Table