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Atomic Theory
Mass of an Atom & Electron Arrangement
Mass of an Atom


The mass of an atom is equal to the number
of protons plus the number of neutrons
Mass number = number of protons + number of neutrons
Information Given by Symbols

Atoms are often written in the following way

From this information we can calculate the
number of protons , the number of electrons and
the number of neutrons.
Calculations

Number of Protons = Atomic Number

Number of Electrons = Number of Protons

Number of Neutrons = Mass
No. – Atomic No.
Isotopes

Isotopes are atoms of the same element
with different mass numbers.

ie : they have different numbers of neutrons.
Example

Lithium has 2 isotopes
Relative Atomic Mass

The Relative Atomic Mass (average atomic
weight) is the average weight of all the
isotopes present in an element.
Example
Chlorine has a relative atomic mass of 35.5 amu.
This is due to the abundance of the 2 different
Isotopes present in Chlorine.
35Cl (75%) & 37Cl (25%)
Electron Arrangement
Electron Arrangement

In an atom Electrons are not the same
distance from the Nucleus

We can now state that electrons are found as
shells at different distances from the nucleus.
Shells


Each shell can hold only a set number of
Electrons.
1st electron shell holds a maximum of 2e2nd electron shell holds a maximum of 8e3rd electron shell holds a maximum of 8e-
Electron Arrangement of Atoms


Examples
Fluorine
9e-
2,7
xx
x
x
x
F
x
xx
x
Examples

[Now complete examples in your note book for the following Elements]

Chlorine

Potassium