Atomic Structure
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An atom is the smallest possible particle of an element that can
exist
Structure of the Atom
The atom consists of a tiny, massive central nucleus surrounded by electrons.
Particle Relative Charge Relative Mass
Proton +1 1
Neutron 0 1
Electron –1 0 (1/1840)
The Nucleus
· Contains the protons and neutrons.
· It is positively charged (protons are positively charged).
· It is the heavy part of the atom (protons and neutrons have much greater mass than
electrons).
· It is small in comparison to the rest of the atom.
The electron shells
· Electrons are arranged in energy levels around the nucleus.
· The electrons are fast moving.
· Each energy level can only contain a set number of electrons. Once an energy level is
full any extra electrons go into the next level.
Atoms of different elements contain different numbers of protons. Atoms have no overall
charge. Therefore
the number of protons is the same as the number of electrons.
Atomic Number and Mass Number
The atomic number is the number of protons in the nucleus.
The mass number is the number of protons and neutrons in the nucleus.
The element will sometimes be represented by the following:
Where: A = the mass number.
Z = the atomic number.
X = the element symbol.
e.g.
This tells us:
The atomic number of sodium = 11
The mass number of sodium = 23
We can now work out the number of protons, neutrons and electrons:
Number of protons = atomic number = 11
Number of neutrons = mass number – number of protons = 12
Number of electrons = number of protons = 11
Isotopes
Isotopes are different atoms of the same element that have the same atomic number but
different mass numbers.
· Isotopes have the same number of protons, but different numbers of neutrons.
· Isotopes have similar chemical properties because chemical reactivity depends upon
the number of electrons but slightly different physical properties (e.g. density).
Calculating the Relative Atomic Mass
Example: Chlorine
Chlorine exists in two forms. These two forms of chlorine differ only in the number of
neutrons in the nucleus.
35
Cl = 75% 37
Cl = 25%
Consider 100 chlorine atoms: 75 will have a mass of 35, 25 will have a mass of 37.
We now need to work out the average mass.
The average mass of an atom =
The Arrangement of Electrons
around the Nucleus
· Electrons are arranged in energy levels (or shells) surrounding the nucleus.
· They do not all have the same energy.
· Electrons with low energy are in energy levels close to the nucleus and ones with high
energy are in energy levels further away from the nucleus.
· There are a limited number of electrons that can be fitted into each energy level.
· This arrangement of electrons surrounding the nucleus is called the electron
configuration.
The first energy level can hold a maximum of 2 electrons.
The second energy level can hold a maximum of 8 electrons.
The third energy level holds 8 electrons (although can hold up to 18)
Once one energy level is full, the next is filled.
e.g. contains 11 protons, 11 electrons and 12 neutrons.
The electron configuration is 2,8,1
Representing electron configuration.
e.g.
Uses of electron configuration
The electron configuration can tell us:
a) Which group of the periodic table the element is in. (The number of electrons in the
highest or outer energy level tells us this.)
b) Which period of the periodic table the element is in. (The number of energy levels tells
us this.)
All elements in the same group of the periodic table have the same number of electrons in
the outer energy level.
Group I II III IV V VI VII VIII
No. of electrons in outer shell 1 2 3 4 5 6 7 8
The Noble Gas Configuration
The Noble gases (group VIII) are the most unreactive elements in the periodic table. They
appear at the end of each period. They are unreactive because they have a full energy level
(shell) of electrons. Other elements in the periodic table react with other elements to gain
the “more stable” noble gas configuration.
Formation of Ions
· Ions are atoms which have gained or lost electrons to obtain a full outer shell. Ions are
charged, because electrons have negative charge.
· Atoms that lose electrons form positive ions because there are now more protons than
electrons. Positive ions are called cations.
· Atoms that gain electrons form negative ions because there are now more electrons
than protons. Negative ions are called anions.
E.g. Sodium
Electron configuration is 2,8,1
To gain a full outer shell the atom can either:
a) lose one electron
b) gain seven electrons
It is easier to lose one electron than to gain seven, so sodium loses an electron to form a
cation (positive) with a charge of +1.
The lost electron will be given to another element (see notes on Ionic Bonding).
So now The number of protons = 11
The number of neutrons = 12
The number of electrons = 10
E.g. Chlorine
The electron configuration is 2,8,7
To gain a full outer shell the chlorine atom can either:
a) lose seven electrons
b) gain one electron
It is preferable to gain one electron than to lose seven, so chlorine gains an electron to form
an anion (negative) with a charge of –1.
The electron is gained from another element.
So now The number of protons = 17
The number of neutrons = 18
The number of electrons = 18
Electron Configuration and the Periodic Table
The chemical properties of an element depend on the number of electrons in the highest
energy level (shell).
· Metals lose electrons to form cations.
· Non-metals gain electrons to form anions.
An easy way to work out the charge on an ion:
Group 1 2 3 4 5 6 7 8
Charge on +1 +2 +3 No Ions –3 –2 –1 No Ions
Ion
