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Chapter 2
Atomic Structure
What particles compose the substances around us?
When you use the term particle to explain the different properties of a solid, liquid or
gas, you are referring to the simplest, most basic unit of that substance. If you took a
sample of that substance and started to divide it into smaller and smaller portions
eventually you would reach this basic unit that still has the properties of you substance.
The type of basic unit (particle) will depend on how the atoms are arranged in your
substance.
For example the simplest unit in a sheet of aluminium foil is the aluminium atom. The
sheet of foil consists of many billions of atoms arranged in a network structure.
Molecules
Molecules are formed in many substances when small groups of atoms join together
with a unique structural conformation. The atoms can be all the same or there can be
several different types. A chemical formula gives information about the types and
numbers of atoms present in each of these molecules.
Examples:
The simplest unit in the
substance water is the water molecule,
H2O which is a cluster of two hydrogen
atoms and one oxygen atom.
Water is therefore composed of 2 different
elements, classifying it also as a compound.
In oxygen gas, O2
there are two oxygen
atoms per molecule.
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Molecules in simple 3-D
Chemical molecules are discrete structures representing everyday substances in their
most simple form (ie; atomic ratio). You will use a molecular model kit a bit later to
represent a variety of different chemical structures.
The lines or “sticks” between individual atoms represent . . . . . . . . . . . . . . . . . . . . . . . . .
..
Water, H2O
CCl4
Ammonia, NH3
Carbon Tetrachloride,
Other substances are made up of a large network of ions. Ions are atoms or groups of
atoms which carry a positive or negative electric charge. Common table salt (which has
the chemical name of sodium chloride) is made of a large network of sodium ions, Na+
and chloride ions, Cl -
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Modelling Atomic Structure
In order to be able to distinguish these different sorts of particles you must understand
some fundamental ideas about the structure of an atom. Since atoms are too small to
be seen even with the best microscopes, scientists have developed models of atoms
to represent them. No model can be exactly like an atom in all respects and so models
are being constantly revised as further research reveals more about atoms. In order to
explain the properties you are observing, use the simplest possible model.
Some models of the atom
A Basic Timeline of historical development of models of the atom
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A simple Bohr Model of the Atom
This model of the atom will be of particular use to you in this module. Atoms are units
composed of much smaller subatomic particles: protons, neutrons and electrons.
Electrons:
orbiting about the nucleus
of the atom
Nucleus:
A dense mass of protons
and neutrons at the
centre of the atom
KEY:
+
Proton (mass = 1 a.m.u.)
Neutron (mass = 1 a.m.u.)
–
•
Electron (mass = 0.00054 a.m.u)
These subatomic particles are measured in arbitrary units called atomic mass
units, shortened to a.m.u. The periodic table lists the atomic weights of all elements
in these units, as individual atoms are far too small and light to weigh individually.
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•
Inside the nucleus are found protons (positive charge) and neutrons (neutral
charge). Almost all the mass of an atom is in the nucleus with an overall positive
charge.
•
Negatively charged electrons orbit the nucleus and define the volume of the atom.
This would indicate that atoms are, by far, mostly empty space! A simple calculation
based on relative atomic masses above will reveal the nucleus as containing around
99.9% by mass of an entire atom!
Data Table of Subatomic Particles:
Subatomic
Particle
Charge
Relative mass
(a.m.u.)
Position
Electron: e –
–1
0.00054
orbiting the nucleus
Proton: p+
+1
1
nucleus
Neutron: n
0
1
nucleus
•
The electrons form a negatively charged cloud around the nucleus.
•
Atoms are neutral, so that the number of positively charged protons must equal the
number to negatively charges electrons.
No of protons (p+) = No of electrons (e –)
•
When electrons are lost an atom becomes a positively charged ion or cation.
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•
When electrons are gained by an atom it becomes a negatively charged ion or
anion.
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Isotopes
Any element is identified by the number of protons in its nucleus, defined as its Atomic
Number. The number of neutrons within an individual element can vary without
changing its identity, hence the concept of isotopes.
Different isotopes of a given element have the same number of protons, but
different numbers of neutrons in the nucleus.
Case Study 1: Carbon (Atomic Number 6)
All atoms of Carbon will be identified by Atomic Number 6, having this number of
protons within its nucleus. The nuclei of most carbon atoms contain also 6 neutrons
giving a combines mass number of 12. But statistically, a very small proportion of
carbon atoms in nature have 8 neutrons, thus giving a combined mass number of 14.
These are two different isotopes of carbon which can be distinguished by their different
masses; C-12 and C-14. C-14 is unstable and radioactive. Radioactivity is a feature of
many different elemental isotopes. The residual intensity of C-14 radioactivity is used for
carbon dating of ancient fossils.
So why do many elements appearing on the Periodic Table have mass number values
listed with decimal places? Fractions of 1 a.m.u. do not occur in nature!
Case Study 2: Chlorine (Atomic Number 17)
Chlorine is listed with an atomic mass of 35.45. All Chlorine atoms contain 17 protons in
their nucleus. From the atomic mass above it could be concluded that a chlorine
nucleus contains 18.45 neutrons; but there is no such thing as 0.45 of a neutron.
•
•
An atomic mass, as appears on the periodic table is an average mass of all
isotopes occurring in nature. This may have decimal places
A mass number is a round figure, summing the protons + neutrons of a single
atom.
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Statistically in nature, chlorine atoms are composed of 2 isotopes:
Chlorine-35 (having 18 neutrons) accounts for around 76% of chlorine atoms in nature.
Chlorine-37 (having 20 neutrons) accounts for around 24% of chlorine atoms in nature.
 Now, given these statistics of two chlorine isotopes, calculate its atomic mass
Quantifying Subatomic Particles in Atoms
The Periodic Table of the Elements is a complete database of known atomic
structures. Atoms are listed in order of atomic number, being the number of protons
contained within the nucleus of the atom. An Element is a substance composed of a
single specific atom. The Periodic Table lists over 100 different elements providing
information on their composition of protons, neutrons and electrons.
FEATURES OF ELEMENTS IN THE PERIODIC TABLE:
1. Chemical Symbol: Shortened form of name.
2. Atomic Number:
3. Mass Number:
Number of Protons in the Nucleus.
Combined mass of Protons and Neutrons.
Atomic Number
20
Atomic Mass
Ca
40.08
Chemical Symbol
Chemical Name of Element
DETERMINING ATOMIC STRUCTURE
Number of Protons = Atomic Number
Number of Electrons = Number of Protons, ie Atomic Number
Number of Neutrons = Atomic Mass (rounded) – Number of Protons
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2.5
How are Electrons organised around the Nucleus?
Protons and neutrons make up the almost the entire mass of each atom and are located
in the nucleus. In our model of the atom, the electrons are arranged in orbits or shells
around the nucleus and equal the number of protons in a neutral atom. However,
because they may be a long way from the nucleus, outer electrons may be readily
removed or added and then an ion is formed.
Electrons which are in the same shell are about the same distance from the nucleus
and they have about the same energy. Shells are sometimes referred to as energy
levels. More complicated models of the atoms refer to subshells, suborbitals, standing
waves and so on. Shells are numbered from the nucleus.
Electron shells (or orbits) around the nucleus
The first four shells contain 2 (first shell), 8, 8 and 18 electrons as a maximum. The next
two shells, contain 18 and 32 electrons.
The rule for assigning electrons to the shells is that the first shell is filled before an
electron is added to the next one.The electron configuration for an element (or ion) is
simply the number of electrons in each shell, separated by a full stop and reading from
the first shell outwards.
Element
Atomic number
Electron configuration
Hydrogen
1
1
Helium
2
2
Lithium
3
2.1
Carbon
6
2.4
Sodium
11
2.8.1
Silicon
14
2.8.4
Chlorine
17
2.8.7
Calcium
20
2.8.8.2
When drawing the electrons of a particular atom, the shells are depicted as concentric
circles around the nucleus, and the electrons as dashes, dots or “e” symbols.
For example, a hydrogen
atom with 1 electron
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Written Exercise: Draw structural models of the following elements:
Aluminium
Beryllium
Calcium
Nitrogen
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THE FIRST 20 ELEMENTS OF THE PERIODIC TABLE
Use a periodic table and your knowledge of electron shell structures to complete the
following table. Some information has been completed for you.
Element
Hydrogen
Symbol
Atomic
Number
Mass
Number
Neutron
Number
7
Beryllium
5
Boron
5
C
Nitrogen
2:4
7
Oxygen
316
Fluorine
10
Neon
10
Na
Magnesium
2:8:1
12
Aluminium
2+
27
Silicon
14
Phosphorous
15
Sulfur
2:8:6
Chlorine
Argon
Potassium
Calcium
Charge of
Ion
 valency
2
Lithium
Sodium
Electron
Configuration
H
Helium
Carbon
Electron
Number
1Ar
19
40
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