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1. The Structure of Atoms - three fundamental particles
WHAT ARE ATOMS? and WHAT DO WE MEAN BY FUNDAMENTAL PARTICLES? (subatomic particles)
Atoms are the smallest particles of matter whose properties we study in Chemistry.
However from experiments done in the late 19th and early 20th century it was deduced that atoms
are made up of three fundamental or sub-atomic particles called protons, neutrons and
electrons, which are listed below with their relative masses and electrical charges.
Portrait picture of an atom
However this diagram, although more realistic in terms of the real size of the nucleus
compared to the atom as a whole, it is not convenient to give a brief diagrammatic picture
of the composition of an atom.
Some more concise and handy styles to show the atomic composition of the same lithium
atom.
WHAT ARE THE CHARACTERISTIC PROPERTIES OF THESE SUB-ATOMIC PARTICLES?
WHAT IS THE NUCLEUS? WHAT ARE NUCLEONS?
The three fundamental particles of which atoms are composed
The table gives the relative mass and electric charge of the three sub-atomic particles
known as the proton, neutron and electron.
What can we say about 'A Portrait of an Atom'? - an image of what you can't see!
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What sub-atomic particles make up atoms? What is their mass and charge?
The diagram below 'portrait of an atom' above, gives some idea on the structure of an
atom (sometimes called the Bohr Atomic Model), it also includes some important
definitions and notation used to describe atomic structure
o The three fundamental particles you need to know are
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o
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proton: mass = 1, charge = +1, the charged particle in the nucleus
neutron: mass = 1, charge = 0, uncharged particle in the nucleus
electron: mass = 1/1850 ~1/2000, charge = -1, NOT in nucleus but exist in an
electronic energy level around the nucleus.
o Incidentally, must of the volume of an atom is where the electrons are, the
nucleus of protons plus neutrons is on about a ten thousandth of the diameter of
an atom.
Protons and neutrons are the 'nucleons' or 'sub-atomic' particles present in the
minute positive nucleus and the negative electrons are held by the positive
protons in 'orbits' called energy levels or shells.
Some important evidence for this 'picture' is obtained from alpha particle scattering
experiments (see Appendix 1).
The atomic/proton number (Z) is the number of protons in the nucleus and is also
known as the proton number of the particular element.
It is the proton/atomic number (Z) that determines the number of electrons an
element has, its specific electron structure and therefore the specific identity of a
particular element in terms of its physical and chemical properties.
It cannot be overemphasised that it is the electronic structure that determines the
chemical character of an element, hence the proton/atomic number determines
everything about a particular element
The mass number (A) is also known as the nucleon number, that is the number of
particles in the nucleus of a particular atom-isotope (notes on isotopes - definition
and examples).
o Therefore the mass/nucleon number = sum of the protons plus neutrons in
the nucleus.
The neutron number (N) = mass number - proton/atomic number
In a neutral atom the number of protons (+) equals the number of electrons (-), that
is the number of positive charges is equal to the number of negative charges.
o If not, and the atom has an overall surplus electrical charge, and is then called an
ion e.g.
 the positive sodium ion Na+ (11 protons, 10 electrons, excess positive
protons),
 or the negative chloride ion Cl- (17 protons, 18 electrons, excess
negative electrons,
 for more details and examples see ionic bonding notes.
Other alternative atomic structure diagrams than the one below are described and
shown in Appendix 2.
In the example below for lithium-7, the nuclide notation states that
o before the chemical symbol of the element Li
o the top left number = nucleon/mass number = 7
o and the bottom left number = proton/atomic number = 3
The electrons are arranged in specific energy levels according to a set of rules
(dealt with in section 3).
This description of an atom consisting of the relatively minute nucleus of protons and
neutrons surrounded by electrons in particular shells or energy levels is sometimes
referred to as the Bohr Model of the atom, after the great Danish scientist Niels Bohr
(1885-1962), one of the brilliant founders of modern atomic theory.
Other examples of interpreting the nuclide notation and definition reminders:
o Top left number is the nucleon number or mass number (A = sum of protons +
neutrons = nucleons)
o Bottom left number is the atomic number or proton number (Z = protons in
nucleus)
o Electrons = protons if the atom is electrically neutral i.e. NOT an ion.
o The neutron number N = A - Z i.e. mass/nucleon number - atomic/proton
number
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Iron atom (iron-56), 26 protons, 30 neutrons (56 - 26), 26
electrons
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Cobalt atom (cobalt-59), 27 protons, 32 neutrons (59 - 27), 27
electrons
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Californium atom (californium-98), 98 protons, 148 neutrons
(246 - 98), 98 electrons
2. ISOTOPES
WHAT ARE ISOTOPES? ARE THEY IMPORTANT?
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Isotopes are atoms of the same element with different numbers of neutrons and
therefore different masses (different nucleon/mass numbers). This gives each isotope
of a particular element a different mass or nucleon number, but, being the same element
they have the same atomic or proton number and are identical chemically.
The phrase 'heavier' or 'lighter' isotope means 'bigger' or 'smaller' mass number.
There are small physical differences between the isotopes e.g. the heavier isotope has a
greater density or boiling point, the lighter the isotope the faster it diffuses.
However, because they have the same number of protons (proton/atomic number)
isotopes of a particular element have the same electronic structure and identical
chemistry.
Examples of isotopes are illustrated and described below.
Caution Note: Do NOT assume the word isotope means the atom it is radioactive,
this depends on the stability of the nucleus i.e. unstable atoms (radioactive) might be
referred to as radioisotopes.
Many isotopes are extremely stable in the nuclear sense and NOT radioactive i.e. most of
the atoms that make up you and the world around you!
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and
are the three isotopes of hydrogen with mass numbers of 1, 2
and 3, with 0, 1 and 2 neutrons respectively. All have 1 proton, since all are hydrogen!
Hydrogen-1 is the most common, there is a trace of hydrogen-2 (sometimes called
deuterium) naturally but hydrogen-3 (sometime called tritium) is very unstable and is
used in atomic bombs - nuclear fusion weapons.
o They are sometimes denoted more simply as 1H, 2H and 3H since the chemical
symbol H means hydrogen and therefore must have only one proton.
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and
or 3He and 4He, are the two isotopes of helium with mass numbers
of 3 and 4, with 1 and 2 neutrons respectively but both have 2 protons. Helium-3 is
formed in the Sun by the initial nuclear fusion process. Helium-4 is also formed in the Sun
and as a product of radioactive alpha decay of an unstable nucleus.
o An alpha particle is a helium nucleus (mass 4, charge +2) and if it picks up two
electrons it becomes a stable atoms of the gas helium. For more details see
Radioactivity Revision Notes Part 4
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and
or 23Na and 24Na, are the two isotopes of sodium with mass
numbers of 23 and 24, with 12 and 13 neutrons respectively but both have 11 protons.
Sodium-23 is quite stable e.g. in common salt (NaCl, sodium chloride) but sodium-24 is a
radio-isotope and is a gamma emitter used in medicine as a radioactive tracer e.g. to
examine organs and the blood system.
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and
are the two nuclear symbols for the two most common and stable
isotopes of the element chlorine. They both have 17 protons in the nucleus and 35-17 =
18 and 37-17 = 20 neutrons respectively.
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and
are the two nuclide symbols for the two most common and stable
isotopes of the element bromine. They both have 35 protons in the nucleus and 79-35 =
44 and 81-35 = 46 neutrons respectively. By coincidence, there are almost exactly 50%
of each isotope present in naturally occurring bromine.
The relative atomic mass of an element is the average mass of all the isotopes
present compared to 1/12th of the mass of carbon-12 atom (12C = 12.00000 i.e. the
standard).
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DENSITY
What is density?
Density is defined as mass per unit volume. In another word, density of any
substance is given by the following density formula.
What is the density formula?
or D equals m divided by V where D stands for Density, m stands for mass, and V
stands for Volume.
What is the unit of density?
The density of a solid is measured in grams per cubic centimeter or g per cm 3
( g/cm 3) because the mass of a solid is measured in grams and volume is measured
in cubic centimeter.
The density of a liquid, however, is often measured in grams per milliliter or g per
mL or (g/mL) because the mass of a liquid is still measured in grams but the volume
of a liquid is measured in milliliter.
That's it! If you want to find the density of any substance, just divide its mass in
grams by its volume in cubic centimeter or millimeter.