Transcript The periodic table

The periodic table
Tim Lehrer
Elements
• An element is a
substance that cannot be
split into simpler
substances by chemical
means
• Robert Boyle from
Lismore in Waterford was
the first man to give a
reasonably adequate
description of an element!
Discovering Elements
• An English Chemist named Humphry Davy
made some of the most important
discoveries in the history of the elements
• After finding that passing electricity
through water split it into Hydrogen and
oxygen he decided to pass electricity
through other compounds and discovered
many elements in this way
• He passed electricity
through molten potassium
hydroxide and isolate
explosive potassium
• The use of electric
current to isolate
elements was a turning
point in chemistry after
1807 many new elements
were discovered
Looking for a pattern
• By 1800 50 elements had been discovered
it was becoming increasingly difficult to
remember long lists of names and
properties some order was needed!
• Examples of properties
4 leading Chemsits
•
•
•
•
•
Johann Dobereiner
John Newlands
Dmitri Mendeleev
Henry Mosely
The work of these men led to the
development of the periodic table of
elements
Dobereiner’s Triads
• 1829 Dobereiner
noticed that bromine
seemed halfway in its
properties between
chlorine and iodine
• He looked at their
atomic weights and
noticed that bromine’s
atomic weight was
halfway between
iodine and chlorine
• He observed similar patterns between
calcium, strontium and barium and also
sulphur , selenium and tellurium
• He called each of these groups of 3 a
triad
• A triad is a group of 3 elements with similar
chemical properties in which the atomic weight
of the middle element is approximately equal to
the average of the other 2
• Because other triads were not detected much of
his work was viewed as coincidence however we
now know it was a forerunner to the periodic
table
Newland’s Octaves
• 1864 John Newlands
arranged elements in
order of atomic weight
• He noticed properties
of the elements
seemed to repeat
after every eight
element
• He stated this in his
law of octaves
Law of Octaves
• Newalnds octaves are groups of elements
arranged in order of increasing atomic
weight, in which the first and eight element
of each group have similar properties
• For example, starting at Li, Be is the
second element, B is the third and Na is
the eighth element.
Problems with the octaves
• Newlands' table showed a repeating or
periodic pattern of properties, but it had
problems. For example, he put iron in the
same group as oxygen and sulphur, which
are two non-metals. As a result, his table
was not accepted by other scientists
• Newland’s did not realise that other
elements had not been discovered and
tried to force all the known elements in
• Another problem was that the noble gases had
not been discovered and when we count these
we know the repetition in properties actually
occurs every 9th element
• Newlands was treated very badly by his fellow
scientists but 20 years later was awarded a
medal when it was found his ideas were
basically correct
Mendeleev’s Periodic Table
• In 1869, just five years after
John Newlands put forward
his Law of Octaves, a
Russian chemist called Dmitri
Mendeleev published a
periodic table.
• Mendeleev also arranged the
elements known at the time
in order of relative atomic
mass, but he did some other
things that made his table
much more successful
• He realised that the physical and chemical
properties of elements were related to
their atomic mass in a 'periodic' way, and
arranged them so that groups of elements
with similar properties fell into vertical
columns in his table.
Mendeleev’s periodic law
• When elements are arranged in order of
increasing atomic weight their properties
vary periodically
Differences between Mendeleev’s
table and Newland’s table
• Mendeleev left gaps in his table to make
the elements fit the proper column
• Instead of seeing this as a problem,
Mendeleev thought it simply meant that
the elements which belonged in the gaps
had not yet been discovered
• He even made predictions about the
properties of these elements and when
they were discovered he was right!
• For example, he predicted the properties
of an undiscovered element that should fit
below aluminium in his table. When this
element, called gallium, was discovered in
1875 its properties were found to be close
to Mendeleev's predictions. Two other
predicted elements were later discovered,
lending further credit to Mendeleev's table.
• Sometimes he reversed the order of the
elements if he felt it was right
• For example we can see that iodine has a lower
atomic weight than tellurium strictly speaking
they should be the other way round
• However iodine’s properties are more similar to
chlorine and bromine so Mendeleev prioritised
the properties and put iodine in the group he felt
it belonged in
Moseley – Atomic Number
• Mendeleev did not understand
why he had to reverse some
elements despite their atomic
number
• Moseley used x-rays emitted
by atoms of various elements
to determine how much
positive charge (thus how
many protons) were in the
nucleus
Atomic Number
• The atomic number of an atom is the
number of protons in the nucleus of that
atom
• Because atomic number is different to
atomic mass Moseley was able to show
that Mendeleev’s table did increase in
order of increasing atomic number
• Using atomic number iodine does come
before tellurium , in fact all the elements
fall naturally into the right group using this
method
The modern periodic table
• The modern periodic table is the
arrangement of elements in order of
increasing atomic number
• New Periodic law
• When elements are arranged in order of
increasing atomic number, the properties
of the elements vary periodically
• Moseley’s work confirmed Mendeleev’s
table
• Poor Moseley was sent to Gallipoli in 1915
and died at the age of 27 missing out on a
nobel prize
Differences between Mendeleev’s
table and the modern table
1. Modern table is arranged in order of increasing
atomic number not atomic mass
2. There are more elements in the modern table
3. There are no gaps in the modern table
4. The transition elements are listed in a separate
block in the modern table they were included in
with other elements in Mendeleev's table
How to use the periodic tale
• Obtain mass numbers and atomic
numbers
• Obtain relative atomic masses
• Write electron configurations
• We have seen the atomic number is the
number of protons in the nucleus
• The mass number of an element is the
sum of protons and neutrons in the
nucleus
• Sodium has 11 protons and 12 neutrons
so its mass number is 23
Find the number of neutrons in the
first 36 elements
• You are required to know the symbols of
the first 36 elements for your leaving
certificate exam
• In the periodic table in the mathematical
tables you can usually get the mass
number by rounding off the relative atomic
mass number to the nearest whole
number
See if you can tell the number of
protons, neutrons and electrons
in the following atoms
Worksheet
Atomic numbers and mass
numbers
• Mass number is the sum of the number of
protons and neutrons in the nucleus of an atom
of an element
• Because they are so small they are not
measured in grams!
• A special unit called the atomic mass unit or
a.m.u or just u
• This can be got by rounding off the relative
atomic mass number which is under the atom in
the periodic table
• An instrument called the mass
spectrometer can measure the mass of an
atom
• To learn how this works lets read the
handout and look at this diagram
The Mass Spectrometer
• When trying to find the mass of neon
atoms Francis William Aston found
something very interesting
• There seemed to be two types of neon
atom
• One had a mass number of 20 the other
had a mass of 22!
• He concluded that one type of neon atom
had 10 neutrons in the nucleus while the
other had 12
Ne20
10
and
Ne22
10
These two varieties are called ISOTOPES
Isotopes
• Isotopes are atoms of the same element
(they have the same atomic number or the
same number of protons) but different
mass numbers due to different amounts of
neutrons in the nucleus
• Other examples of isotopes are Carbon
and chlorine see p31 above fig 4.2
Relative atomic mass
• The average of the mass numbers of the
isotopes of the element
• As they occur naturally
• Taking their abundances into account
• Expressed on a scale in which the atom of the
carbon 12 isotope have a mass of exactly 12
units
• NB Relative atomic mass is written as Ar and
has no units
Try the following
• Read example 4.2 and try part l p 42
Writing Electron configurations