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 atomsWorksheet

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 and Ne22

10 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