Chemistry
Elements and
Periodic Table of Elements
The Basics
• Element = all the atoms are the same (JC)
• Element – cannot be broken down chemically into simpler substances
• (nuclear reactions are a different story!)
How do we know what’s an element?
• Periodic Table of ……
Elements
• Need to know
• symbols of elements 1- 36
• C • is for Carbon
• but – watch out for ………
• Potassium is K
• Phosphorus is P
• learn the rest as we go through the course
Be careful
• Symbol
• can only have one capital letter
• Na
• not NA• not NA
History
• Greeks – 4 elements – were they right?
• Boyle – (2000 ish years later!) – better idea of element – can’t be broken down into simpler substance
Davy
• used electricity to break down compounds into their elements
(JC – you saw water broken into H and O)
• he isolated Na, K, Ba, Sr, Ca, Mg
Moseley
• used x-rays
• showed that the nucleus had positive charge
• each element had different positive charge on its nucleus
• Element …….
• all elements have the same atomic number (no. of protons
Periodic Table
• Vertical Lines are called
Groups
• Horizontal Lines are called
Periods
• Metals on left , non- metals in right
• division begins at Boron
• (check with Aluminium Al)
Same number of electrons in outside
shell
Same number of shells
Learning Check
• Do I know….
(LC) definition of an element?
4 steps in how idea of element developed?
Where is a group on PTE?
What a group tells you?
Where is a period on PTE?
What this tells you?
Correct way to write the symbols ?
Periodic Table History
Dobereiner
• He put elements with similar chemical properties together
• They went into in groups of 3
• He noticed these trends in groups of elements such as:
Can you name these
elements ?
Atomic Weight of “middle” element
is average of other two –
approx.
Dobereiner’s Law of Triads
• A triad is a group of three elements • with similar chemical properties in which • the atomic mass of the middle element is
approximately equal to the average of the other two.
• Only worked for very few elements of the 50 or so that were known at the time
• BUT • He was first to make a link between atomic weight
and properties
John Newland
• Arranged the 60 known elements in order of increasing atomic weight
• His pattern was…
• Every 8th element was a repeat
(i.e. had 7 groups)
• His Law was called…
Law of Octaves
• An octave is a group of elements arranged in order of increasing atomic weight, in which the first and the eighth element of each group have similar properties.
• The properties repeat every 8 as noble gases hadn’t been discovered yet!
• Only worked for 17 out of 60 elements• There were several problems such as iron being
grouped with oxygen and sulphur. • Laughed at but was basically correct
Mendeleev
• Arranged the known elements in order of increasing atomic weight
• His Law was called…
Mendeleev’s Periodic Law:
• When elements are arranged in order of increasing atomic weight (relative atomic mass), the properties of the elements vary periodically.
Mendeleev:
• Put elements with the same properties in the same vertical group.
• Reversed the order of some elements (Te/I) so that their properties matched their group.
• Left gaps to make the elements fit into the proper column (group).
• Predicted that elements (eg. Germanium and Gallium) would be discovered to fill these gaps. Predicted their properties correctly.
Mendeleev’s Modern
Periodic Table V Periodic Table • Gaps left for
undiscovered elements• Group 0 elements
missing• Elements discovered
since 1869 missing• D-block elements
arranged as subgroups beside each main group
• Elements arranged in order of increasing atomic weight in almost all cases.
• Gaps have been filled• Group 0 elements
included• Elements discovered
since 1869 included• D-block elements
arranged as subgroups in a separate block
• Elements arranged in order of increasing atomic number
Mendeleev’s table
Moseley – Atomic Number
• The atomic number of an atom is the number of protons in the nucleus of that atom.
• Moseley used x-rays to find out how much positive charge each nucleus had
• In other words the difference between the
elements is the number of protons in the nucleus.
• Once the atomic number was known it was seen that Mendeleev’s table was in order of increasing atomic number – not weight.
Modern Periodic Table
• In order of increasing atomic number.
• There were 63 elements in Mendeleev’s table – now 109
• No gaps
• The transition elements are listed separately.
• Today’s table 1940 – Glenn Seaborg
Do I know it???
• Who proposed Law of Triads?• Who proposed Law of Octaves?• Who proposed Periodic Law?• Can I explain each ?• Can I name 3 triads?• What was Moseley’s contribution to the Periodic
Table development?• Four differences between Mendeleev’s Table
and modern table?
• Group 1
• Form +1 ions
• Very reactive
• Low density
• Soft-can be cut with a knife
• Burn in air to form oxides
• React with water to form hydroxides
Alkali metals
4 Groups Periodic Table
• Group One – Alkali metals
• Group Two– Alkaline earth metals
• Group 7– Halogens
• Group 8 (O)Noble Gases
Alkaline Metals
Demonstration of the reaction with water of lithium, sodiumand potassium.
Sodium
• Sodium + Oxygen Sodium oxide
• Sodium + Water Sodium Hydroxide + Hydrogen gas
• Stored in oil to prevent them from reacting with air or water
Group 2 Alkaline Earths
• Group 2
• Reactive• ions with a +2 Mg +2
• Harder than group one
• Burn in air to form oxide
• React with water to form hydroxides
Beryllium
Barium
calcium
Magnesium
Strontium
Radium
Halogens
• Group 7
• Form -1 ions Cl-1
• Reactive
• Coloured
• Chlorine, Bromine, Iodine
• React with hydrogen to form acids
Chlorine
• Green gas
• Chlorine + Hydrogen Hydrogen Chloride gas
• Hydrogen chloride gas dissolves in water to form Hydrochloric acid.
• Chlorine reacts with sodium to form Sodium Chloride [Table Salt]
bromine
iodine
Group 18: Noble Gases
Group VIII (group 0) of the periodic tableHelium and neonused to make lighted signs, refrigerants, and lasersDon’t form ionsnot reactive - because they have little tendency to gain or lose electrons.
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