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.