2.5Chapter 2: Elements, Compounds and Mixtures

Organising the elements — Alhussan Chemistry - Stage 7 Revision Notes & Topical Questions

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2.5 Organising the elements — Groups, Periods and Mendeleev — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe how the periodic table is arranged and how Mendeleev and other scientists developed it.Order — the modern table arranges the elements in order of …

Metals and non-metals
Special groups
Outer electrons
John Dalton
Stanislao Cannizzaro
Arrangement

Part 1 of 6Layout of the periodic table

Layout of the periodic table

Order

Order the modern table arranges the elements in order of increasing atomic number.

Groups

Groups vertical columns, and elements in the same group have similar chemical properties.
Group 1 metals (lithium, sodium, potassium) all react with water to give an alkaline solution and hydrogen.

Periods

Periods horizontal rows, and elements in the same period have the same number of electron shells.

Metals and non-metals

Metals and non-metals metals are on the left and in the centre, non-metals are on the right, and a stepped line separates them.

Special groups

Group 0 (or 18), the noble gases, are very unreactive, and Group 7, the halogens, are reactive non-metals.

Outer electrons

Outer electrons the number of outer-shell electrons equals the group number for Groups 1 to 7,
- which explains why elements in a group behave similarly.

Part 2 of 6Building up knowledge

By 1860

By 1860 scientists had discovered around 60 elements and knew some of their properties, but the knowledge was a jumble of facts.
They asked what patterns exist in properties, how many more elements there are, and whether patterns could help find new elements.

John Dalton

John Dalton in the early 1800s he studied experimental evidence and suggested that atoms of each element have a different mass.

Stanislao Cannizzaro

Stanislao Cannizzaro this Italian scientist worked out the masses and in 1860 gave data on the atomic masses of known elements at a conference in Germany.

Mendeleev

Mendeleev the Russian scientist Dmitri Mendeleev picked up a copy of the data.

Other scientists

Other scientists several tried to use the mass data to show patterns in properties, but nobody found the perfect arrangement.

Part 3 of 6Mendeleev's periodic table

1869

Mendeleev wrote each element on a card with its atomic mass and properties and tried sorting the cards in different ways.

Arrangement

Arrangement he found one with the elements in atomic mass order and elements with similar properties grouped together.
Elements with similar properties repeat at intervals, which is why it is called periodic.
The first periodic table was written on the back of an envelope.

Gaps

Gaps he was confident enough to leave gaps for elements he predicted should exist but had not been discovered, and predicted their properties.

Swaps

Swaps he swapped the order of a few elements, such as tellurium and iodine, so they fitted the pattern of properties.

Part 4 of 6Finding the missing elements

1874

1874 a French scientist found the missing element beneath aluminium, named gallium, with properties as Mendeleev predicted.

Scandium and germanium

Swedish scientists soon discovered more of the missing elements, and their properties were also as predicted.

1925

1925 most gaps were filled, but there were spaces beneath manganese for two more elements.
Ida Tacke and Walter Noddack extracted 1 g of a new substance from 660 kg of rock, and called it rhenium.

Acceptance

Acceptance scientists accepted the table because it explained patterns in properties and its predictions turned out to be correct.

Date	Contribution
Early 1800s	Dalton: atoms of each element have a different mass
1860	Cannizzaro gives atomic mass data; about 60 elements known
1869	Mendeleev arranges elements by mass and leaves gaps
1874 onwards	Gallium, scandium and germanium discovered
1925	Tacke and Noddack discover rhenium

Part 5 of 6From Mendeleev to today

Atomic number

Atomic number later work on atomic structure showed the order should follow the number of protons, not atomic mass,
which explained why Mendeleev had to swap some elements.

Noble gases

Noble gases they were discovered after his time and fitted into a new group.

Feature	Mendeleev	Modern table
Order	Atomic mass	Atomic number
Gaps	Left for unknown elements	No gaps for elements up to 118
Noble gases	Not included	Group 0

Part 6 of 6Patterns in groups

Patterns in groups

Group 1

Group 1 soft metals that react with water, with reactivity increasing down the group,
so potassium reacts more vigorously than sodium and sodium more than lithium.

Group 7

Group 7 coloured non-metals, with reactivity decreasing down the group, so chlorine is more reactive than bromine.

Group 0

Group 0 all unreactive, so they are used in lamps and balloons.

Position

Position sodium is in Period 3, Group 1, so the period gives the row and the group gives the column.

Key terms

Term	Meaning
Group	A vertical column of the periodic table.
Period	A horizontal row of the periodic table.
Atomic number	The number of protons in an atom.
Atomic mass	The mass of an atom compared with others.
Prediction	A statement of what will be found, based on a pattern.
Noble gas	An unreactive element in Group 0.