C3aL1  Classifying Elements

Key Words

Atom - smallest particle an element can be broken down into

Atomic Mass - the mass of the atom.

Element  - a substance consisting of one kind of atom only.

Group - a family of elements that has similar chemical properties.

Halogens - Group 7, consisting of fluorine, chlorine, bromine, etc.

Period - The rows of the periodic table

Transition Elements - elements between Groups 2 and 3 in the periodic table

Transuranic elements - elements of atomic number 92 to 102.

Grade E

A number of Nineteenth Century chemists independently worked on the classification of elements:

• John Newlands;
• Lothar Meyer;
• Dmitri Mendeleev.

Some earlier attempts at classification were made by Antoine Lavoisier (whose work was prematurely stopped by an appointment with the guillotine), and John Dalton.

Newlands set out the elements in increasing order of atomic mass, and noticed a periodicity or repetition of properties, every eight elements or so.  The table was called Newlands' Octaves.

Mendeleev and Meyer worked independently.  The elements that had similar properties were placed into columns called Groups.  Mendeleev published his work a year before Meyer, so his name is remember, but both chemists' tables were very similar and formed the basis of the periodic table today.

Grade C

Mendeleev's table had Groups of elements with similar properties.  For example, in Group I there were:

• Hydrogen;
• Lithium;
• Sodium;
• Potassium;
• Copper;
• Rubidium;
• Silver.

In Group VII (note the Roman Numerals that are always used to number groups), there were:

• Fluorine;
• Chlorine;
• Manganese;
• Bromine;
• Something else not known;
• Iodine.

The elements in italics indicate elements that shared the same cell of the table (lucky things).

We now know  these two groups as the alkali metals and the halogens.

Notice that there seem to be some very odd bed-fellows.  Manganese is a metal, while the other halogens are gases.  Copper does not fit in at all well with the Group I metals; it is very unreactive, while the other alkali metals are very reactive.  Silver is also out of place, although it forms single positive ions.  Both silver and copper are transition elements.

Grade A

Mendeleev's Table showed other oddities like Iron, Cobalt, and Nickel as being in Group VIII.  They bear no resemblance whatever to the Noble gases, but these had not yet been discovered.  Gaps of undiscovered elements also existed.

We now know that many elements fit into a class called the transition elements, placed between Groups II and III.

In the 1940s, the transuranic elements (atomic numbers 94 to 102) were proposed and discovered by Glen Seaborg who had discovered plutonium.  All of these are radioactive, heavy metals.