The Periodic Table
- In I/GCSE Chemistry, the periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number (number of protons), electron configurations, and recurring chemical properties. This ordering shows periodic trends, such as elements with similar behavior in the same column.
- It is called the periodic table of elements because elements with similar properties occur at regular intervals.
- The elements are arranged in order of atomic number (proton number) so that very similar elements end up in vertical columns called groups AND they have the same number of outer electrons (except H and He) which gives them very similar chemical properties.
- Most elements are metals on the left and lower parts of the periodic table. Non-metals are on the right and towards the top of the periodic table. The thick diagonal zig–zag black line shows the main division between metals on the left of the periodic table and non–metals on the right of the periodic table. However, note that the metallic elements and non–metallic elements adjacent to this zig–zag line can show a 'mixture' of properties.
Dmitri Mendeleev's Periodic Table
In I/GCSE Chemistry, what is Dmitri Mendeleev's periodic table?
Mendeleev arranged the elements, known at that time, in a periodic table by using properties of these elements and their compounds. Mendeleev used his table to predict the existence and properties of some elements not then discovered. However, Mendeleev had arranged elements in order of increasing relative atomic mass but this didn't always give the correct order in the periodic table because of the relative abundance of isotopes of some pairs of elements in the periodic table.
- Mendeleev laid out all the known elements in order of 'atomic weight' (what we now call relative atomic mass) except for several examples like tellurium (Te) and iodine (I) whose order he reversed because chemically they seemed to be in the wrong vertical column.
- He arranged similar elements into vertical columns (we now call groups) and switched to the next row (we now call periods) when the next similar element appeared when laid out in atomic number (left to right and then down).
- He wrote out all the known properties of at least 50 elements and looked for deeper and more meaningful patterns than his predecessors like Newland.
- He recognized that even more elements could be fitted into groups of elements with similar properties, and these he arranged as vertical columns, which we now call groups (of the periodic table).
In Mendeleev's periodic table the Group 1 metals (Li, Na, K, Rb) and Group 2 metals (Be, Mg, Ca, Sr) are all in place in columns 1 and 2, three of group 3 (B, Al, In), three of group 4 (C, Si, Sn) in place, 4 of group 5 (N, P, As, Sb), 4 of group 6 (O, S, Se, Te), 4 of group 7 (F, Cl, Br, I), but his group 8 (where group 0 Noble Gases are) are metals from the first and second transition metals series.
He predicted the existence of gallium (in group 3) and germanium (he called it ekasilicon, because he expected to have much in common with silicon in Group 4).
Therefore, where he thought elements were missing, he not only predicted their existence, he also predicted some of the physical properties like melting point and density, and chemical properties such as the formula of the oxide.
Similar properties would involve physical properties like melting point, boiling point, density, appearance and chemical properties would include formula of compounds like oxides, chlorides etc.
He left gaps between elements, he reasoned from the patterns of physical and chemical properties of similar elements of a 'group' that there must be one or more elements missing from the table.
That's all!
