History of Development
Johann Dobereiner – Law of Triads
- Nature contained triads of elements
- The middle element had properties that were an average of the other two members when ordered by the atomic weight (the Law of Triads).
John Newlands – Law of Octaves
Any given element will show similar behavior to the eighth element following it in the table.
Lothar Meyer – Meyer’s Curve
- Plotted the atomic volumes of the elements against the atomic weight, and found that the chemical properties of the element recur periodically.
- Also notice that the elements occupying the corresponding position of the curve show similar chemical properties.
Mendeleev – First Periodic Table
- arrange the elements according to the ascending order of atomic mass and put all the elements that have same chemical properties in the same group.
- left empty space in the periodic table for elements that haven’t been discovery at that time.
- Although he arranged the elements in ascending order of the atomic mass, he changed the order if the chemical properties of the element did not match its group.
- From the empty space present in his table, he even predicted the existence and properties of unknown elements which he called eka-aluminum, eka-boron, and eka-silicon.
H.J.G. Moseley – Modernized the Periodic Table by Arranging The Elements According to the Ascending Order of Atomic Number (Proton Number)
- managed to measure the proton number of atoms.
- arranged the elements in the periodic table according to the ascending order of the atomic number (proton number), but not the atomic mass, as done by Mendeleev.
- managed to predict the existence of four undiscovered elements from the proton number.
Modern Periodic Table
- In modern periodic table, the elements are arranged in ascending order of proton number.
- The vertical columns of the periodic table is called GROUPs whereas the horizontal rows is called the PERIODs.
- The vertical columns of the periodic table are called Group.
- There are 18 groups in the periodic table.
- All the elements in the same group have same number of valence electron.
- Thus, elements in the same group exhibit similar chemical properties.
- The Group is named according to their position in the periodic table. For example, the first group is called Group 1, the second group is called Group 2 and so on.
- Group 3 to Group 12 are categorised to another big group, called the Transition Metal Group.
- Group 1, 2, 17 and 18 have trivial name, as shown in the table below.
|Group 1||The Alkali Metals|
|Group 2||The Earth Alkaline Metals|
|Group 17||The Halogens|
|Group 18||The Noble Gases|
- The horizontal rows is called the PERIOD.
- There are 7 periods in period table.
- The first period only has 2 elements only.
- The second and third period consist of 8 elements, are called the short period.
- The forth and the fifth period consist of 18 elements, are called the long period.
- The sixth and the seventh period has 32 elements.
Periodic Table and Electrons Arrangement
- The classification of Group and Period are totally related to the electron arrangement of the atoms. As shown in the table above, the classification of elements in a periodic table is as follows:
- All the elements in a same group have equal number of valence electron(s) (outer most electron)
- All the elements in a same period have equal number of electron shell(s) (orbit).
- For example, the electron arrangement of calcium is 18.104.22.168. It has 2 valence electron and 4 electron shell. Therefore, it is placed in Group 2, period 4.
- Hydrogen, the simplest element atom, with 1 electron, does not fit into any group. (Even though some of the periodic table place it in Group 1, due to its 1 (and the only one electron) valence electron.
|Number of Valence Electron in a Group|
Number of Valence Electron