ZON PENA

History of Periodic Table

Antoine Lavoisier

Antoine Lavoisier

• Antoine Lavoisier is believed to be the first scientist who classify elements into a few groups.
• The following table shows the classification of elements (Including light and heat) given by Antoine Lavoisier.
• In his table, he differentiated metal and non-metal.
• He had mistakenly classified light and heat as matter and baryta (barium hydroxide), lime and silica as element.

Johann Dobereiner--Law of Triads

Johann Dobereiner

• In 1817 Johann Dobereiner noticed that the atomic weight of strontium is equal to the average atomic weight of calcium and barium, and these 3 elements possessing similar chemical properties.
• He grouped these 3 elements together and name it as a triad.
• In 1829, he discoverd 2 more triads: the halogen triad composed of chlorine, bromine, and iodine and the alkali metal triad of lithium, sodium and potassium.
• After this discovery, he proposed that nature contained triads of elements, and named this as the Law of Triads.
• According to the Law of Triads, the atomic mass of the middle element is equals to the average of the other two members and all 3 elements show similar chemical properties.
• The table below shows the example of triads found by Dobereiner.

John Newlands-Law of Octaves

• In 1864, John Newlands arranged the elements according to the ascending order of their atomic mass, and managed to publish his version of periodic table.
• He grouped the elements together base on their chemical properties and found that the chemical properties of element changes when the atomic mass increases but similar chemical properties will repeat every 7 elements.
• For example, he found that the chemical properties of the 8th element is equals to the 1st element, and the chemical properties of the 9th element is equals to the 2nd.
• Base on this discovery, he proposed the Law of Octaves (by analogy with the seven intervals of the musical scale).
• This law stated that any given element will exhibit similar chemical properties to the eighth element following it in the table.

Lothar Meyer-Meyer’s Curve

Lothar Meyer

• Meyer estimated the volume of atom by using an equation .
• He plotted the atomic volumes of the elements against the atomic weight, and found that the chemical properties of the element recur periodically.
• He also notice that the elements occupying the corresponding position of the curve show similar chemical properties.
• For example, all the elements located at the peak of the graph (Lithium, sodium and potassium)show similar chemical properties.
• In 1868, Meyer constructed a periodic table which he gave to a colleague for evaluation. (This may be consider as the first but not published periodic table.)
• Unfortunately for Meyer, Mendeleev's table became available to the scientific community via publication (1869) before Meyer's appeared (1870).

An introduction to Lothar Meyer's Atomic Volume Curve Julius Lothar Meyer – the first identifier of periodicity? More detail discussion on Meyer's Periodic table

Mendeleev-First Periodic Table

Mendeleev

• Mendeleev published his periodic table & law in 1869.
• He 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.
• He 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.
• Later when elements gallium, scandium and germanium were found, scientist found that Mendeleev's prediction was excellently accurate.
• Comparison of Mendeleev's prediction of eka-aluminium with later found galium is shown in the table below.

H.J.G. Moseley-Modernized the Periodic Table

Henry Moseley

• Moseley managed to measure the proton number of atoms.
• He then 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.
• This correction was considered as a very important step in modernisation of periodic table.

He managed to predict the existence of four undiscovered elements from the proton number.

Modern Periodic Table

• The idea of the Periodic Table is to arrange the elements in a way that enables chemists to understand patterns in the properties of the elements.
• According to www.webelements.com, there are 118 known elements now.
• Part of it exist in the nature, the others were synthetic elements.
• In modern periodic table, the elements are arranged in ascending order of Atomic(proton) Number.
• The vertical columns of the periodic table is called GROUP whereas the horizontal rows is called the PERIOD.

Group

• The vertical columns of the periodic table is called Group.
• There are 18 groups in the periodic table.
• All the elements in the same group have same number of valence electron.
• Since the chemical properties of an element is determined by its electron arrangement, particularly the number of valence electron(s). Thus, elements in the same group exhibit similar chemical properties.
• Nevertheless, the reactivity of the elements in the same group changes when move down the group.
• The elements in most of the groups (but not all) also exhibit similar physical properties, and the physical properties varies gradually down the group.

Name of the Group

• 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.

Period

• 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.

Lanthanide and Actinide Series

• The sixth period has 32 elements. Due to short of space, 14 elements in the transition metal group are removed from the same horizontal row and is placed below the main table, These elements are called the Lanthanide Series.
• The seven period also has 32 elements. With the same reason, 14 elements are removed from the same horizontal row and is placed below the main table, These elements are called the Actinide Series.

Periodic Table and Electronic Configuration

• The similarities and differences in the chemical properties of elements can be explained by the electron arrangement of the atoms.
• 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:

1. All the elements in a same group have equal number of valence electron(s) (outer most electron)
2. All the elements in a same period have equal number of electron shell(s) (orbit).

• Table below shows the number of valence electron in a different group. In the SPM syllabus, we ignore the arrangement of electron of the transition metal.
• For example, the electron arrangement of calcium is 2.8.8.2. 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.