Historical Development of the Periodic Table

5.1 INTRODUCTION

 

                As scientists were trying to unravel the nature of matter through their studies on the subatomic particles and the Quantum Theory, the search for new compounds still continue. From ancient times to 1899, 83 elements were already discovered. These elements needed to be arranged in such a way that they can be easily studied. Unlike the periodic table shown above, the elements were not organized and were difficult to study, thus, paved the way to the development of the Modern Periodic Table of Elements.

The modern periodic table is a chart where the elements are arranged according to increasing atomic number and are grouped according to similar physical and chemical properties. The physical and chemical properties of elements can be observed according to columns called Groups or Families or through rows called Periods.

 

o   Group – column of elements that have similar properties

o   Period – row of elements that tells us the valence shell of the element

Periods are numbered 1 to 7 while Groups are numbered 1 to 8, classified as either A or B. Groups are labeled in two ways: the number-letter symbol, which is commonly used by chemists, and the Group numbers from 1 to 18. For convenience, groups are also given special names that depict their physical and chemical properties, which will be discussed later in the chapter.

Elements are classified in to three types: Metals, Metalloids (semimetals), and Nonmetals.

o   Metals -  are good conductors of heat and electricity

- elements on the left side of the “staircase” on the periodic table

o   Nonmetals – are poor conductors of heat and electricity

   - elements on the right side of the “staircase” on the periodic table

o   Metalloids – elements having intermediate properties of metals and

nonmetals

Some of the elements classified as metalloids are Boron, Silicon, Germanium, Arsenic, Antimony, and Tellurium. In some books, Polonium and Astatine are considered as metalloids but are still under debate. Metalloids have valuable importance in the computer industry. Some metalloids do not conduct electricity at normal conditions, but when certain conditions are met like temperature or amount of electricity, they allow the flow of electricity, making them valuable components of the circuitry in computers.

5.2 HISTORICAL DEVELOPMENT OF THE PERIODIC TABLE

Since the age of antiquity, many elements were already known to man, some of which are gold, lead, silver, copper, tin, iron, and mercury. The first scientific discovery of an element was done by a German merchant Hennig Brand who discovered phosphorus by distilling human urine. This was an accidental discovery because he was actually looking for the Philosopher’s Stone, the mythical substance that could turn anything into gold, to save him from his bankruptcy. After that, many more scientists followed in discovering new elements.

The earliest attempt to arrange the elements was made by Johann Wolfgang Döbereiner, a German chemist who proposed the Law of Triads.

o   Law of Triads – a group of elements having the same properties where thesecond element of the triad has almost exactly the average atomic mass of the first and third.

                Below are examples of triads given by Döbereiner:

Element	Atomic Mass	Density
Chlorine Bromine Iodine	35.45 g/mol 79.90 g/mol 126.90 g/mol	0.0032 g/ml 3.1028 g/ml 4.933 g/ml
Calcium Strontium Barium	40.09 g/mol 87.62 g/mol 137.33 g/mol	1.55 g/ml 2.54 g/ml 3.59 g/ml

                After Döbereiner’s attempt, another scientist in the name of John Newlands proposed the Law of Octaves.

o   Law of Octaves – when elements are arranged in increasing atomic mass, the physical and chemical properties of elements tend to repeat after every eighth element or after an interval of 7 elements.

John Newlands likened his arrangement of the elements to the octaves in music as shown below:

1	2	3	4	5	6	7	8
do	re	mi	fa	sol	la	ti	do
Li	Be	B	C	N	O	F	Na
Na	Mg	Al	Si	P	S	Cl	K

Lithium in the “lower do” and Sodium in the “higher do” have the same physical and chemical properties. The same rule applies in the “lower notes” in Newlands’ arrangement. The problem with this arrangement was that the pattern was no longer applicable to elements beyond Calcium.

 

 As you can observe from his arrangement, after Fluorine and Chlorine, the elements Sodium and Potassium followed immediately. This is because during those times, scientists haven’t included the noble gases yet to the periodic table as a new class of elements. Newlands made his Law of Octaves in 1865 while Lord Rayleigh and William Ramsay proved the existence of the noble gases and got it included in Mendeleev’s periodic table in 1902.

Newlands’ Law of Octaves received negative feedbacks from his colleagues for likening his arrangement of the elements to the musical scale but later came into importance when Gilbert Lewis proposed his Valence Bond Theory and Irving Langmuir’s  with his Octet Rule.

The most successful attempt to arrange the elements was done by Dmitri IvanovichMendeleev. He was a Russian chemist who, like others before him, arranged the elements according to increasing atomic mass. He grouped the elements based on their physical and chemical properties and made some predictions on the properties of some elements that were not yet discovered during those times. A contemporary of Mendeleev who had the same idea as his was the German chemistJulius Lothar Meyer. He arranged the elements the same way Mendeleev did. But what made Mendeleev’s arrangement different was that he left gaps in his table, leaving spaces for future elements to be discovered. One example is his prediction of an unknown element that has almost the same properties as aluminum. He called this element as eka-aluminum (Ea)(eka- from Sanskrit which means “first”, meaning that eka-aluminum is the first element beneath Aluminum).

	Eka-Aluminum	Gallium
Atomic Mass	68 amu	69.9 amu
Melting Point	Low	30.15oC
Formula of Oxide	Ea2O3	Ga2O3
Density	5.9 g/cm3	121

                Though they had virtually the same arrangement of elements, the larger credit was also given to Mendeleev because of him taking the risk of predicting new elements and publishing his work a few months before Meyer did.

                Further refinements were made in Mendeleev’s periodic table. He noticed that there were anomalies in the properties of elements if the basis of the arrangement is the atomic mass. An example would be Nickel and Cobalt:

                 In the modern periodic table, Cobalt comes first followed by Nickel. But if you will look at the atomic masses of the two elements, Cobalt is heavier than Nickel, therefore, in Mendeleev’s periodic table, he placed Nickel before Cobalt. This arrangement posed a problem because the elements do not fit the properties of the other elements within the group. Mendeleev made a bold decision to switch the position of the two elements, thus, its current position. This problem can also be seen between ₁₈Ar (At. Mass = 39.948) and ₁₉K     (At. Mass = 39.098). This made scientists realize that the atomic mass of elements is not the basis of periodicity.

                This problem was later on resolved by Henry Moseley, a 25-year old English scientist who discovered the Atomic Number. This atomic number corresponds to the nuclear charge of the atom or the number of protons inside the nucleus. He made his discovery by bombarding metals with high energy electrons. Through his discovery of the atomic number, they were able to prove that Cobalt (Atomic number of 27) should come first before Nickel (Atomic number of 28) despite their atomic masses. This led to the formulation of the Periodic Law.

o   Periodic Law – states that the physical and chemical properties of elements are periodic functions of increasing atomic number.