Periodic Classification of Elements

Periodic Classification of Elements (By Ajay Raj)

Introduction 

Scientists have discovered 111 chemical elements till date. Some of these elements occur in free state and some in combined state. But all of these elements were not discovered in a day. When a very few elements were known, studying them separately was not a problem. But when a large number of elements had been discovered, it became difficult to study the properties of all of them separately. So, attempts were made, from time to time, to sort out the elements into groups so as to follow their behaviour in an orderly manner. The study of the properties of a typical element of a particular group enables scientists to predict roughly the properties of other elements of that group. We will now briefly discuss the various attempts made to classify elements. 

Early attempts at Classification 

(1) Lavoisier’s Classification

(2) Dobereiner’s Classification

(3) Newlands' Classification

(4) Mendeleev's Classification

(5) Henry Moseley Classification

(6) Niels Bohr Classification

(1) Lavoisier’s Classification 

Lavoisier classified elements into metals and nonmetals. This classification was based on certain distinctive physical properties such as hardness, malleability and lustre. On the basis of these properties, sodium and lead were classed together as belonging to the group of metals. 

Limitations 

(i) Hardness, malleability and lustre were found to be the only common properties of sodium and lead, otherwise the two elements were entirely different. 

(ii) In such a classification there was no place for elements with properties resembling those of metals as well as nonmetals. Therefore, Lavoisier’s classification was found to be inadequate. 

(2) Dobereiner’s Classification 

Law of triads In 1817, German chemist Johann Dobereiner classified elements having similar chemical properties into groups of three. These groups were called triads. He proposed a law known as Dobereiner’s law of triads. According to this law, when elements are arranged in the order of increasing atomic mass in a triad, the atomic mass of the middle element was found to be approximately equal to the arithmetic mean of the atomic masses of the other two elements.

The classification of elements into triads was very successful in predicting the atomic mass and properties of the middle element. Further, this classification showed that there exists some relationship between the properties of elements and their atomic masses. This paved the way for future attempts at classification of elements. 

Limitation:- All the elements could not be grouped into triads.

(3) Newlands’ Classification  

Law of octaves In 1864, John Newlands, an English chemist, showed that when elements are  arranged in the order of their increasing atomic masses, the eighth element, starting from a given  element, was a kind of repetition of the first one, like the eighth note in an octave of music, i.e.,  

sa re ga ma pa dha ni sa,  

where the first and the eighth note are the same.  

A part of Newlands’ classification is given below where the figures under the symbols show the atomic masses.

Starting from lithium (Li) the eighth element is sodium (Na). The eighth element starting from sodium is potassium. The properties of lithium, sodium and potassium are similar. The properties of beryllium, magnesium and calcium are similar too. 

Limitations:- 

(i) This law worked well for lighter elements (up to calcium), but it could not be applied to heavier ones (elements of higher atomic masses) because starting from calcium every eighth element was found to have properties different from those of the first element. 

(ii) Newlands emphatically said that only 56 elements do exist in nature and no more element is likely to be discovered in future. But this concept was later on found to be untrue with the discovery of many new elements which defied the law of octaves. 

(iii) In arranging elements in the form of a table, Newlands clubbed two elements together at the same place and in the same column. Not only this, he also placed some dissimilar elements in the same column. For example, cobalt (Co) and nickel (Ni) were clubbed together in the column of fluorine (F), chlorine (Cl) and bromine (Br) (under sa/do). We know that cobalt and nickel have properties entirely different from those of fluorine, chlorine and bromine. It is also known that cobalt and nickel have properties similar to those of iron. But iron (Fe) was  placed in a column (under ni/ti) different from the column of cobalt and nickel.  

However, this law lent support to the idea that the properties of elements depend upon the  atomic masses. It also showed that the properties of elements are repeated after a certain interval,  i.e., the properties of elements are periodic in nature.  

(4) Mendeleev's Classification

While working systematically on the physical and chemical properties of elements, Dmitri Ivanovich Mendeleev noticed that properties of elements varied regularly with the atomic mass.  He arranged the 63 elements then known in a table on the basis of similarities in properties. It was  found that most of the elements occupied places in the table in order of their increasing atomic  masses. In 1869, Mendeleev formulated a law, now known as the periodic law. The law is stated  as follows.  

The properties of elements are periodic functions of their atomic masses. This means, if the  elements are arranged in order of increasing atomic masses then those with similar properties are  repeated at regular intervals.  

On the basis of the periodic law, Mendeleev presented his classification in the form of a table,  now known as Mendeleev’s periodic table. A simplified version of this periodic table is given below. In this table, copper, silver and gold find places in groups I as well as VIII.

This table consists of vertical columns called groups and horizontal rows called periods. 

There are only eight groups in the table. Mendeleev left some vacant places (shown by question marks) for the yet undiscovered elements. Noble gases were not discovered then. So, he did not provide any place for them in his periodic table. 

Mendeleev’s idea was remarkable in that he used a fundamental atomic property (atomic mass) as the basis of classification. While classifying elements he laid special emphasis on two factors. 

1. Similar elements were grouped together. 

2. Elements were arranged in order of increasing atomic masses. 

Modified Version of Mendeleev’s Periodic Table 

The elements which were undiscovered and for whom Mendeleev had left vacant places were discovered later. Some of these are scandium (Sc), gallium (Ga) and germanium (Ge). These elements were accommodated in their proper places in the table. The elements helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) became known only towards the end of the nineteenth century. These elements, called noble gases, were placed in the table as a separate group, called 0 (zero) group. The periodic table had to be modified then. The modified version of the table is shown below:-

Features of the modified version of Mendeleev’s periodic table 

(a) Groups into subgroups:- 

Each group of this periodic table is further divided into two subgroups A and B. The properties of elements within a subgroup resemble more markedly but they differ from those of the elements of the other subgroups. For example, lithium (Li), sodium (Na), potassium (K), etc., of subgroup IA have close resemblance of properties but they have hardly any resemblance to the coinage metals (Cu, Ag and Au) of subgroup IB. Mendeleev allowed the subgroups to be represented within the same group. 

(b) Prediction of errors:- 

This periodic table could predict errors in the atomic masses of some elements on the basis of their position in the periodic table. For example, when the periodic table was published, the experimental value of the atomic mass of beryllium (Be) was supposed to be 13.65 and its valency, 3. So, the position of Be should have been somewhere else, but Mendeleev placed it at its appropriate position on the basis of its properties. He further suggested that the atomic mass of Be needed correction. Mendeleev predicted its atomic mass to be 9.1 and valency, 2. Later investigations proved him right. Similarly, the atomic mass of uranium was corrected from 120 to 240. Corrections were also made in the atomic masses of gold, platinum, etc. 

(c) Predictions of properties of hitherto undiscovered elements:- 

We know that Mendeleev classified the elements in order of their increasing atomic masses. However, this order had to be ignored at some places to make sure that the elements with similar properties fell in the same group. In doing so, he left some vacant places in the table. These vacant places were kept reserved for elements not discovered till then. Mendeleev was confident that these elements would be discovered later and they would occupy these vacant places. Not only this, he also predicted the properties of these undiscovered elements on the basis of his study of the properties of the neighbouring elements. Amazingly, when the missing elements of Mendeleev’s periodic table were discovered subsequently, their properties were found to be very similar to those predicted by Mendeleev. 

The elements scandium, gallium and germanium were not known in 1871 but their existence was predicted by Mendeleev. He named these elements as eka-boron, eka-aluminium and eka-silicon. When these elements were discovered, they were named scandium, gallium and germanium respectively and their properties were found to be in good agreement with those predicted by Mendeleev. Properties of eka-aluminium (predicted by Mendeleev) and those of the  gallium (discovered later) are given in Table below:-

Considering its atomic mass, titanium (Ti) should have been placed below aluminium in the periodic table, but Mendeleev placed it below silicon (Si) because the properties of titanium were similar to those of silicon. Thus, a gap was left below aluminium in the periodic table. This gap was filled up by gallium which was discovered later. The properties of gallium (Ga) were found to be similar to those of boron and aluminium. 

(d) Basic features intact:-

All the basic features of Mendeleev’s periodic table are intact even today. Even when a new class of elements, i.e., noble gases, were discovered, they found place in a separate group called the zero group. The existing order of the periodic table was not at all disturbed. 

Discrepancies in Mendeleev’s periodic table 

Mendeleev’s periodic table has the following defects. 

(a) Position of hydrogen:- 

The position of hydrogen in the periodic table is anomalous. Hydrogen resembles alkali metals (Li, Na, K, etc.) in certain properties. Hence, it is placed in group IA along with the alkali metals. But certain properties of hydrogen resemble those of halogens (F, Cl, Br, etc.). So it may be placed in the group of the halogens (VII A). 

(b) Position of lanthanides and actinides:- 

The elements from atomic number 57 to 71 are collectively known as lanthanides. They do not have a proper place in the periodic table. They all have been placed at the same position in group III and period 6. Similarly, the actinides (atomic numbers 89–103) also have no proper place in the periodic table. These elements have also been placed in the same position, in group III and period 7. 

(c) Some similar elements are separated, while some dissimilar elements have been placed in the same group:- 

Some similar elements are separated in the periodic table. For example, copper (Cu) and mercury (Hg), silver (Ag) and thallium (Tl), and barium (Ba) and lead (Pb). On the other hand, some dissimilar elements have been placed together in the same group. For example, copper (Cu), silver (Ag) and gold (Au) have been placed in group I along with the alkali metals. Similarly, manganese (Mn) is placed in the group of the halogens. 

(d) Presence of some anomalous pairs of elements:-

In Mendeleev’s periodic table, the elements are arranged in order of increasing atomic mass. In some places, this order has been ignored. 

(i) The atomic mass of argon is 40 and that of potassium is 39. But argon is placed before potassium in the periodic table. 

(ii) The positions of cobalt and nickel are not in proper order. Cobalt (at. mass = 58.9) is placed before nickel (at. mass = 58.6). 

(iii) Tellurium (at. mass = 127.6) is placed before iodine (at. mass = 126.9). 

(iv) Thorium (at. mass = 232.12) is placed before protactinium (at. mass = 231).

(v) Position of isotopes:-

The isotopes of an element have no place in the periodic table.  The failure of Mendeleev’s periodic law to explain the wrong order of the atomic masses of  some elements and the position of isotopes led scientists working in this field to conclude that  atomic mass cannot be the basis for the classification of elements. There must be a more  fundamental property of elements which can be the basis of classification. 

Salient Features of the Modern Periodic Table:-

Group 2 (s-block):-
What are Alkaline earth metals?
Elements whose atoms have their s-subshell filled with their two valence electrons are called alkaline earth metals. Their general electronic configuration is [Noble gas] ns2. They occupy the second column of the periodic table and so-called as group two metals also.
Examples of Alkaline earth Metals: Beryllium (Be), Magnesium(Mg), Calcium (Ca), Strontium (Sr), Barium(Ba) and Radium (Ra).

Metals	Beryllium	Magnesium	Calcium	Strontium	Barium
Atomic Number	4	12	20	38	56
Configuration	[He]2s2	[Ne]3s2	[Ar]4s2	[Kr]5s2	[Xe]6s2
Abundance (ppm)	6	20900	36300	300	250
Atomic size (pm)	112	160	197	215	222
Density g/cm3	1.85	1.74	1.55	2.63	3.62
Ionization energy kJ/mol	899 & 1757	737 &1450	590&1146	549&1064	503&965
Hydration enthalpy kJ/mol	-506	-406	-330	-310	-276
Reduction potential (v)	-1.7	-2.37	-2.87	-2.89	-2.9
Flame colour	–	–	Brick red	Crimson red	Apple green

The name “alkaline” comes from the fact that compounds of these elements form basic (pH greater than 7) or alkaline solutions when dissolved in water.

Group 3-12 (d-block element):-

Transition elements:-

Transition elements (also known as transition metals) are elements that have partially filled d orbitals. IUPAC defines transition elements as an element having a d subshell that is partially filled with electrons, or an element that has the ability to form stable cations with an incompletely filled d orbital.

What are the General Characteristics of Transition Elements?

The d-block elements are known for their:

(a) Large charge: radius ratios

(b) High melting points and boiling points

(c) High densities and hardness.

(d) Formation of paramagnetic compounds

(e) Formation of coloured ions/compounds

(f) Ability to form stable complexes

Group 13 (p-block)

Boron family – Boron is the first member of the group

Boron got its name from the Arabic word ‘buraq’ which is the name of borax. It belongs to the 13th group of the p block element. The elements of the 13th group are boron, aluminium, gallium, indium, and thallium. They all are metallic in nature except boron which is a metalloid. All of them have 3 electrons in the outermost shell which has the electronic configuration of ns2np1. There are two oxidation states (+3 and +1) of boron family. 

Why Boron is a metalloid?

Boron resembles with both metals and non-metals therefore, it is metalloid. 

Boron acts as non-metal when it reacts with highly electro-positive metals as Na, K etc. & B acts as metal when it reacts with F (to produce BF3).