Classification Of Elements

HISTORY OF DEVELOPMENT OF THE PERIODIC TABLE

All earlier attempts on the classification of the elements were based upon their atomic weights.

Dobereiner's Triads:

Dobereiner classified the elements with similar properties in the groups of three elements (Triads). He could succeed in making only a few triads. In the triads of elements the atomic weight of the middle element was the arithmetic mean of the atomic weights of the other two. Some of the triads are as under.

Li Na K Ca Sr Ba

7 23 39 40 88 137

P As Sb Cl Br I

31 75 120 35.5 80 123

The major drawback of Dobereiner's classification was that the concept of triads could be applied only for the limited number of elements.

Newland's Law of octaves:

Newland pointed out that, if the elements are arranged in order of their increasing atomic weights, every eighth element had similar properties to first one like the first and eighth note of the musical scale. For example

sa re ga ma pa dha ni sa

Li Be B C N O F Na

Na Mg Al Si P S Cl K

This generalization was also discarded since it could not be applied to elements having atomic weight greater than that of calcium i.e. 40 a.m.u Furthermore with the discovery of noble gas, the properties of the eighth elements were no longer similar to the first one.

Lothar Meyer's Atomic Volume Curve:

Lothar Meyer a German chemist plotted a graph between atomic weight and atomic volume (i.e. atomic weight in solid state/density) and he find out that elements with similar properties occupied the similar positions on the graph. Strong electropositive elements of IA group except Li i.e. Na, K, Rb, Cs etc. occupied the top position on the graph. IIa group elements like Be, Mg, Ca, Sr, Ba etc. occupied the positions on the ascending part of the graph. Inert gases except He occupied the positions on the descending part of the graph. Halogens also occupied the descending part of the graph.

Mendeleev's Periodic Law:

Mendeleev, a Russian Chemist used broader range to physical and chemical properties to classify the elements. The physical and chemical properties of elements are periodic functions of their atomic weights. If the elements are arranged in the order of their increasing atomic weights, after a regular interval elements with similar properties are repeated (inert gases were not discovered till then)

MENDELEEV'S PERIODIC TABLE

Mendeleev arranged the discovered 63 elements in the periodic table into 7 horizontal rows known as periods and 8 vertical columns known as groups numbered 1 to 8.

Mendeleev Suggested:

$\bullet$He excluded certain elements and assigned them a separate independent position.

$\bullet$Leaving gaps for the then undiscovered elements

$\bullet$When the properties of elements did not correspond to what is expected of the group they were named by prefixing Eka to the preceding element e.g. Eka boron (Silicon); Eka silicon (Germanium), Eka aluminum (Gallium); Eka Maganese (Technitium)

Uses of Mendeleev's Periodic Table

$\bullet$Systematic study of the elements

$\bullet$Atomic weights of elements were determined with the help of periodic table. Atomic weight = Valency ´ Equivalent weight = Group number ´ Equivalent weight.

$\bullet$Atomic weight of elements were corrected. Atomic weight of Be was calculated to be 3 x 4.5 = 13.5 by considering its valency 3. Mendeleev calculated it 2 x 4.5 = 9.

$\bullet$Discovery of new elements – In Mendeleev's periodic table two consecutive members differs by two or three units in the atomic weight. Where this gap was more, the gaps were left in the periodic table.

Defects of Mendeleev's Periodic Table:

$\bullet$Position of hydrogen is uncertain. It has been placed in IA and VIIA groups because of its resemblance with both the groups.

$\bullet$No separate positions are given to isotopes.

$\bullet$It is not clear to which group lanthanides and actinides belong to.

$\bullet$Although there is no resemblance except valency of subgroups A and B, they have been put in the same group.

$\bullet$Order of increasing atomic weights is not strictly followed in the arrangement of elements in the periodic table. For e.g. – Co (At.wt. 58.9) is placed before Ni (58.7) and Ar (39.9) is placed before K (39)

LONG FORM OF THE PERIODIC TABLE OR MOSELEY'S PERIODIC TABLE

The table which is most commonly used these days is called long form of the periodic table which consists of eight vertical columns and seven horizontal rows. Moseley studied the frequency of

X-rays produced by the bombardment of a strong beam of electrons on a metal target. He found that the square root of the frequency of X-rays$\left(\sqrt{v}\right)$ is directly proportional to the number of nuclear charge (Z) of the metal. $\sqrt{v}=a\left(Z-b\right)$where a and b are constants. Nuclear charge of metal is equal to the atomic number. So Moseley related the properties of elements with their atomic number and gave the new periodic law.

Moseley's Periodic Law or Modern Periodic Law: Physical and chemical properties of elements are the periodic functions of their atomic numbers. If the elements are arranged in order of their increasing atomic number, after a regular interval, elements with similar properties are repeated.

Characteristics of Periods:

$\bullet$First period is called shortest period and contains only two elements. Second and third periods are called short periods containing eight elements each. Fourth and fifth periods are long periods containing eighteen elements each. Sixth period is longest period with thirty-two elements. Seventh period is an incomplete period.

$\bullet$Lanthanide and actinide series containing 14 elements each are placed separately below the main periodic table. These are related to sixth and seventh periods of

III B group respectively

$\bullet$Elements of third period from sodium (Na) to Chlorine (Cl) are called typical elements.

$\bullet$Valency of an element in a period increases from 1 to 7 with respect to oxygen.

Na₂O MgO Al₂O₃ SiO₂ P₂O₅ SO₃ Cl₂O₇

1 2 3 4 5 6 7

$\bullet$Elements of second period are called bridge elements.

Characteristic of Groups:

$\bullet$Moseley's periodic table contains sixteen groups. These are represented by Roman numerals I, II, III, IV, V, Vi, VII, VIII and zero. Groups I to VII are divided into two subgroups A and B, Group VIII consists of three sets, each one containing three elements.

$\bullet$Inert gases are present in zero group.

$\bullet$The valency of an element in a group is equal to the group number.

$\bullet$There is no resemblance in the elements of subgroups A and B of same group, except valency

$\bullet$The elements of the groups which resemble with typical elements are called normal elements. For example IA, IIA, IIIA, IVA, VA, VIA, VIIA group elements are normal elements or main group elements or representative elements.

$\bullet$Those elements of the groups which do not resemble with typical elements are called transition elements. For example- IB, IIB, IIIB, IVB, VB, VIB, VIIB, and VIII group elements are transition elements.

Illustration 1. There are 2, 8 and 8 elements in first, second and third periodic of periods table respectively. Explain

Solution: In first period 1s is completed. Its capacity is of two electrons. In second period 2s 2p and in third period 3s 3p are completed. The capacity of these shells is of 8 electrons each. Thus, 2, 8 and 8 elements are present in first, second and third periods respectively.

CAUSE OF PERIODICITY

According to the Modern Periodic Law, the Properties of the elements are repeated after certain regular intervals when these elements are arranged in order of their increasing atomic number. Therefore the cause of periodicity in properties is the repetition of similar outer electronic configuration at certain regular intervals.

All the elements of group IA i.e. alkali metals have similar outer electronic configuration i.e. ns¹, which are given below

Thus it is because of similarity in electronic configuration that all the elements have similar properties

NOMENCLATURE OF ELEMENTS WITH ATOMIC NO > 100

IUPAC approved the names based on the following points.

$\bullet$The names are derived directly from the atomic numbers using numerical roots for 0 and numbers from 1-9 and adding the suffix ium

$\bullet$ In certain cases, the names are shortened. For example biium & triium are shortened to bium & trium

Roots for IUPAC nomenclature of elements are given below.

The recommended and official names of some super heavy elements are given below

Illustration 2. The element 119 has not been discovered. What would be the IUPAC name and symbol for this element? On the basis of the Periodic Table, predict the electronic configuration of this element and also the formula of its most stable chloride and oxide.

Solution: Ununennium (Uue)

Electronic Configuration

1s², 2s²2p⁶, 3s² 3p⁶3d¹⁰,4s²4p⁶4d¹⁰ 4f¹⁴, 5s² 5p⁶ 5d¹⁰ 5f¹⁴, 6s² 6p² 6d¹⁰, 7s²7⁶, 8s¹

Oxide = Uue₂O, chloride = UneCl

CLASSIFICATION OF ELEMENTS ON THE BASIS OF THEIR ELECTRONIC CONFIGURATION

On the basis of electronic configuration, the elements may be divided into four groups, that is s, p, d & f. This division is based on the name of orbital which receives the valence electron.

s- Block elements:

Elements in which the last electron enters the s- orbital or their respective outermost shells are called s- block elements. These are present in the left part of the periodic table in group IA & IIA i.e. 1 & 2 group in modern periodic table. Electronic configuration of valence shell is ns^1-2

(n 1 to 7)

Characteristic of s – block elements:

$\bullet$They are soft metals with low melting & boiling points

$\bullet$They posses metallic character & reactivity of metal increases down the group

$\bullet$They are highly electropositive and having low ionization enthalpies

$\bullet$They have valency +1(in case of alkali metals) &+2 (in case of alkaline earth metals)

$\bullet$Most of the metals of this block impart characteristic color to the flame

$\bullet$They are strong reducing agents & are good conductor of heat & electricity.

p - block elements:

These are present in right part of the periodic table and constitute the groups IIIA to VIIA and zero groups i.e. group 13 to 18 of the modern periodic table. Most of these elements are metalloids & non metals but some of them are metals also. The last electron enters in p - orbital of valency shell and electronic configuration of valency shell is ns² n^1-6(n=2 to 7)

Characteristics of p - block elements:

$\bullet$It contains both metals & non metals. The metallic character decrease from left to right along the period and metallic character increases from top to bottom within a group

$\bullet$They mostly form covalent compounds

$\bullet$Ionization energy is higher as compared to s – block elements.

$\bullet$Reducing character increases from top to bottom in a group & oxidizing character increases left to right in a period.

d- Block elements:

These are present in the middle part of the periodic table (between s & p block elements) and constitute IIIB to VIIB, VIII, IB & IIB i.e. 3 to 12 groups of the modern periodic table. The outermost electronic configuration is (n-1) d^1-10 ns^1-2 (n=4 to 7). There are four series of d-block elements, which are 3d series-Sc (21) to Zn (30) 4d series – Y( 39) to Cd(48) & 5d series

–La (57), Hf (72) to Hg(80) 6d series Ac(89), Rf(104) ………………………………. Unb (ununbium) 112 (incomplete)

Characteristics of d – block elements:

$\bullet$These are hard, ductile & malleable metals with high melting & boiling points

$\bullet$Ionization energy is between s & p- block elements

$\bullet$They show variable oxidation states and are good conductors of heat & electricity.

$\bullet$They form both ionic & covalent compounds and compounds are generally coloured and paramagnetic in nature

$\bullet$Most of the transition metals form alloys.

f- Block elements:

These are placed separately below the main periodic table and are mainly related to IIIB i.e. group 3 of the periodic table. There are two series of f–block elements, which are

4f series – Lanthanides 14 elements Ce(58) to Lu(71) and 5f series Actinides 14 elements Th(90) to Lr(103). Their outermost electronic configuration is (n-2)f^1-14 (n-1)s²(n-1)p⁶ (n-1) d^0-1 ns².

(n = 6 & 7)

Characteristics to f – block elements:

$\bullet$They are heavy metals with high melting & boiling points

$\bullet$They show variable oxidation states & their compounds are generally coloured

$\bullet$Most of the elements of the actinide series are radioactive

PREDICTION OF GROUP, PERIOD AND BLOCK OF A GIVEN ELEMENT

We can easily predict the group, period & block of any element from its electronic configuration by the following ways

$\bullet$Principal quantum number of the valence shell corresponds to the period of an element.

$\bullet$The orbital containing the last electron indicates the block of an element.

$\bullet$The group of an element is predicted from the number of electrons in the valence shell or a penultimate shell (n-1) as follows

(a) For s – block elements, group number is equal to the number of valence electrons.

(b) For p – block elements group number is equal to 10 + number of electrons in the valence shell.

(c) For d – block elements, group number is equal to the number of electrons in

(n-1) d – subshell + number of electrons in valence shell (n^th shell)

Alternately you can calculate group number = Number of electrons in (penultimate shell + valence shell) - 8

(d) For f block elements, group number is always III^rd /3B

BOHR'S CLASSIFICATION OF ELEMENTS

Elements may be classified into four groups on the basis of number of incomplete shells present in them

$\bullet$Inert gases

(a) s- and p-orbitals of the outer most shell of these elements are completely filled. The outermost electronic configuration is ns²np⁶.

(b) Helium is also inert gas but its electronic configuration is 1s²

$\bullet$Representative or Normal Elements:

(a) Outermost shell of these elements is incomplete. The number of electrons in the outermost shell is less than eight.

(b) Inner shells are complete

(c) s-and p-block elements except inert gases are called normal or representative elements.

$\bullet$Transition Elements:

(a) Last two shells of these elements namely outermost and penultimate shells are incomplete

(b) The last shell contains one or two electrons and the penultimate shell may contain more than eight up to eighteen electrons.

(c) Their outermost electronic configuration is similar to d-block elements i.e.

(n-1)d^1-10 ns^1-2.

(d) According to the latest definition of transition elements those elements which have partly filled d-orbitals in neutral state or in any stable oxidation state are called transition elements. According to this definition Zn, Cd and Hg (IIB group) are d-block elements but not transition elements because these elements have d¹⁰ configuration in neutral as well as in stable +2 oxidation state.

$\bullet$Inner Transition Elements:

(a) In these elements last three shells i.e. last, penultimate and pre-penultimate shells are incomplete.

(b) These are related to IIIB i.e. group 3.

(c) The last shell contains two electrons. Penultimate shell may contain eight or nine electrons and pre-penultimate shell contains more than 18 upto32 electrons.

(d) Their outermost electronic configuration is similar to ¦-block elements i.e. (n-2)¦^1-14

(n-1)s² (n-1)p⁶ (n-1)d^0-2ns²

(e) Elements of the seventh period after atomic number 92 (i.e. actinides) are synthetic elements and are called transuranic elements.