CBSE Class 11 Chemistry Notes For Chapter 3 Classification Of Elements And Periodicity In Properties

CBSE Class 11 Chemistry Notes For Chapter 3 Classification Of Elements And Periodicity In Properties Introduction

At present, 118 elements are known to us. It is Almost an impossible task to remember the individual properties of these elements And A larger number of compounds derived from them. Several attempts were made by former scientists to arrange the elements in a coherent and orderly manner. After Dalton’s Atomic theory, attempts were made to establish a correlation between the atomic masses of various elements and their properties.

But until a method for the estimation of correct atomic masses of elements was innovated, the work on the proper classification of elements could not make any significant progress. However, after the atomic masses of elements were correctly determined, the attempts for the classification of elements received particular attention. The way of arranging similar elements together and separating them from dissimilar elements is called the classification of elements.

Historical Background Of The Classification Of Elements Based On Atomic Weight

Dobereiner’s Law Of Triads

In 11117, German scientist Doberelnor stated that in a group of three chemically similar elements, called a triad, the atomic weight of (the middle element of each triad Is very close to the arithmetic mean of those of the other two elements.

This was called Oohereiner’s law of triads. Some familiar triads, based on lids law, are shown below:

From the table, it is observed that the atomic weight of sodium (Na) is the average of the atomic weights of lithium (Li) and potassium (K)

⇒ \(\left[\frac{7+39}{2}=23\right]\)

This relationship is only applicable to a limited number of elements and hence fails to classify all the known elements.

However, it cannot be denied that it indicated the existence of an inter-relationship between the properties and atomic weights of elements.

Law of Telluric Screw

In 1862, Chancourtois attempted to classify the elements based on atomic mass. He took a vertical cylinder with 16 equidistant lines drawn on its surface (lines are parallel to the axis of the cylinder). He drew a spiral line or helix on the surface making an angle of 45° to the axis of the cylinder.

The atomic weights were plotted vertically along the spiral line. He arranged the elements on the helix in order of their increasing atomic weights. It was observed that in the telluric screw, the elements that differed from each other in atomic weight by 16 or multiples of 16 fell on the same vertical line. The elements lying on the same vertical line showed nearly the same chemical properties. However, this concept did not attract much attention.

Newlands’ Law Of Octaves

Arranging the known elements in the ascending order of their atomic weights, Newlands, observed (1865) that properties of the eighth element, starting from a given one, is a kind of repetition of the first, like the eighth note in an octave of music. He called this regularity the law of octaves.

Starting from Li, the eighth element is Na and the eighth element following Na is K. There exists a striking resemblance in properties among these elements. Similarly, F shows similarity with the eighth element Cl following it in properties. The law of octaves was found to be satisfactory in the case of lighter elements from hydrogen (H) to calcium (Ca). However in the case of heavier elements beyond calcium, it lost its validity and hence, the law was discarded.

Lothar Meyer Arragngement

In 1869, Lothar Meyer, a German scientist, studied the different physical properties of the known elements and plotted a graph of atomic volume (atomic weight divided by density) against the atomic weight of various elements. He noticed that the elements with similar properties occupied similar positions on the curve. Based on this observation, Lothar Meyer concluded that the physical properties of the elements are a periodic function of their atomic weights.

Periodic Law

In 1869, Russian chemist, Dmitri Mendeleev, examined the relationship between the atomic weights of the elements and their physical and chemical properties. From his studies, Mendeleev pointed out that the physical and chemical properties of elements are periodic functions of their atomic weights. This generalisation is called Mendeleev-Lothar Meyer Periodic Law or simply Mendeleev’s Periodic Law.

Mendeleev’s Periodic law:

The physical and chemical properties of elements are a periodic function of their atomic weights. This law implied that if the elements are arranged in the order of increasing atomic weights, the physical & chemical properties of the elements change regularly from one member to another and get repeated after a definite interval. This recurrence of properties ofthe elements at definite intervals is called the periodicity of elements.

Periodic classification and periodic properties:

Based on the periodic law, the classification of elements according to the increasing atomic weight is called periodic classification. The properties of the elements which are directly or indirectly related to their electronic configurations and show a regular gradation when we descend in a group or move across a period in the periodic table are called periodic properties.

For example:

The size of atoms or atomic radii, ionic radii, atomic volume, metallic character, ionisation enthalpy, electron affinity, electronegativity, melting point, boiling point, valency etc. Radioactivity is not a periodic property of elements: Radioactivity is neither directly nor indirectly related to the electronic configuration of atoms. It depends on the ratio between the number of neutrons and protons present in the atom.

Classification Of Elements Based On Outer Electronic Configurations

Based on electronic configurations of the ultimate and penultimate shell of the atoms, Bohr divided the elements into four classes viz., gas elements,

1. Representative elements,
2. Transition elements and
3. Inner-transition elements.

Inert gas elements

S and p -subshells of the outermost shell of the elements of this class are filled.

Except He (electronic configuration: Is²)

• All other inert gas elements have the valence shell electronic configuration: ns²np⁶.
• All these elements are stable and chemically inert as their outermost shells contain octets of electrons.
• They do not normally participate in chemical reactions because the gain or loss of electrons by their atoms would disturb their stability. So, they are called inert gas elements.
• Their valency being zero, they find a place in group ‘0’ or ’18 These elements act as a bridge between highly electropositive alkali metals and strongly electronegative halogens.

Representative Elements

Elements present in s – and p -blocks (except group-1) of the periodic table are known as representative elements. The electronic configuration of the outermost shell of these elements varies from ns1 to ns²np⁵. These consist of some metals, all non-metals and metalloids.

The name ‘representative’ has been assigned to the elements because of their frequent occurrence nature and because they typify the properties of all other members of the group to which they belong.

All the elements of groups 1A, 2A and from 3A to 4LA are included in this class.

These elements are very reactive Chemical reactivity of these elements can be ascribed to the ability of their atoms to attain inert gas electronic configuration (ns²np⁶ or Is²) either by gaining or losing electron(s) or by sharing one or more electron pairs with other atoms. These elements are also known as typical elements.

Transition Elements

Elements of this class are characterised by the presence of atoms in which the inner d -subshell is not filled. According to the modified definition, the elements in which atoms in their ground state or any stable oxidation state contain incompletely filled d -subshell are known as transition elements. Atoms of the elements in this class have the general electronic configuration: (n-1)d^1-10ns^1-2

Cu, Ag and Au, despite having filled d orbitals, are regarded as transition elements. This is because at least in one stable oxidation state of these elements, d subshell remains incompletely filled.

There are four transition series corresponding to the filling of 3d,4d,5d and 6d orbitals These four series belong to the 4th. 5th. 6th and the period of the periodic table.

Each series begins with a member ofthe group-3 and ends with a member of the group-12.

Characteristics:

• All transition elements are metallic.
• They have more than one oxidation state or valency.
• Their ions are coloured.
• They form complex compounds.
• Elements of group-12 (11B) (Zn, Cd, Hg) are not considered transition elements because they have no partially filled d -d-orbitals in any of their oxidation states.
• Moreover, they do not form stable complexes and do not show characteristic colour and paramagnetism.
• However, their tendency to form complex is much greater than that of the representative elements.
• They exhibit properties of both transition representative elements.

Differences between typical and transition elements:

• During the building up of an atom of a typical element by the filling of electrons in its various orbitals, the last electron goes to s -or p -orbital of the outermost shell (n).
• However, in the case of transition elements, the last electron enters the inner d -d-orbital of(n- 1) th shell.
• For the representative elements, atomic volume or radius decreases but ionisation enthalpy and electro negativity go on increasing with the increase in atomic number across a period.
• In the case of the transition elements, as the last electron enters the inner (n- 1)d -orbital, the extent of change is relatively small.
• Most of the representative elements exhibit only one valency. Some elements, of course, show more than one valency.
• But transition elements show 2 or more valencies through the participation of inner d -d-orbital electrons.
• In the case of representative elements, the tendency to form complex compounds is almost negligible while transition elements are found to show a strong tendency to produce complex compounds due to the presence of incompletely filled d -d-orbital.
• Compounds formed by representative elements are, in general, colourless but the compounds of transition elements are mostly coloured.
• Due to the absence of odd electron(s), compounds formed by representative elements are diamagnetic while transition metal compounds, because of the presence of odd electrons, are paramagnetic.
• Many of the transition metals and their compounds act as catalysts in chemical reactions. Such a tendency is seldom observed in the case of representative elements.

Inner-transition elements

Elements of this class are also transition elements, although they may be distinguished from the regular transition series by their electronic configurations.

• Atoms of these elements not only contain incompletely filled d-subshell [(n-1)d] but also contain incompletely filled f-subshell [(n- 2)f].
• These elements comprise a transition series within a transition series and hence, they are called Inner-transition elements.
• The two series of inner-transition elements are O lanthanoids (rare earth elements) and actinoids.

In the case of 14 elements i.e., cerium (Cel to lutetium (₇₁Lu) following lanthanum (₅₇La), 4f- and 5d subshells remain incompletely filled. These are called lanthanoids.

Their general electronic configuration is:

4f^1-14 5d^0-16s² With the increase in atomic number (58-71). the differentiating electrons of these elements enter the 4f- subshell, despite the presence of a partially filled 5d -subshell. The total electron-accommodating capacity of the f-subshell Is 14.

So the number of lanthanoids is also 14. Likewise, 14 elements after actinium (₈₉Ac), from thorium (₉₀Th) to lawrencium (₁₀₃Lr) are called actinoids. Their general electronic configuration is 5f1’14 6d01 7s2. With the increase in atomic number (90-103), the differentiating electrons enter the 5f-subshell, despite the presence of an incompletely filled 6d -6d-subshell. Hence, like the lanthanoids, the number of actinoids Is also 14.

Lanthanoid contraction

In the case of lanthanoids (₅₈Ce – ₇₁Lu), it is observed that with an increase in atomic numbers, atomic and ionic size (M³⁺) go on decreasing, although the decrease in Ionic radii is much more regular than that of atomic radii.

This decrease in atomic and ionic radii with an increase in atomic number in the case of lanthanoids, is known as lanthanoid contraction.

Cause of lanthanoid contraction:

• The general electronic configuration of lanthanoids is 4f^1-14 5d^0-16s². The differentiating electrons of these elements enter the 4f-subshell.
• Now due to their diffused shape, f-orbitals have a very poor shielding effect.
• Thus with the gradual addition of the f- electrons, the atomic number increases by one unit while the shielding effect does not increase appreciably; i.c., there is a gradual increase in the effective nuclear charge acting on the outermost electrons.
• Consequently, the attraction of the nucleus for the electrons in the outermost shell increases, causing the electron cloud to shrink although its magnitude is small.
• Thus, there is a gradual shrinkage in the atomic and ionic radii with an increase in atomic number.
• Precisely speaking, f-orbitals are too diffused to screen the outermost electrons effectively against the attractive force of the nucleus. Thus, there is a slow contraction in atomic and ionic radii (lanthanoid contraction).
• In the same way, the d -contraction due to the accommodation of die electrons in (n- 1) d -subshell in the transition series can be interpreted.
• But d -d-orbitals are more effective in screening compared to tyre f-orbitals.
• So this effect is less pronounced in the case of transition elements.

Classification of elements as metals, non-metals and metalloids

All the known elements can be divided into three classes— metals, non-metals and metalloids based on their properties.

Metals

About 78% of the known elements are metals. They appear mainly on the left side and central portion of the long form ofthe periodic table.

Examples are:

1. Alkali metals,
2. Alkaline earth metals,
3. D -block elements,
4. F-block elements,
5. Higher members of p -block elements.

Metals have the following characteristics:

• They are solids at room temperature. Mercury is an exception, which is a liquid at ordinary temperature.
• Gallium (melting point 30°C) and caesium (melting point 29°C) are also liquids above 30°C.
• They usually have high melting and boiling points.
• They are good conductors of heat and electricity.
• They are malleable (can be flattened into thin sheets) and ductile (can be drawn out into wires).

Non-metals

There are only about 20 non-metals discovered so far. They are located towards the top right-hand side of the periodic table. Hydrogen and some p-block elements are non-metals.

• Six of the non-metals (C, B, P, S, Se and I) are solid.
• Bromine is the only liquid non-metal.
• The remaining non-metals (N, O, F, Cl, H and inert gases) are gases.
• Non-metals have low melting and boiling points (boron and carbon are exceptions).
• They are poor conductors of heat and electricity (graphite is a good conductor of electricity).
• Nonmetallic solids are usually brittle and are neither malleable nor ductile.

Metalloids

• There are some elements which have certain characteristics common to both metals and non-metals.
• These are called semimetals or metalloids. Examples are—silicon (Si), germanium (Ge), arsenic (As), antimony (Sb) and tellurium (Te).
• In most of their properties (both physical and chemical), metalloids behave as non-metals. However, they somewhat resemble the metals in their electrical conductivity. They tend to behave as semiconductors.
• This property is found particularly in the case of silicon and germanium.
• These two metals are mainly responsible for the remarkable progress in the past five decades in the field of solid-state electronics.

IUPAC Nomenclature Of Transuranic Elements (Atomic Number More Than 100)

The elements beyond fermium (100) are called transfermium elements. They have atomic numbers above 101.

Fermium (100), mendelevium (101), nobelium (102), and lawrencium (103) are named after eminent scientists. Some of the elements with atomic numbers higher than 103 were synthesized and reported simultaneously by scientists from the USA and the Soviet Union.

Each group proposed different names for die same element, e.g., an element with atomic number 104 was named Rutherfordium by USA scientists while Soviet scientists named it Kurchatovium.

To overcome such controversies, the IUPAC (1977) has recommended a new method of naming these elements. This is discussed here.

1. The digits expressing the atomic number of an element are represented serially (from left to right) by using the numerical roots given below.

2. The successive roots are written together and the name ends with ‘mum’ To avoid the repetition of some letters, the following procedure is adopted.

• If ‘enn’ occurs before ‘nil; the second ‘n’ of ‘enn’ is dropped.
• Similarly the letters ‘i’ of ‘bi’ and ‘tri’ are dropped when they occur before ium,bi+ium= bium, tri+ium= trium, enn+nil= ennil etc.

3. The symbol of an element is derived by writing successively the initial letters (z.e., abbreviations) of the numerical roots which constitute the name.