Classification of Elements and Periodicity:Class 11 Chemistry NCERT Chapter 3
Key Features of NCERT Material for Class 11 Chemistry Chapter 3 – Classification of Elements and Periodicity
In the previous chapter 2:Structure of atom,we have studied about the structure and classification of an atom.In this chapter 3:Classification of elements and periodicity we will study about the historical development of the periodic table.
Quick revision notes
Everything, including the food we eat to the air around us, an issue is composed of several elements, which go about as the basic building blocks. Get any item around you, and you would be surprised at the composition of that article’s issue. Diving deep into the study of the issue around us reveals numerous novel and interesting facts about the composition of the issue.
Elements
If it is to be put in easy words, elements are the basic building blocks of the issue around us. In regular intervals, new elements have been discovered which have uncovered numerous properties of objects around us.
As fresher elements share their direction practically speaking knowledge of humanity, it becomes difficult to study every single element in individual terms. Thus, it makes sense to sort these into several subgroups, so that their study can be carried out efficiently. This is the reason the idea of classification comes into shape.
Genesis of Periodic Classification Dobereiner’s Triads
Dobereiner, In 1829, arranged certain elements with similar properties in groups of three in a manner that the atomic mass of the middle element was about the same as the normal atomic masses of the elements preceding and succeeding it. A few triads proposed by him are underneath.
Drawbacks of Dobereiner’s Triads
The triads proposed by Dobereiner were helpful in grouping some elements with similar characteristics together, yet he could not organize all the elements into the groups of three.
John Newlands’ Law of the Octaves
John Newlands stated the law of octaves by saying that when elements are arranged by increasing atomic masses, each eighth element has properties similar to the first. Newlands called it the law of octaves because the same relationship exists in the musical notes also.
This can be illustrated as:
Limitations of Newlands’ Law of Octaves
(I) This classification was successful just up to the element calcium. After that, every eighth element does not possess the same properties as done by the above seven elements.
(ii) The inclusion of noble gases when they discovered disturbed the whole structure of the table.
Mendeleev’s Periodic Table
Mendeleev’s Periodic Law: The chemical and physical properties of the elements are a periodic
The function of their atomic masses.
Mendeleev arranged the elements known around in increasing atomic masses.
And this course of action was called The periodic table that has all the elements.
Vertical rows have elements with similar characteristics called groups.
Rows were called as Periods.
Description of Mendeleev’s Periodic Table
(I) In the periodic table, vertical rows have the elements arranged known as groups, and horizontal rows are often known as Periods.
(ii) Indicated by roman numerals, here are nine groups I, II, III, IV, V, VI, VII, VIII, and zero. Group VIII consists of nine elements that are arranged in three triads. The zero group contains elements with inert gasses or noble gasses and elements present with zero valencies.
(iii) Seven periods are there (numbered from 1 to 7) or even rows in the Mendeleev’s periodic table.
Significance of Mendeleev’s Periodic Table
(I) This made the study of the elements very systematic in the sense that if the properties of one element in a specific group are known, those of others can be predicted.
(ii) This helped by and large in the discovery of these elements at a later stage.
(iii) Mendeleev corrected the atomic masses of specific elements with the assistance of their expected positions and properties.
Defects in Mendeleev’s Periodic Table
(I) Hydrogen has been placed in group IA alongside antacid metals. Yet, it also resembles halogens of group VII An in numerous properties. Hence, its position in the Mendeleev’s periodic table has always been a topic of controversy.
(ii) Although their atomic masses have arranged the elements in the Mendeleev’s periodic table, however in some cases, the element with higher atomic mass precedes the element with lower atomic mass.
(iii) We realize that the isotopes of an element have different atomic masses, but the same atomic number. Since the periodic table has been made on the basis of increasing atomic masses of the elements, different positions must have been allotted to all the isotopes.
(iv) As per Mendeleev, the elements placed in the same group must have the same properties as those in which it is placed. Yet, there is no similarity among the elements in the two sub-groups.
(v) In some cases, different groups have been allotted to the elements with similar properties.
(vi) Lanthanoids and actinoids were given a position at the base of the table in two separate rows of the periodic table without assigning a legitimate reason.
(vii) No legitimate clarification has been offered for the fact that why the elements placed in the group show resemblance in their properties.
Modern Periodic Law
The periodic function of their atomic numbers is the physical and chemical properties of the elements.
- Present Form of the Periodic Table (A prolonged form of Periodic Table)
In the Modern Periodic Table also called the prolonged form of the periodic table, the elements have been arranged from top to bottom based on the increase in their atomic numbers.
- Nomenclature of Elements whose atomic masses of more than 100
- Structural Features of the Periodic Table
Groups
The prolonged form of periodic table also comprises the vertical rows called groups. There are in every one of the 18 groups in the periodic table. In contrast to the Mendeleev periodic table, each group is said to be the independent group.
Characteristics of groups:
(I) The elements present in a single group have the same electronic configuration.
(ii) The separation of the elements in a group is made by keeping definite gaps of atomic numbers (2, 8, 8, 18, 18,32) in mind.
(iii) The element’s atomic size in a group increased down the group due to an increase in the shells’ quantity.
(iv) Elements with the physical properties such as solubility, melting point, boiling point, etc., and so on, follow a systematic example.
(v) The elements in each group have commonly similar chemical properties.
Periods
Level rows in a periodic table are known as periods.
There are in each of the seven periods in the long form of the periodic table.
Features of row/period:
(I)The electrons are in the same valence shell in all the elements present in a period.
(ii)In a horizontal plane, moving left to right, the atomic size decreases.
s-Block Elements
General electronic configuration: ns1-2
Characteristics of s-block elements:
(I) All the elements are soft metals.
(ii) They have low liquefying and breaking points.
(iii) They are profoundly responsive.
(iv) Most of them bestow colors to the flame.
(vi) They have high conduction ability.
p-Block Elements
General electronic configuration: ns2np1-6
Features of p-block elements:
(I) The compounds formed by these elements are mostly covalent.
(ii) They show variable oxidation states.
(iii) In moving from left to directly in a period, the non-metallic character of the elements increases.
(iv) The reactivity of elements in a group by and significant decreases downwards.
(v) At the end of every period is a noble gas element having a closed valence shell of configuration ns2 np6.
(vi) As we move top to bottom in a row, Metallic character increases.
d-Block Elements
Elements belonging to d block have (n – 1) d1-10 ns0-2 general electronic configuration.
Characteristics of d-block elements:
(I) They are mainly metals with high liquefying and breaking points.
(ii) The compounds of the elements are commonly paramagnetic.
(iii) They mostly form colored ions and show variable oxidation states.
(iv) They are often used as catalysts.
f-Block Elements
Electronic configuration of f-block elements is (n – 2) )f1-14) (n – 1) (d0-1) ns2
They are known as internal transition elements because, in the transition elements of d-block, the electrons are filled in (n – 1) d sub-shell while in the inward transition elements of f-block the filling of electrons takes place in (n – 2) f subshell, which happens to be one inward subshell. Characteristics of f-Bloc ask elements:
(I) The two rows of elements at the base are known as the Actinoids Th(Z = 90) -Lr(Z = 103) and Lanthanoids Ce (Z = 58) – Lu (Z = 71
(ii) These two series of elements are referred to as Inner Transition Elements (f-Block Elements).
(iii) They are, for the most part, metals. Inside every series, the properties of the elements are very similar.
(iv) Most of the elements of the actinoid series are radio-dynamic.
- Metals
(I) Metals comprise over 78% of every known element and show up on the left side of the Periodic Table.
(ii) Metals are solids at room temperature.
(iii) Metal usually has high breaking and softening limits.
(iv) They have good conduction ability.
(v) They are pliable and malleable.
- Non-metals
(I) Non-metals are placed at the upper right-hand side of the periodic table..
(ii) Non-metals are often solids or gases at low temperatures with low liquefying and breaking points.
(iii) They are insulators and hence do not conduct.
(iv) When we move from top to bottom in a table, the non-metallic character tends to increase.
(v) Most non-metallic solids are weak and non-ductile.
- Metalloids
The elements (e.g., silicon, germanium, arsenic, etc.) having the characteristics of both metals and nonmetals are known as metalloids or semimetals.
- Inner Transition elements
– These elements occupy group 18 of the periodic table.
– Each period ends with a noble gas at last.
–They have gaseous nature, and because of the presence of all the occupied filled orbitals, they have almost no tendency to participate Actinoids Th (Z = 90) – Lr (Z = 103) and Lanthanoids Ce (Z = 58) – Lu (Z = 71)
(ii) These two particular series of elements are called Inner Transition Elements (f-Block Elements).
(iii) They are, for the most part, metals. Inside every series, the elements exhibit similar properties.
(iv) Most of the elements of the actinoid series are radio-dynamic.
- Noble Gases
– This category comprises the elements that fall in group 18 of the periodic table.
– Each period ends with the noble gas element.
– All the members are gaseous, and because of the presence of all the occupied filled orbitals, they have almost no tendency to take an interest
- Representative Elements
The elements of group 1, 2,and group 13 to 17 all together fall into this category. They are mainly the elements belonging to s-block and p-block of the perodic table.
- Transition Elements
All the d-block elements fall into transition elements, and they are present in the focal point of the periodic table among s and p-block elements.
- Inner Transition Elements
Actinoids and Lanthanoids (the fourteen elements after actinium) are generally called the inner transition elements and are the f-block elements.
After the uranium, the elements falling are called transuranic elements.
Periodic Trends in Properties of Elements
Variation in Physical Properties
Atomic Radii:The distance from the nucleus to the outermost shell having the electrons is termed as atomic radii .On the basis of the category that if the element is metal or nonmetal, the atomic radii is further subdivided into four types listed below.
(a) Metallic Radius
(b) vander Waals Radius
(c) Covalent radium
(d) Ionic radius.
(a) Covalent Radius: It is equivalent to half of the distance between the centers of the cores of two atoms held together by a simply covalent single bond.
(b) Ionic Radius: It is the effective distance from the nucleus of an element to a point till which it can create an bond.
(c) van der Waals Radius: Noble gases’s atoms are binded together by feeble van der Waals forces of fascination. The van der Waals radius is half the distance between the focal point of cores of atoms of noble gases.
(d) Metallic Radius: The distance between the nucleus of two adjacent metal ions in the metallic grid is termed as metallic radius.
- Atomic Radius in the Periodic Table
Variety in a Period: During a period, the atomic radii of the elements, for the most part, decreases from left to right.
Variety in a group: On moving downwards, The atomic radii of the elements in each group of the periodic table increases.
- Ionic Radius
By measuring the distances among cations and anions in ionic crystals, the ionic radius is calculated. As a rule, the ionic radii of elements display the same trend as the atomic radii.
Cation: The expulsion of an electron from a molecule results in the formation of a cation. The radius of a cation is always smaller than that of the particle.
Anion: Whenever an electron is gained, an anion is formed. The radius of the anion is always more significant than that of the molecule.
Isoelectronic Species:Atoms and ions having the same number of electrons are termed to be as isoelectronic species. For instance, O2-, F–, Na+, and Mg2+ have the same number of electrons (10). Due to their different nuclear charges, their radii would not be the same.
- Ionization Enthalpy
It is the vitality required to expel an electron from an isolated gaseous particle in its ground state.
M (gas) + I.E — – >M+ (gas) + e–
Ionization enthalpy’s unit is kJ mol-1, and the unit of ionization potential is electron volt per particle.
Successive Ionization Enthalpies
Successive Ionization enthalpies is a phenomena If a gaseous particle lose more than one electron, it can be removed in a steady progression.
- Ionization Enthalpies in the Periodic Table:
Ionization Enthalpy Along a Period
The ionization enthalpies of the elements decrease in moving from top to the base in any group. The underneath factors accounts for the decrease in Ionization enthalpy down the group
(I) There is an increase in the quantity of the fundamental vitality shells (n) moving from one element to the next.
(ii) Due to the gradual increase in the quantity of inner electrons, an increase in the magnitude of the screening effect can also be seen .
- Electron Gain Enthalpy
Electron Gain Enthalpy is the vitality released when an electron is added to an isolated gaseous particle to change over it into a negative particle. The process is represented as:
For the dominant part of the elements, the electron gain enthalpy is negative. For instance, halogens have profoundly negative electron gain enthalpy due to the reason that they can attain the nearest noble gas configuration by tolerating an additional electron.
Besides, noble gases accounts for positive electron gain enthalpies because of their stable configuration.. The electron added must be placed in the higher quantum energy level, subsequently producing profoundly unstable electronic configuration.
Successive Electron Gain Enthalpies
We have studied that electrons from gaseous atoms are lost in succession (i.e., consistently). Similarly, these are also accepted consistently, i.e., in succession.
After the addition of one electron, the molecule becomes negatively charged, and the second electron is to be added to a negatively charged particle. However, the addition of the second electron is opposed by electrostatic repulsion, and subsequently, the vitality has to be supplied for the second electron acquisition. and hence, the second electron gain enthalpy turns out to be positive for an element.
For instance, when an electron is added to an oxygen molecule to form an O–particle, vitality is released. In any case, when another electron is added to 0-particle to form O2-particle, vitality is absorbed to defeat the strong electrostatic repulsion between the negatively charged 0–particle and the second electron being added. Thus, the first electron gain enthalpy:
Dependence of Electron Gain Enthalpy
(i) Atomic size:As the size of a molecule increases, the distance between its nucleus and the approaching electron also increases, and electron gain enthalpy becomes less negative,
- ii) Nuclear charge:( With the increase in nuclear charge, a force of fascination between the nucleus and the approaching electron increases, and thus electron gain enthalpy becomes more negative.
(iii) The symmetry of the Electronic Configuration: The atoms with symmetrical configuration (having filled or half-filled orbitals in the same subshell) do not have any inclination to take up additional electrons because their configuration will get unstable.
In that case, the vitality will be needed, and electron gain enthalpy (Δ eg, H) will be positive. For instance, noble gas elements have positive electron gain enthalpies.
Variety of Electron Gain Enthalpy Across a Period
Electron gain enthalpy becomes more negative with an increase in the atomic number across a period.
Variety of Electron Gain Enthalpy in a Group
As we surf top to bottom, Electron gain enthalpy becomes less negative.
- Electronegativity
A subjective measure of the capacity of a molecule in a chemical compound to draw in shared electrons to itself is called electronegativity. In contrast to ionization enthalpy and electron gain enthalpy, it’s anything but a measurable amount.
In any case, various mathematical scales of electronegativity of elements viz, Pauling scale, Milliken-Jaffe scale, Allred Kochow scale have been developed. The electronegativity of some random element is not constant; it varies depending on the element to which it is bound.
Across a Period
On a horizontal plane from left to right, Electronegativity increases
In a Group
It decreases down a group.
- Periodic Changes in Chemical Properties along a Period
(I) Metallic character: Decrease across a period greatest on the extraordinary left (alkali metals).
(ii) Non-metallic character: Increases while surfing in a horizontal plane.
(iii) Basic nature of oxides: Decreases when travelling in a horizontal plane
(iv) Acidic nature of oxides: Increases from left
- Variation in a vertical plane
(I) Metallic character. For the most part, it increases because of the increase in atomic size and subsequently decreases in the ionization vitality of the elements in a group through and through.
(ii) Non-metallic character. For the most part, it decreases down a group. As electronegativity of elements decreases, start to finish in a group.
(iii) Basic nature of oxides. Since metallic character or electropositivity of elements increases in going start to finish in a group, the basic nature of oxidizing increases typically.
(iv) Acidic character of oxides. By and large decreases as the non-metallic character of elements decreases in going start to finish in a group.
(v) Reactivity of metals. By and large increases down a group. Since the tendency to lose electrons increases.
(vi) Reactivity of non-metals. By and large decreases down the group, Higher the electro-cynicism of non-metals, more prominent is their reactivity. Since electronegativity of non-metals in a group decreases through and through, their reactivity also decreases.
- Anomalous Properties of Second Period Elements
The first element of every one of group 1 (lithium) and 2 (beryllium) and group 13-17 (boron to fluorine) differs in many respects from different members of their respective groups. For instance, lithium is not at all like other alkali metals, and beryllium is not like other alkaline earth metals.
Form compounds which have significant covalent character; different members of these groups, predominantly form ionic compounds.
It has been noticed that some elements of the second period show similarities with the elements of the third period placed diagonally to one another, however having a place with different groups.
For instance,
This similarity in properties of elements placed diagonally to one another is called a diagonal relationship.
- Mendeleev’s Periodic Law. Periodic function of their atomic masses determines the properties of an element(chemical and physical both)
- Modern Periodic Law. Periodic function of their atomic numbers determines the properties of an element(chemical and physical both)
- Groups. There are 18 groups. These are vertical rows.
- Periods. There are seven periods. These are level rows.
- Representative Elements. The Representative element takes in account the S and P block elements.
- Transition Elements. They are also known as d-block elements. Their basic electronic configuration is (n – 1) d1-10 (ns0-2.)
- Inner Transition Elements. Actinoids and Lanthanoids are the inner transition elements having (n – 2) f1-14(n – 1) d0-1 ns2. as basic electronic configuration.
They are also called f-block elements.
- Metals. Present on the left portion of the periodic table. Comprise more than 78% of the known elements.
- Non-metals. Mostly located on the right-hand side of the periodic table.
- Metalloids. Elements which line as the borderline among metals and non-metals (e.g., Si, Ge, As) are called metalloids or semimetals.
- If a trip is taken down the group, Atomic Radii and Ionic Radii increase.
- Ionization Enthalpy. Ionization enthalpy decreases if we move top to bottom but increases if we move left to right in a group.
- Noble Gas Elements. Elements with symmetrical configuration are chemically inert.
- Electric Nuclear Charge. Z = Nuclear charge – Screening constant.
- Electronegativity. Increases along with a period decrease down the group,
- Chemical Reactivity. Chemical reactivity is highest at the two extremes of a period and lowest in the inside.
- Oxides of Elements. Elements on the left form basic oxides Oxides and elements on right forms acidic oxides.Oxides of elements in the inside are amphoteric or impartial.
Questions:
Q1: What are London dispersion forces?
Ans: London dispersion forces are those that happen due to the brief imbalances of charge happening inside an iota. The convergence of charge of the atoms undergoes constant changes as these electrons are always moving. This creates a quick shift in the general charge distribution of the molecule.
At the point when this molecule happened to interact with another, the transitory lopsidedness of charge will result in a fascination with positive and negative charges. These are the London dispersion forces.
Q2: Describe the physical characteristics of the modern periodic table. Is there any limitation to this table?
Ans: According to the modern periodic table, there are up to 18 groups and seven periods in the periodic table. Flat rows of the periodic table are called periods. The first period contains two elements and is the shortest period. The second period and third period have the same number of elements. Both fourth and fifth periods have 18 elements in it and thus, are known as extensive stretches.
With 32 elements, the sixth period becomes the longest in the whole table. Infact, the seventh period which has 19 electrons is still deficient.. Every vertical section is known as a group. Halogens, ore-forming elements, noble gases, alkali metals, and alkaline earth metals are also included in these groups.
The prolonged periodic table bears several limitations. One of them is the placement of hydrogen as In the table, the position of hydrogen continues to stay ambiguous. Without having a legitimate reason, the actinoids and the lanthanoids(inner transition elements) are placed at the bottom of the table as they do not have any specific position in the table. Ultimately, the element and it’s position in the periodic table can be found easily. You can do this using the electronic configuration of the element and the quantity of electrons in the valence shell. As an outcome, it makes it easy to find the period of the elements.