Revision: Class 11 >> Classification of Elements and Periodicity in Properties NEET (UG) Classification of Elements and Periodicity in Properties

Definitions [14]

Definition: Modern Periodic Table

A tabular arrangement of elements in groups (vertical columns) and periods (horizontal rows), highlighting the regular trends in properties of elements, is called a Periodic Table.

The classification of elements resulting from an arrangement of the elements in an increasing order of their atomic numbers is the modern periodic table.

Define the atomic size.

Atomic size is the distance between the center of an atom i.e., from the nucleus to the outermost shell (valence shell) of that atom.

Define periods of modern periodic table.

The horizontal rows are called periods. The table consists of 7 periods, numbered from 1 to 7 from top to bottom.

Define:

Periodic table

The periodic table is a tabular arrangement of elements in horizontal rows, called periods, and vertical columns, called groups, to classify elements and facilitate their systematic study.

Define groups of modern periodic table.

In the modern periodic table, groups (also known as families) are the 18 vertical columns that organize chemical elements by shared properties. Unlike periods, which are horizontal rows representing electron shells, groups are vertical columns of elements with similar chemical properties.

Definition: Elements

Elements are pure substances made up of one type of atom.

Definition: Ionic Compounds

The compounds formed in this manner by the transfer of electrons from a metal to a non-metal are known as ionic compounds or electrovalent compounds.

Define electronegativity.

Electronegativity is defined as the relative tendency of an element present in a covalently bonded molecule, to attract the shared pair of electrons towards itself.

Definition: Atomic Radius

The average distance between center of nucleus of an atom to outermost shell electron is called as Atomic Radius.

Definition: Ionic Radius

The ionic radius is the average distance between the center of nucleus of an ion and its outermost shell containing electron.

Definition: Ionization Enthalpy

The minimum amount of energy required to remove an electron from the isolated gaseous atom is called ionisation enthalpy.

Definition: Electron Gain Enthalpy

Electron gain enthalpy is the enthalpy change that occurs when an isolated gaseous atom accepts an electron to form an anion.

Definition: Electronegativity

The tendency of an atom in a molecule to attract the shared pair of electrons towards itself is called its electronegativity.

Definition: Atomic Size (Atomic Radius)

It is the distance between the centre of the nucleus of an atom and its outermost shell.

Theorems and Laws [1]

Law: Modern Periodic Law

The modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers.
Atomic number is more fundamental than atomic mass.
The modern periodic table is based on electronic configuration.
It explains the periodic repetition of properties after regular intervals.
Elements are arranged in increasing order of atomic number.
This law removed the defects of Mendeleev’s periodic table.

Key Points

Key Points: Significance of Classification of Elements

Classification arranges elements systematically according to similar properties.
It makes the study of a large number of elements easier.
Elements with similar chemical behaviour are placed in the same group.
It helps in predicting properties of unknown or newly discovered elements.
It shows periodic trends such as atomic size, ionisation enthalpy and electronegativity.
Modern classification is based on atomic number and electronic configuration.

Key Points: Genesis of Periodic Classification

Early classification was based on atomic masses and similar properties.
Dobereiner arranged elements in triads.
Newlands proposed the Law of Octaves.
Mendeleev arranged elements in order of increasing atomic mass.
The modern periodic table arranges elements according to increasing atomic number.
The modern periodic table removed many defects of older classifications.

Key Points: Modern Periodic Table

Dobereiner grouped elements in threes (triads) with similar properties and a pattern in atomic masses.
Newlands found that every 8th element shared similar properties (the Law of Octaves).
Mendeleev arranged elements by atomic mass and predicted new elements, but couldn’t explain isotopes and rare earths.
Moseley fixed the flaws by arranging elements by atomic number, forming the modern periodic table.
Bohr proposed the long-form periodic table based on electron arrangement.

Key Points: Insights into Mendeleev’s Periodic Table

Mendeleev corrected atomic masses to correctly place elements.
He predicted new elements like Sc, Ga, and Ge with correct properties.
He later added a zero group for noble gases.
Co and Ni have similar masses, which can cause confusion about their order.
Isotopes couldn’t be placed due to different masses.
Atomic mass gaps are uneven, so new elements couldn’t be predicted.
Hydrogen's position is unclear—it’s like both alkali metals and halogens.

Key Points: Electric Configurations of Elements and the Periodic Table

Electronic configuration determines the position of an element in the periodic table.
Elements in the same group have similar valence-shell configurations.
Similar valence-shell configurations give similar chemical properties.
The period number is determined by the highest principal quantum number n present.
The block is determined by the subshell that receives the last electron.

General electronic configurations:

s-block: ns¹ or ns²
p-block: ns²np¹ to ns²np⁶
d-block: (n−1)d¹⁻¹⁰ns⁰⁻²
f-block: (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns²

Key Points: Electronic Configurations

Metals react by losing electrons to form positively charged ions (cations), while non-metals gain electrons to form negatively charged ions (anions), in order to attain a stable electronic configuration (octet or duplet).
Ionic compounds consist of aggregates of oppositely charged ions held together by strong electrostatic forces of attraction, and do not exist as molecules.

Key Points: The s-Block Elements

Includes Group 1 and Group 2 elements.
Group 1 elements are alkali metals.
Group 2 elements are alkaline earth metals.
General configuration: ns¹ or ns².
They are highly electropositive and metallic.
They form basic oxides and ionic compounds.
They have low ionisation enthalpy.
Reactivity generally increases down the group.

Key Points: The p-Block Elements

Includes Groups 13 to 18.
General configuration: ns²np¹ to ns²np⁶.
Contains metals, non-metals and metalloids.
Group 17 elements are halogens.
Group 18 elements are noble gases.
Shows variable oxidation states.
Metallic character decreases from left to right.
Non-metallic character increases from left to right.

Key Points: The d-Block Elements

Elements of Groups 3 to 12 are d-block elements.
General configuration: (n−1)d¹⁻¹⁰ns⁰⁻².
They are called transition elements.
They show variable oxidation states.
They form coloured compounds.
They often act as catalysts.
They form complex compounds.
They are generally hard metals with high melting points.

Key Points: The f-Block Elements

The f-block includes lanthanoids and actinoids.
Last electron enters the f-orbital.
General configuration involves (n−2) f orbitals.
Lanthanoids have atomic numbers 58–71.
Actinoids have atomic numbers 90–103.
They show variable oxidation states.
Actinoids are mostly radioactive.
They are placed separately to keep the periodic table compact.

Key Points: Metals, Non-metals and Metalloids

Metals are generally present on the left side of the periodic table.
Non-metals are mainly on the right side.
Metalloids lie along the zig-zag line.
Metals lose electrons and form cations.
Non-metals gain electrons and form anions.
Metallic character decreases across a period.
Metallic character increases down a group.
Metalloids show intermediate properties.

Key Points: Periodic Trends in Properties

Periodic trends are regular changes in properties across periods and down groups.
These trends depend on atomic number and electronic configuration.
Across a period, the effective nuclear charge increases.
Down a group, the number of shells increases.
Important trends include atomic radius, ionic radius, ionisation enthalpy, electron gain enthalpy and electronegativity.
Periodic trends help predict chemical reactivity.

Key Points: Atomic Radius

Atomic radius is the distance from the nucleus to the outermost shell.
Across a period, atomic radius decreases.
This is due to increase in effective nuclear charge.
Down a group, atomic radius increases.
This is due to addition of new shells.
Types include covalent radius, metallic radius and van der Waals radius.
Van der Waals radius is usually larger than covalent radius.

Key Points: Ionization Enthalpy

Factors affecting ionisation enthalpy:

Factor	Change	Effect on Ionisation Enthalpy
Atomic size	Increases	Ionisation enthalpy decreases
Atomic size	Decreases	Ionisation enthalpy increases
Nuclear charge	Increases	Ionisation enthalpy increases
Nuclear charge	Decreases	Ionisation enthalpy decreases
Screening effect	Increases	Ionisation enthalpy decreases
Screening effect	Decreases	Ionisation enthalpy increases
Penetration effect of electrons	Increases	Ionisation enthalpy increases
Penetration effect of electrons	Decreases	Ionisation enthalpy decreases

Key Points: Electronegativity

Electronegativity is the ability of an atom to pull shared electrons; it is highest for fluorine (4.0).
It increases across a period (left to right) and decreases down a group (top to bottom).
Non-metals have high electronegativity (gain electrons), while metals have low electronegativity (lose electrons).
Greater electronegativity difference increases ionic character.

Key Points: Valency

Valency is the combining capacity of an atom, determined by valence electrons.
Valence electrons are electrons present in the outermost shell.
For s- and p-block elements, valency = number of valence electrons or 8 valence electrons.
For d- and f-block elements, valency depends on both valence shell and inner d/f electrons.
Transition elements (d-block) commonly show variable valency, but general values are 2 and 3.

Key Points: Structure of the Modern Periodic Table

The modern periodic table has 7 periods, 18 groups, and 118 elements.
A zig-zag line separates metals, metalloids, and nonmetals.
Periods are horizontal rows.
Groups are vertical columns.
Period number represents the number of shells.
The group number generally represents the valence electrons of s- and p-block elements.
Elements are divided into s, p, d and f blocks.
Lanthanoids and actinoids are placed separately at the bottom.

Key Points: Physical Properties of Halogen Derivatives

Alkyl halides are colourless when pure but develop colour on exposure to light; they have a sweet smell
Polarity: Moderately polar
Solubility: Insoluble in water, soluble in organic solvents

Bond Length

Increases in order: R–F < R–Cl < R–Br < R–I

Bond Strength

Decreases in order: R–F > R–Cl > R–Br > R–I

Boiling Point

Order: R–I > R–Br > R–Cl > R–F
Increases with an increase in molecular mass
Decreases with branching

Dipole Moment

Decreases as the electronegativity of halogen decreases

Isomeric Dihalobenzenes

Boiling point: nearly the same
Melting point: para > ortho/meta

Key Points: Atomic Size

Atomic radius is the distance from the nucleus to the outermost shell of an atom.
Across a period, atomic size decreases due to increase in effective nuclear charge.
Down a group, atomic size increases due to addition of new shells.
Atomic size depends on number of shells and nuclear charge.
Cations are smaller than their parent atoms due to loss of electrons.
Anions are larger than their parent atoms due to gain of electrons.
In isoelectronic species, greater nuclear charge leads to smaller size.
Noble gases show larger atomic size due to consideration of van der Waals radius.

Redox Reactions and Electrochemistry Purification and Characteristics of Organic Compounds

Revision: Class 11 >> Classification of Elements and Periodicity in Properties NEET (UG) Classification of Elements and Periodicity in Properties

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Definitions [14]

Theorems and Laws [1]

Key Points

Concepts [29]

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