Revision: Classification of Elements and Periodicity in Properties JEE Main Classification of Elements and Periodicity in Properties

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.

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

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

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

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

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

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.

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.

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.

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.

OR

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

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

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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

• 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.

Factors affecting ionisation enthalpy:

• 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.

• 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.