Periodic Table: Groups, Periods and Chemical Classification of Elements

The Periodic Table arranges all 118 known chemical elements systematically based on atomic number and recurring chemical properties. Elements lie within 7 horizontal periods and 18 vertical groups. The Modern Periodic Law states that physical and chemical properties of elements are periodic functions of their atomic numbers. This structure groups elements into four distinct electronic blocks according to the subshell hosting their valence electrons.

Organization of Periods and Groups

The layout of the modern periodic table reflects the underlying atomic structure and electron shell filling order governed by the Aufbau principle.

Horizontal Periods

• Period 1: Contains 2 elements (H and He); fills the 1s orbital.
• Periods 2 and 3: Known as short periods; contain 8 elements each, filling 2s, 2p and 3s, 3p subshells.
• Periods 4 and 5: Known as long periods; contain 18 elements each, introducing the 3d and 4d transition series.
• Periods 6 and 7: Known as very long periods; contain 32 elements each, accommodating the 4f (Lanthanoids) and 5f (Actinoids) inner transition series.

Vertical Groups

The 18 vertical columns group elements with identical valence shell electronic configurations and similar chemical reactivities.

Classification by Subshell Blocks

The periodic table splits into four primary blocks based on the orbital receiving the last valence electron.

s-Block Elements

• Comprises Group 1 (Alkali Metals) and Group 2 (Alkaline Earth Metals), plus Hydrogen and Helium.
• All s-block elements are electropositive metals (except Hydrogen and Helium).
• They have low ionization enthalpies and act as strong reducing agents.
• They impart characteristic flame colors due to low excitation energies.

p-Block Elements

• Comprises Groups 13 to 18.
• Contains a mix of metals, all known metalloids, and all non-metals.
• Elements exhibit the inert pair effect, where heavier elements prefer oxidation states two units lower than their maximum group valence.

d-Block Elements

• Comprises Groups 3 to 12.
• Known as transition metals because they bridge the electropositive s-block and electronegative p-block.
• They possess unpaired electrons in (n-1)d subshells, leading to paramagnetism, high tensile strength, high melting points, and interstitial compound formation.

f-Block Elements

• Located separately at the bottom in two horizontal rows: Lanthanoids (Z = 58 to 71) and Actinoids (Z = 90 to 103).
• Lanthanoids show the Lanthanoid Contraction, where poor shielding by 4f electrons causes a steady decrease in atomic and ionic radii.
• All Actinoids are radioactive, and all elements beyond Uranium (Z > 92) are synthetic transuranic elements.

Chemical Classification by Metallic Character

Elements divide into three broad categories based on electrical conductivity, ionization potential, and physical properties.

Periodic Trends Across Rows and Columns

Systematic variations in atomic and electronic configurations dictate the direction of periodic properties.

• Atomic and Ionic Radii: Increase down a group due to the addition of electron shells; decrease across a period from left to right due to increased effective nuclear charge (Z_eff).
• Ionization Enthalpy: Decreases down a group; increases across a period.
• Electronegativity: Decreases down a group; increases across a period (Fluorine is the most electronegative element with a Pauling value of 4.0).
• Electron Gain Enthalpy: Becomes more negative across a period; Chlorine has a more negative electron gain enthalpy than Fluorine due to less inter-electronic repulsion in its 3p orbital.
• Metallic Nature: Increases down a group; decreases across a period. Non-metallic character increases across a period.

Important Facts

• Dmitri Mendeleev published the first widely accepted Periodic Table in 1869 based on atomic mass, correctly predicting the properties of undiscovered elements like Eka-Boron (Scandium), Eka-Aluminium (Gallium), and Eka-Silicon (Germanium).
• Henry Moseley established in 1913 that atomic number (Z), rather than atomic mass (A), is the fundamental property determining periodicity through X-ray spectra studies (sqrt{nu} = a(Z – b)).
• Glenn T. Seaborg redesigned the periodic table in 1944 by placing the Actinoid series below the Lanthanoid series.
• Francium (Fr) is the most electropositive naturally occurring element, while Fluorine (F) is the most electronegative element.
• Mercury (Hg) is the only liquid metal at standard room temperature (25°C), while Bromine (Br) is the only liquid non-metal.
• Gallium (Ga) and Cesium (Cs) have melting points slightly above room temperature and turn into liquids just above 28°C to 30°C.
• Osmium (Os) and Iridium (Ir) have the highest calculated densities among all elements, each exceeding 22.5 g/cm³.
• Helium (He) possesses the highest first ionization enthalpy among all known chemical elements.
• Promethium (Pm, Z = 61) and Technetium (Tc, Z = 43) are the only elements below atomic number 83 that have no stable isotopes.
• Element 118, Oganesson (Og), is the heaviest element confirmed on the periodic table and completes the 7th period.
• The diagonal relationship occurs in the periodic table between second and third period elements (such as Li–Mg, Be–Al, and B–Si) due to similar charge-to-radius ratios.
• Hydrogen has no fixed single position; it resembles Group 1 alkali metals in forming unipositive ions and Group 17 halogens in forming diatomic molecules and hydride ions.