The classification of elements into metals and non-metals is based on their physical and chemical properties, which are determined by their electronic configurations. In the modern periodic table, a jagged diagonal line of metalloids separates metals (located on the left and center) from non-metals (located on the upper right).
Electronic Configuration and Chemical Nature
Metals (Electropositive Elements)
Metals typically possess 1, 2, or 3 electrons in their outermost valence shell (with exceptions like Hydrogen and Helium). To achieve a stable noble gas configuration, they readily lose these valence electrons, forming positively charged ions called cations. This characteristic makes them highly electropositive.
Non-metals (Electronegative Elements)
Non-metals generally have 4, 5, 6, or 7 electrons in their valence shell. They tend to gain or share electrons to complete their octet, forming negatively charged ions called anions. This tendency to attract electrons makes them highly electronegative.
Physical Properties: A Comparative Analysis
The structural differences at the atomic level translate into contrasting physical characteristics.
| Property | Metals | Non-metals |
| Physical State | Solid at room temperature (except Mercury, which is liquid). | Can be solids (Iodine, Sulfur), liquids (Bromine), or gases (Oxygen, Nitrogen) at room temperature. |
| Lustre | Possess a metallic shine and can be polished. | Generally dull and do not reflect light (except Diamond and Iodine). |
| Malleability & Ductility | Highly malleable (beaten into sheets) and ductile (drawn into wires). Gold and Silver are the most malleable. | Brittle in solid form; they break into pieces when stressed. They are neither malleable nor ductile. |
| Conductivity | Excellent conductors of heat and electricity due to the presence of free mobile electrons. | Poor conductors of heat and electricity due to the absence of free electrons (except Graphite). |
| Density & Melting Points | High density and high melting/boiling points (except alkali metals like Sodium and Potassium). | Low density and low melting/boiling points (except Diamond, a form of Carbon). |
| Sonority | Sonorous; they produce a ringing sound when struck. | Non-sonorous. |
Chemical Properties: Reactivity and Bonding
The chemical behavior of these elements dictates how they interact with oxygen, water, acids, and each other.
Reaction with Oxygen
Metals react with oxygen to form basic oxides. For example, Magnesium burns in oxygen to form Magnesium Oxide (MgO), which turns red litmus paper blue. Some metals like Aluminum and Zinc form amphoteric oxides that react with both acids and bases. Non-metals react with oxygen to form acidic or neutral oxides. Sulfur dioxide (SO2) is acidic, whereas Carbon Monoxide (CO) and Nitrous Oxide (N2O) are neutral.
Reaction with Water and Acids
Reactive metals displace hydrogen from water and dilute mineral acids (like HCl and H2SO4) to release Hydrogen gas (H2). Non-metals do not displace hydrogen from water or dilute acids because they cannot donate electrons to reduce hydrogen ions (H^+) into hydrogen gas.
Nature of Chemical Bonding
When metals react with non-metals, transfer of electrons takes place, resulting in the formation of strong ionic (electrovalent) bonds. When non-metals react with other non-metals, they share electrons, resulting in the formation of covalent bonds.
UPSC Prelims Facts and Exceptional Cases
- Alkali Metals Softness: Sodium (Na) and Potassium (K) are metals but are so soft that they can be easily cut with a knife. They also have low densities and float on water before reacting.
- The Exceptional Non-metal Conductor: Graphite, an allotrope of carbon, is a non-metal that conducts electricity efficiently due to its hexagonal layered structure containing delocalized free electrons.
- Highest and Lowest Melting Points: Tungsten (W) has the highest melting point among all metals (≈ 3422°C), making it ideal for bulb filaments. Carbon (in the form of diamond) has the highest melting point among non-metals (≈ 3550°C).
- Liquid Non-metal: Bromine (Br) is the only non-metal that exists as a reddish-brown liquid at room temperature.
- Anomalous Lustre: Iodine (I) is a non-metal belonging to the halogen group, yet it exhibits a distinct, shiny metallic lustre.
- Oxidizing vs Reducing Nature: Due to electron loss, metals act as excellent reducing agents (Lithium is the strongest in aqueous solution). Non-metals act as excellent oxidizing agents (Fluorine is the strongest).