What determines whether an oxide is acidic or basic?

The acidic or basic nature of an oxide is primarily determined by the electronegativity of the element bonded to oxygen. Generally, oxides of non-metals are acidic because the non-metal atom strongly attracts electrons, making the oxygen-non-metal bond covalent and polarizing the O-H bond in the corresponding oxyacid, releasing $H^+$. Oxides of metals are typically basic because metals have low electronegativity, leading to an ionic M-O bond. In water, this bond breaks to release $OH^-$ ions. Elements with intermediate electronegativity form amphoteric oxides, exhibiting both acidic and basic properties.

Why do some halides hydrolyze in water while others do not?

Hydrolysis of a halide in water depends on the nature of its bond and the availability of vacant orbitals on the central atom. Ionic halides (e.g., $NaCl$) generally dissolve without hydrolysis because the strong ionic bonds are simply solvated by water molecules. Covalent halides, especially those with a central atom having vacant d-orbitals (e.g., $SiCl_4$, $PCl_5$), readily hydrolyze. Water molecules act as nucleophiles, attacking the electron-deficient central atom and displacing the halogen, leading to the formation of oxyacids or hydroxides and hydrogen halides.

How do Fajan's rules apply to the properties of halides?

Fajan's rules help explain the degree of covalent character in compounds that are nominally ionic. For halides, these rules state that covalent character increases with: (1) smaller size and higher charge of the cation (increased polarizing power), and (2) larger size and higher charge of the anion (increased polarizability). For example, $AlF_3$ is largely ionic, but $AlCl_3$ exhibits significant covalent character due to the larger, more polarizable $Cl^-$ ion and the relatively small, highly charged $Al^{3+}$ cation, leading to its dimerization in the vapor phase.

What is the difference between a peroxide and a superoxide?

The key difference lies in the oxidation state of oxygen and the structure of the anion. In peroxides, oxygen has an oxidation state of -1, and the anion is $O_2^{2-}$ (e.g., $H_2O_2$, $Na_2O_2$). This ion has a single O-O bond. In superoxides, oxygen has an oxidation state of -1/2, and the anion is $O_2^-$ (e.g., $KO_2$). The superoxide ion has an unpaired electron, making it paramagnetic, and it is generally formed with larger alkali metal cations that can stabilize its larger size and lower charge density. Both are strong oxidizing agents.

Why does the basicity of metal hydroxides increase down a group in the periodic table?

As we move down a group, the atomic size of the metal increases, and its electronegativity decreases. This leads to a weaker attraction between the metal nucleus and the valence electrons, making the M-O bond in $M-OH$ compounds weaker and more ionic. Consequently, the $OH^-$ group is more easily released into solution, increasing the concentration of hydroxide ions and thus enhancing the basicity. For example, $CsOH$ is a stronger base than $LiOH$ because the $Cs-O$ bond is weaker and more ionic than the $Li-O$ bond.

Oxides, Hydroxides, Halides

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Oxides are binary compounds formed when an element reacts with oxygen. They are ubiquitous in nature and exhibit a wide range of chemical properties, from highly acidic to strongly basic, and even amphoteric or neutral. Hydroxides are compounds containing the hydroxyl group (OH $^-$ ) bonded to a metal or a non-metal, typically formed by the reaction of metal oxides with water or direct reaction of …

Quick Summary

Oxides are binary compounds of oxygen with another element, classified into acidic (non-metal oxides, e.g., $CO_2$ ), basic (metal oxides, e.g., $Na_2O$ ), amphoteric (e.g., $Al_2O_3$ ), and neutral (e.g., $CO$ ). Their nature depends on the electronegativity of the central atom; acidity increases across a period and basicity increases down a group. Peroxides ( $O_2^{2-}$ , O.S. -1) and superoxides ( $O_2^-$ , O.S. -1/2) are special types of oxides.

Hydroxides contain the $OH^-$ group. Metal hydroxides are typically basic, with basicity increasing down a group and decreasing across a period. Amphoteric hydroxides like $Al(OH)_3$ react with both acids and bases. Non-metal hydroxides are generally acidic (oxyacids).

Halides are compounds of elements with halogens. They can be ionic (e.g., $NaCl$ ) or covalent (e.g., $CCl_4$ ), depending on the electronegativity difference. Ionic halides are high-melting solids, while covalent halides are often liquids or gases.

Many covalent halides hydrolyze in water, especially if the central atom has vacant d-orbitals. Fajan's rules help predict the degree of covalent character based on ion size and charge. Understanding these classifications and trends is crucial for NEET.

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Key Concepts

Identifying Oxide Types

Classifying oxides is fundamental. Acidic oxides are typically non-metal oxides ( $CO_2$ , $SO_3$ ) or high…

Predicting Basicity of Hydroxides

The basicity of metal hydroxides is determined by the ease of releasing $OH^-$ ions. This is related to the…

Covalent Character in Halides

Even in compounds considered 'ionic', there's often some covalent character. Fajan's rules are key here. A…

Oxides: — Binary compounds with oxygen.

- Acidic: Non-metal oxides ( $CO_2$ , $SO_2$ ). - Basic: Metal oxides ( $Na_2O$ , $CaO$ ). - Amphoteric: $Al_2O_3$ , $ZnO$ . - Neutral: $CO$ , $NO$ , $N_2O$ . - Peroxides: $O_2^{2-}$ (O.S. -1), diamagnetic ( $Na_2O_2$ ). - Superoxides: $O_2^-$ (O.S. -1/2), paramagnetic ( $KO_2$ ).

Hydroxides: — Contain

OH^-

- Basicity: Increases down a group, decreases across a period. - Amphoteric: $Al(OH)_3$ , $Zn(OH)_2$ .

Halides: — Compounds with halogens.

- Ionic: Large electronegativity difference (e.g., $NaCl$ ). High MP/BP, soluble in water, no hydrolysis. - Covalent: Small electronegativity difference (e.g., $CCl_4$ ). Low MP/BP, may hydrolyze. - Hydrolysis: Occurs if central atom has vacant d-orbitals (e.g., $SiCl_4$ , $PCl_5$ ). $CCl_4$ does not hydrolyze. - Fajan's Rules: Covalent character $\propto$ (cation charge / cation size).

For Amphoteric oxides, remember Aluminum, Zinc, Lead, Sn (tin), Beryllium, Gallium: 'Always Zebras Look So Beautifully Gray'.