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Nature of Matter: Elements, Compounds, and Mixtures - Elements

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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An element is a pure substance that consists of only one type of atoms and cannot be broken down into simpler substances by physical or chemical means.

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Elements are categorized into three main types: Metals, Non-metals, and Metalloids.

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Metals are generally solids (except Mercury, HgHg), lustrous, malleable, ductile, and good conductors of heat and electricity.

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Non-metals can be solids, liquids, or gases; they are non-lustrous (except Iodine, I2I_2, and Graphite), brittle, and poor conductors (except Graphite).

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Metalloids (e.g., Silicon SiSi, Germanium GeGe) exhibit properties of both metals and non-metals.

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Atomicity refers to the number of atoms present in one molecule of an element. Examples include monoatomic (HeHe), diatomic (O2O_2), triatomic (O3O_3), and polyatomic (P4P_4, S8S_8).

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Chemical symbols are abbreviations used to represent elements. For example, Hydrogen is HH, Sodium is NaNa (from Latin Natrium), and Iron is FeFe (from Latin Ferrum).

📐Formulae

Atomicity=Molecular MassAtomic Mass\text{Atomicity} = \frac{\text{Molecular Mass}}{\text{Atomic Mass}}

Representation of an atom: ZAX\text{Representation of an atom: } ^{A}_{Z}X

where A=Mass Number, Z=Atomic Number\text{where } A = \text{Mass Number, } Z = \text{Atomic Number}

💡Examples

Problem 1:

Identify the atomicity of Oxygen (O2O_2), Phosphorus (P4P_4), and Helium (HeHe).

Solution:

Oxygen: Diatomic; Phosphorus: Tetra-atomic; Helium: Monoatomic.

Explanation:

Oxygen exists as a molecule with 22 atoms, Phosphorus with 44 atoms, and Helium exists as single atoms.

Problem 2:

If the atomic mass of Nitrogen is 14 u14\text{ u} and its molecular mass is 28 u28\text{ u}, calculate its atomicity.

Solution:

Atomicity=2814=2\text{Atomicity} = \frac{28}{14} = 2

Explanation:

By dividing the molecular mass by the atomic mass, we find that a Nitrogen molecule contains 22 atoms, making it diatomic (N2N_2).

Problem 3:

Classify the following as Metal, Non-metal, or Metalloid: Copper (CuCu), Silicon (SiSi), and Chlorine (ClCl).

Solution:

Copper: Metal; Silicon: Metalloid; Chlorine: Non-metal.

Explanation:

CuCu is a good conductor and malleable; SiSi has intermediate properties; ClCl is a gas and a poor conductor.