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Elements, Compounds and Mixtures - Chemical Formulae of Compounds

Grade 6ICSE

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

🔑Concepts

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A Chemical Formula is a symbolic representation of one molecule of an element or a compound. It uses symbols of elements and numerical subscripts to show the proportion of atoms.

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A Compound is a substance formed when two or more elements combine chemically in a fixed ratio by mass. For example, H2OH_2O always contains two atoms of Hydrogen for every one atom of Oxygen.

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The Subscript in a formula (the small number written at the bottom right of a symbol) indicates the number of atoms of that specific element present in the molecule. If no number is written, it is assumed to be 11.

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Atomicity refers to the total number of atoms present in one molecule of an element or compound. For example, the atomicity of Ozone (O3O_3) is 33.

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In a chemical formula, the metal (or more electropositive element) is usually written first, followed by the non-metal. For example, in Sodium Chloride (NaClNaCl), Sodium (NaNa) is the metal.

📐Formulae

H2O (Water)H_2O \text{ (Water)}

CO2 (Carbon Dioxide)CO_2 \text{ (Carbon Dioxide)}

NaCl (Sodium Chloride - Common Salt)NaCl \text{ (Sodium Chloride - Common Salt)}

MgO (Magnesium Oxide)MgO \text{ (Magnesium Oxide)}

NH3 (Ammonia)NH_3 \text{ (Ammonia)}

CH4 (Methane)CH_4 \text{ (Methane)}

CaCO3 (Calcium Carbonate)CaCO_3 \text{ (Calcium Carbonate)}

HCl (Hydrochloric Acid)HCl \text{ (Hydrochloric Acid)}

💡Examples

Problem 1:

How many atoms of each element are present in a molecule of Sodium Carbonate (Na2CO3Na_2CO_3)?

Solution:

Sodium (NaNa) = 22 atoms, Carbon (CC) = 11 atom, Oxygen (OO) = 33 atoms.

Explanation:

The subscripts tell us the quantity: NaNa has a subscript of 22, CC has no subscript (which means 11), and OO has a subscript of 33.

Problem 2:

Calculate the total atomicity of a Glucose molecule, represented by the formula C6H12O6C_6H_{12}O_6.

Solution:

Atomicity = 6+12+6=246 + 12 + 6 = 24

Explanation:

To find the atomicity, we sum the number of atoms of all elements: 66 Carbon atoms + 1212 Hydrogen atoms + 66 Oxygen atoms = 2424 atoms.

Problem 3:

Write the chemical formula for a compound containing one atom of Magnesium and two atoms of Chlorine.

Solution:

MgCl2MgCl_2

Explanation:

We write the symbol for Magnesium (MgMg) first and Chlorine (ClCl) second. Since there are two Chlorine atoms, we add the subscript 22 to ClCl.