Atoms and molecules and chemical reactions

Na

Step 1: Some elements have symbols derived from their Latin or ancient names rather than their English names. Step 2: Sodium's symbol 'Na' comes from its Latin name 'Natrium'. Step 3: This follows the rule established by Berzelius for creating element symbols. Step 4: The symbol is always written with the first letter capitalized and second letter (if any) in lowercase. Step 5: Other examples include Au for gold (Aurum), Fe for iron (Ferrum), and K for potassium (Kalium).

Combination reaction

Step 1: In this reaction, two reactants (magnesium and oxygen) combine to form a single product (magnesium oxide). Step 2: A combination reaction occurs when two or more substances react to form one new compound. Step 3: The general pattern is A + B → AB, which matches our reaction pattern. Step 4: When magnesium burns in air, it combines with oxygen to produce the white powder magnesium oxide. Step 5: Other reaction types are incorrect: decomposition breaks compounds apart, displacement involves one element replacing another, and double displacement involves exchange of ions.

It decomposes into calcium oxide and carbon dioxide

Step 1: When calcium carbonate (limestone) is heated, it undergoes thermal decomposition. Step 2: The reaction is: CaCO₃ → CaO + CO₂ (in presence of heat). Step 3: This breaks down one compound into two simpler substances - calcium oxide (quicklime) and carbon dioxide gas. Step 4: The CO₂ gas can be tested by bringing a burning match stick near it - the flame gets extinguished. Step 5: This is a decomposition reaction because one reactant breaks down into two or more products when energy (heat) is supplied.

18u

Step 1: Molecular mass is the sum of atomic masses of all atoms in a molecule. Step 2: Water (H₂O) contains 2 hydrogen atoms and 1 oxygen atom. Step 3: Calculate: 2 × (atomic mass of H) + 1 × (atomic mass of O). Step 4: Substituting values: 2 × 1u + 1 × 16u = 2u + 16u = 18u. Step 5: Therefore, the molecular mass of water is 18 unified mass units. This means 18g of water contains one mole of water molecules.