(a) An element X is represented as \(^{40}_{20} X\) (i) How many electrons and how many neutrons are present in the atom of X? (ii) Write the electronic con...

(a) The element X, represented as ⁴⁰₂₀X (atomic number 20, mass number 40)

• (i) Electrons and neutrons: number of electrons = atomic number = 20. Number of neutrons = mass number - atomic number = 40 - 20 = 20.
• (ii) Electronic configuration: 2, 8, 8, 2 (that is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²). X is calcium.

(b) Reaction with chlorine (atomic number 17)

• (i) The bond formed is an electrovalent (ionic) bond.
• (ii) X has 2 electrons in its outermost shell and each chlorine atom needs 1 electron to complete its octet. The X atom loses its 2 outer electrons to become X²⁺, and two chlorine atoms each gain one electron to become Cl^-. The oppositely charged ions are held together by strong electrostatic attraction.
• (iii) Formula of the compound: XCl₂ (calcium chloride, CaCl₂). Three properties: it is a crystalline solid with a high melting point; it is soluble in water; and it conducts electricity when molten or in aqueous solution.

(c) Possible relative molar masses of a chlorine molecule

Chlorine exists as Cl₂. Using isotopes of mass 35 and 37 the possible molecular masses are:

• 35 + 35 = 70
• 35 + 37 = 72
• 37 + 37 = 74

(d) Mass of one atom of carbon

1 mole (6.02 × 10²³ atoms) weighs 12.0 g, so:

Mass of one atom = 12.0 ÷ (6.02 × 10²³) = 1.99 × 10^-23 g