(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 configuration of the atom
(b) Chlorine, whose atomic number is 17, reacts with the element X to form a compound.
(i) What type of bond is formed between X and chlorine?
(ii) Explain how the bond between X and chlorine is formed
(iii) Write the formula of the compound formed and statelhree properties of the compound.
(c) Chlorine has two isotopes of mass numbers 35 and 37 respectively. Suggest the possible relative molar masses of a chlorine molecule.
(d) Calculate the mass of one atom of carbon, given that one mole of carbon weighs 12.0g. (L = 6.02 x 10\(^{23}\))
(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
(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
