Page 32 - ACE YR IGCSE A TOP APPROACH TO CHEM

(b) Period number can be shown by the number of Even though uranium is a metal, it is a poor
shells. [1] conductor of electricity.
The electron distributions of all elements show (ii) Basic / dissolve in acid / reacts with acid [1]
that there are three shells. [1] Uranium is a metal. Metal oxide is basic.
(c) The noble gas [1] 8 (a) Metal ions [1]
Z = 2,8,8. Elements of Group 0. are arranged in lattice [1]
(d) Has stable outer shell [1] with a sea of electrons / delocalised electrons.
(e) (i) Number of protons and number of [1]
neutrons [1] + – + – – + – + – + – +
– + + – + – + – + – +
(ii) 14 [1] – – – – – – – –
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27 – 13 = 14 – + – + – + – + – – + – + – –
(iii) 13 [1] + – + + – + – + +
Number of protons = number of electrons (b) They have mobile electrons. [1]
(iv) Any one from: [1] The moving electrons allow the current to pass
• High melting point / boiling point through.
• Strong / hard (c) Chlorine / Cl [1]
• Shiny At room temperature, halogens are all diatomic.
• Malleable / Ductile Same to these common elements: O , N , H .
• Sonorous 2 2 2
• Conducts electricity (d) Has stable electron arrangement. [1]
Valency is the number of electrons its atoms lose,
7 (a) (i) Krypton [1] gain or share, to form a compound. Argon has a full
Uranium has the atomic number of 92. outer shell. It does not lose, gain, or share.
Uranium is split into barium and Z.
Barium has the atomic number of 56. (e) Atoms are bonded with strong covalent bonds
92 – 56 = 36 [1]
Refer to the Periodic Table, the element with in a giant lattice / macromolecule / giant
the atomic number of 36 is krypton. structure [1]
(ii) Period 4 [1] (f) Any two from: [2]
Refer to the Periodic Table, krypton is located • NaCl is an ionic compound and PCl is a
3
in Period 4. covalent compound.
(iii) 8 [1] • The ionic bond is strong in NaCl and the
Krypton is in Group VIII. So, it has 8 electrons intermolecular force is weak between PCl 3
in its outer shell. molecules.
For questions (a)(ii) and (a)(iii), the electronic • PCl reacts with water and NaCl does not.
configuration can show the answer. 3
The electronic configuration for krypton is (g) MgO with a dilute HCl / H SO / HNO / Acid /
3
2
4
2,8,18,8 (total is 36). Any acidic oxide and NaOH / Alkali / Base /
There are 4 numbers in the configuration Any basic oxide [1]
means it has 4 shells. Thus, it is located in MgO will neutralise the acid but not NaOH /
Period 4. Alkali / Base / Any basic oxide [1]
The last number of electronic configuration Note: Must always include the expected result.
shows the number of outer shell electrons.
(h) 2+ 2–
(b) (i) Radioisotopes [1]
(ii) 235 U: 92, 143, 92 [1] Mg O
238 U: 92, 146, 92 [1]
In an atom, the number of protons and the Mg with 8 electrons (represented by dots) in
number of electrons must be the same. the outer shell. [1]
Calculate the number of neutrons: O with 6 electrons (represented by crosses) and
235 – 92 = 143 2 electrons gained (represented by dots) [1]
238 – 92 = 146 Correct charge [1]
(c) (i) Any two from: [2] Note: Mark all the electrons using either dots or
• High melting point / boiling point crosses.
• Shiny 9 (a) C [1]
• Malleable Ionic bond is strong. [1]
• Ductile Ionic compounds do not conduct electricity in
• High density solids because ions cannot move. [1]
• Hard

Cambridge IGCSE TM
144 Ace Your Chemistry

Answers.indd 144 3/4/22 3:54 PM