Chemistry entrance exam, 2015 E.C. (2023)

Isotopes are atoms of the same element with the same number of protons and electrons but a different number of neutrons. Because chemical behaviour is decided by the electron arrangement, all isotopes of an element react the same way, so D is true.

A is wrong because the proton and electron counts are the same. B is wrong because the mass numbers differ. C is wrong because it is the neutron count that changes.

Textbook: Chemistry Grade 9, Unit 3 Structure of the Atom, section 3.5 Composition of an Atom and the Isotopes (page 87).

In the quantum mechanical model we cannot say exactly where an electron is. We can only give the probability of finding it in a region of space called an orbital, so A is correct.

B is Dalton's solid sphere model, C is Thomson's plum pudding model, and D is Bohr's model with fixed circular orbits.

Textbook: Chemistry Grade 11, Unit 1 Atomic Structure and Periodic Properties of the Elements, section 1.6 The Quantum Mechanical Model of the Atom (page 34).

For chlorine-35 the atomic number is 17 and the mass number is 35. The correct diagram must show 17 protons and 18 neutrons in the nucleus (35 minus 17 = 18) and 17 electrons arranged in shells as 2, 8, 7.

Any diagram with a different proton, neutron or electron count is wrong. The recorded answer is diagram C.

Textbook: Chemistry Grade 9, Unit 3 Structure of the Atom, section 3.5 Composition of an Atom and the Isotopes (page 87).

CaCl2 is made of a metal (calcium) and a non-metal (chlorine). Calcium gives away its two valence electrons to two chlorine atoms, forming Ca2+ and Cl- ions held together by electrostatic attraction. That is an ionic compound.

CO2, PCl3 and H2O2 are each built from non-metal atoms only, which share electrons, so they are covalent compounds.

Textbook: Chemistry Grade 9, Unit 5 Chemical Bonding, section 5.2 Ionic Bonding (page 146).

Group VIIA, period 3 is chlorine, a halogen and a reactive non-metal. Non-metal oxides are acidic, and chlorine forms acidic oxides such as Cl2O7, which gives HClO4 in water. So C is correct.

A is wrong because a halogen gains one electron and forms a 1- ion, not 2-. B and D are wrong because halogens have high ionization energy and high electron affinity.

Textbook: Chemistry Grade 9, Unit 4 Periodic Classification of Elements, section 4.4 The Major Trends in the Periodic Table (page 128).

Dissolving a non-volatile solute changes the solvent's physical properties: the vapor pressure and chemical potential fall, the boiling point rises and the freezing point drops.

Dissolving is a physical process, so the solvent molecules themselves are unchanged. The chemical identity of the solvent stays the same, which makes C the property that is not affected.