S1.1 Introduction to the particulate nature of matter

In the notation ${}_{27}^{59}\text{Co}$, the bottom number (27) is the atomic number (protons) and the top number (59) is the mass number (nucleons = protons + neutrons).

Number of neutrons = mass number − atomic number = $59-27=32$ neutrons.

The isotope contains 27 protons and 32 neutrons, which matches the option '27 protons and 32 neutrons'.

We need to correctly interpret the nuclear notation ${}_{27}^{59}\text{Co}$ in terms of protons and neutrons.

This confuses the mass number with the neutron number. The mass number (59) equals protons plus neutrons, not neutrons alone. This option is incorrect.

This reverses the values. The atomic number (bottom) gives protons = 27, not 32. This option is incorrect.

While 59 nucleons is correct, 27 refers to protons, not neutrons. The neutron count is $59-27=32$. This option is incorrect.

'27 protons and 32 neutrons' correctly identifies the atomic number as 27 and calculates neutrons as $59-27=32$.

According to the kinetic molecular theory, liquids have fixed volume but no fixed shape. Their particles are close together but not in a regular arrangement.

Liquid particles experience moderate intermolecular forces — strong enough to keep them close, but weak enough to allow movement. They slide past each other, which explains why liquids flow.

The statement 'Particles are close together in a disordered arrangement and can slide past one another' accurately describes a liquid's particle behaviour.

We need to identify which description matches the particle behaviour of a liquid, not a solid or gas.

This describes a gas, where intermolecular forces are negligible and particles are widely spaced.

This describes a solid, where strong intermolecular forces lock particles into a lattice structure.

This is internally contradictory — regular packing suggests a solid, but moving freely throughout describes a gas. Neither description fits a liquid.

'Particles are close together in a disordered arrangement and can slide past one another' correctly describes the liquid state.