Question

Which of the properties of isotopes of an element is different? (a) First ionisation enthalpy (b) Effective nuclear charge (c) Electron affinity - (d) Melting point and boiling point

Short answer

The properties of isotopes of an element that are different are their melting point and boiling point (d), because these properties are influenced by atomic mass which varies between isotopes.

Step-by-step solution

Understanding Isotopes

Isotopes are variants of a particular chemical element that differ in neutron number, while the number of protons (which defines the element) and the number of electrons (in a neutral atom) remains the same. This means isotopes of an element have the same electronic structure and similar chemical properties.

Comparing Chemical Properties

The first ionisation enthalpy, effective nuclear charge, and electron affinity depend primarily on the electronic configuration and the charge of the nucleus which remains the same for isotopes. Thus, isotopes typically have very similar values for these properties.

Considering Physical Properties

The melting point and boiling point are influenced by the mass of the atoms. Since isotopes differ in neutron number, they have different atomic masses, which can lead to differences in their physical properties such as melting points and boiling points.

Key concepts

First Ionisation Enthalpy

First ionisation enthalpy refers to the energy required to remove the outermost electron from a neutral atom in its gaseous state. This process forms a positively charged ion, known as a cation. Isotopes have the same number of protons and electrons but can differ in neutron numbers; however, this does not significantly affect the first ionisation energy because this energy is more a reflection of the attraction between the nucleus and the outermost electron.

Isotopes, sharing identical electronic configurations, experience similar attractive forces from the nucleus onto their electrons, consequently resulting in similar first ionisation energies. Interestingly, slight variations can occur due to differences in mass and volume of isotopes which affect electron shielding and penetration, but these variations are generally minute.

Effective Nuclear Charge

Effective nuclear charge (ENC) is the net positive charge experienced by an electron in a multi-electron atom. The ENC takes into account both the actual nuclear charge (number of protons) and the shielding effect of the electrons located between the nucleus and the electron in question. Since isotopes of an element have an identical number of protons and a similar distribution of electrons, the ENC experienced by the electrons remains constant among isotopes.

Effective nuclear charge can be calculated using Slater's rules, which estimate the screening effect that inner electrons have on outer electrons. Because the number of protons and the structure of the electron cloud remain unchanged in isotopes, the shielding effect and thus the ENC don't differ appreciably.

Electron Affinity

Electron affinity measures the energy change when an electron is added to a neutral atom, forming a negative ion. It reflects how strongly an atom attracts additional electrons. Isotopes of an element, having the same electronic structure, tend to exhibit similar electron affinities because the electron affinity is principally impacted by the arrangement of electrons around the nucleus.

However, there could be subtle distinctions due to isotopic mass effects, which alter the atom's volume and thereby the distance between the nucleus and the added electron. Nevertheless, these differences are typically very minor and often negligible when considering electron affinity amongst isotopes.

Melting Point and Boiling Point

The melting point is the temperature at which a solid becomes a liquid, and the boiling point is the temperature at which a liquid becomes a gas. These physical properties are influenced by intermolecular forces and the mass of particles. Isotopes, having varied masses due to differences in neutron numbers, can exhibit different melting and boiling points.

Differences in mass affect the vibrational energy of the atoms in a solid or liquid state, thus affecting the energy required to change phases. For example, in isotopic substances, the heavier isotopes usually have higher melting and boiling points due to the larger mass causing stronger intermolecular forces and decreased vapor pressures. This is significant, for example, in heavy water (deuterium oxide) compared to regular water, where heavy water has distinctively higher melting and boiling points.