Question

Write short note on (a) lanthanide contraction (b) stability of isotopes of plutonium

Short answer

Lanthanide contraction is due to the poor shielding of nuclear charge by 4f electrons, causing a decrease in atomic radii of lanthanides from Cerium to Lutetium. Plutonium isotopes, such as Pu-238, -239, -240, -241, and -242 are unstable because of their large nuclear size, emitting radiation over time to achieve stability.

Step-by-step solution

Writing a short note on lanthanide contraction

(a) Lanthanide contraction refers to the phenomenon where the atomic radii of lanthanides, elements 57-71 on the periodic table, decrease with increasing atomic number. This phenomenon is due to the poor shielding of nuclear charge by 4f electrons, leading to an increase in effective nuclear charge with an increase in atomic number. Therefore, the size of lanthanide elements decreases from cerium (Ce) to lutetium (Lu). It has significant influence on the properties of elements and their compounds.

Writing a short note on the stability of plutonium isotopes

(b) Plutonium is a radioactive chemical element with multiple isotopes, most common of which are Pu-238, Pu-239, Pu-240, Pu-241, and Pu-242. Plutonium isotopes are unstable because their large atomic nuclei have too many protons, which causes a strong repulsive force. Over time, to achieve stability, plutonium isotopes undergo radioactive decay, a process in which they spontaneously emit radiation in the form of alpha, beta, or gamma rays. Pu-239, in particular, is of interest as it can sustain a nuclear chain reaction and is used in nuclear weapons and reactors.

Short summary

To sum up: (a) Lanthanide Contraction is the phenomenon of contraction in size of the atoms in the series of lanthanides elements due to the inadequate shielding of one electron by another in 4f orbital. (b) Plutonium isotopes are unstable due to their large atomic number leading to excessive repulsive forces. To stabilize, these isotopes undergo radioactive decay.

Key concepts

Stability of Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This variance in neutron number gives rise to isotopes with different atomic masses. Not all isotopes are stable. In fact, the stability of an isotope depends on the ratio of protons to neutrons in its nucleus.
For plutonium, a well-known element for its radioactive properties, there are several isotopes, such as Pu-238 and Pu-239. The uneven proton to neutron ratio in these isotopes means that their nuclei are packed with protons that repel each other due to similar charges.
To counteract this instability, these isotopes undergo processes like radioactive decay. Over time, they emit radiation, which can take the form of alpha particles, beta particles, or gamma rays, as they transition into stable elements or isotopes. Understanding the stability of plutonium isotopes is essential for applications ranging from energy production in nuclear reactors to nuclear weapons development.

Atomic Radii of Lanthanides

Lanthanides are a series of 14 elements ranging from cerium ( Ce) to lutetium (Lu), known for their unique chemical and physical properties in the periodic table. A notable characteristic of lanthanides is their atomic radii, which surprisingly decrease across the series. This is known as the lanthanide contraction.
The decreasing trend in atomic radii as you move from one lanthanide to the next is attributed to the ineffective shielding provided by the 4f electrons. With each additional proton, these electrons fail adequately to screen the nucleus's increasing positive charge, resulting in an increase in effective nuclear charge. Consequently, the electrons are pulled closer to the nucleus, leading to smaller atomic radii. Lanthanide contraction influences many aspects such as the element’s ionization energy, bonding characteristics, and material properties.

Radioactive Decay

Radioactive decay is the process by which unstable atomic nuclei lose energy by emitting radiation. This decay can occur in various forms, including alpha, beta, and gamma decay. Each type of decay involves a different mechanism of energy release:

• Alpha decay involves the emission of an alpha particle, which consists of 2 protons and 2 neutrons.
• Beta decay involves the conversion of a neutron to a proton (or vice versa) and the emission of an electron or positron.
• Gamma decay involves the release of energy in the form of gamma rays, without changing the number of protons or neutrons.

Plutonium isotopes, with their large unstable nuclei, often undergo alpha decay to release the strong repulsive forces created by their large numbers of protons. Understanding radioactive decay is crucial, especially when handling or working with materials that can emit potentially harmful radiation.

Effective Nuclear Charge

The concept of effective nuclear charge ( Z_eff) is central to understanding many properties of elements across the periodic table. It reflects the net positive charge experienced by an electron in an atom and is influenced by two main factors – the actual nuclear charge and the shielding effect of inner-shell electrons. In simple terms, although atoms have a specific nuclear charge based on their number of protons, not all of this charge influences outer electrons equally. Inner electrons can shield outer electrons from the nuclear charge, reducing its impact. In systems like the lanthanides, where 4f electrons do not sufficiently shield the outer electrons, the Z_eff becomes higher across the series. This increased Z_eff is why the atomic radii of lanthanides decrease, resulting in what is known as lanthanide contraction. Effective nuclear charge plays a significant role in determining the chemical reactivity and bonding properties of elements.