Question

Which of the following statements regarding an anion is not true? (a) The gain of an electron leads to the formation of an anion. (b) The radius of the anion is larger than the atomic radius of its parent atom. (c) The effective nuclear charge increases when an anion is formed. (d) Electron cloud expands due to increased repulsion among the electrons.

Short answer

The incorrect statement regarding an anion is (c) 'The effective nuclear charge increases when an anion is formed.'

Step-by-step solution

Understanding Anion Formation

An anion is formed when an atom gains one or more electrons. This gain of electrons results in a negative charge as the number of electrons exceeds the number of protons.

Compare Anion Radius to Parent Atom Radius

When an atom gains an electron and becomes an anion, the electron-electron repulsion within the electron cloud increases. As a result, the electron cloud expands, and so the radius of the anion is larger than that of the parent atom.

Analyzing Effective Nuclear Charge

The effective nuclear charge (ENC) is the net positive charge experienced by an electron in a poly-electronic atom. The ENC is not increased upon the addition of an electron to form an anion; rather, it may decrease because of the increased shielding and electron repulsion.

Identifying the Incorrect Statement

Given that anions are formed by the gain of electrons and that they have a larger radius than their parent atoms due to electron cloud expansion, the only incorrect statement is (c) 'The effective nuclear charge increases when an anion is formed.' The nuclear charge remains the same as protons are not added, but the effective nuclear charge experienced by each electron actually decreases due to increased electron repulsion and shielding effect.

Key concepts

Gain of Electrons

When atoms engage in chemical reactions, one key process is the gain of electrons, which leads to the formation of negatively charged ions known as anions. In atoms, the number of protons (positively charged) typically equals the number of electrons (negatively charged), resulting in a net charge of zero. However, when an atom gains extra electrons, it becomes an anion because the negative charges (electrons) now outnumber the positive charges (protons).

This addition of electrons can happen during various types of chemical interactions, particularly when atoms have a high affinity for electrons. These atoms are often found on the right side of the periodic table, such as the nonmetals. Because electrons carry a negative charge, the gain of electrons into an atom creates an overall negative charge, making the atom an anion. This process is crucial in the formation of ionic bonds, where anions and cations (positively charged ions) are held together by electrostatic forces.

Anion Radius

Understanding the concept of anion radius involves recognizing how the physical size of an ion compares to its neutral atom counterpart. Generally, when an atom gains electrons and becomes an anion, its radius increases. This increase in size is due to the electron-electron repulsion that becomes more pronounced as additional electrons are added to the electron cloud. In turn, the electrons spread out more to minimize repulsion, causing an expansion in the electron cloud and thus increasing the ion's radius.

This expansion contradicts the notion that adding more negative charge to a region would constrict it due to the attraction to the nucleus. However, as electrons repel each other, the net effect is a larger, more diffuse electron cloud compared to the parent atom. The radius of anions is a crucial characteristic that affects how these ions interact in the formation of ionic compounds and their resulting properties, such as solubility and melting points.

Effective Nuclear Charge

The effective nuclear charge (ENC) is another fundamental concept in chemistry that represents the net positive charge an electron experiences from the nucleus, taking into account both the actual charge of the nucleus and the repulsive effects of other electrons in the atom. The ENC depends on the number of protons in the nucleus—the actual nuclear charge—and the electron distribution around that nucleus.

As an atom gains electrons to become an anion, the ENC on each electron does not increase, contrary to what some might think. While the actual nuclear charge remains constant (since the number of protons does not change), the additional electrons contribute to a greater shielding effect. This means that the inner electrons block some of the nucleus's pull on the outer electrons, thus reducing the ENC. This reduced ENC makes it easier for other elements (especially metals that easily lose electrons) to bond with the anion, which is why anions can readily form ionic compounds.