Question

Why do halogens often form ionic bonds with alkaline earth metals? A. The alkaline earth metals have much higher electron affinities than the halogens. B. By sharing electrons equally, the alkaline earth metals and halogens both form full octets. C. Within the same row, the halogens have smaller atomic radii than the alkaline earth metals. D. The halogens have much higher electron affinities than the alkaline earth metals.

Short answer

D. The halogens have much higher electron affinities than the alkaline earth metals.

Step-by-step solution

Understand Ionic Bonds

Ionic bonds are formed when one atom donates an electron to another atom, resulting in the formation of ions. Typically, metals lose electrons to become positively charged cations, while non-metals gain electrons to become negatively charged anions.

Characteristics of Halogens

Halogens are located in Group 17 of the periodic table and have high electron affinities, meaning they readily gain electrons to achieve a full octet.

Characteristics of Alkaline Earth Metals

Alkaline earth metals are located in Group 2 of the periodic table. They have low electron affinities and tend to lose electrons to achieve a full octet.

Compare Electron Affinity

Compare the electron affinities of the two groups. Halogens have much higher electron affinities compared to alkaline earth metals. This makes halogens more inclined to gain electrons.

Identify the Correct Answer

Given the properties of halogens and alkaline earth metals, the correct reason why they form ionic bonds is due to the high electron affinity of halogens, which allows them to gain electrons from alkaline earth metals that readily lose them.

Key concepts

Halogens

Halogens are a fascinating group of elements found in Group 17 of the periodic table. These elements include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Halogens are known for their high reactivity, especially with metals. They possess seven valence electrons, which means they need only one more electron to achieve a stable configuration known as an 'octet'. They achieve this by attracting electrons from other elements, which gives them high electron affinity.
Halogens have several important properties:

• High electronegativity: They strongly attract electrons.
• Highly reactive: Especially with metals, forming salts.
• Nonmetals: Except for astatine, which is a metalloid.

The high reactivity and electron affinity of halogens make them prone to forming ionic bonds with metals, particularly alkaline earth metals.

Alkaline Earth Metals

Alkaline earth metals are located in Group 2 of the periodic table. This group includes elements like beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These metals are characterized by having two electrons in their outermost shell. To achieve a full octet, alkaline earth metals tend to lose these two electrons, becoming positively charged cations.
Some key features of alkaline earth metals include:

• Low electron affinity: They do not attract electrons strongly.
• Low ionization energy: They easily lose their valence electrons.
• Reactiveness: They react readily with nonmetals, especially halogens.

Because of their tendency to lose electrons, alkaline earth metals readily form ionic bonds with halogens, which are eager to gain electrons.

Electron Affinity

Electron affinity refers to the amount of energy released when an electron is added to a neutral atom in the gas phase. It is an important property that explains the reactivity of elements, especially nonmetals like halogens.

• High electron affinity: Nonmetals such as halogens release more energy when they gain an electron, making them highly reactive.
• Low electron affinity: Metals such as alkaline earth metals release less energy when gaining an electron, making them less reactive in this way.

High electron affinities in halogens are key to their ability to form ionic bonds. They readily accept electrons from metals, which have lower electron affinities and are thus inclined to lose electrons. The difference in electron affinities between halogens and metals underpins the formation of ionic bonds.

Octet Rule

The octet rule is a fundamental concept in chemistry that describes the tendency of atoms to prefer having eight electrons in their valence shell. By achieving an octet, atoms attain a more stable, lower-energy configuration. This rule primarily applies to main-group elements:

• Metals, like those in the alkaline earth group, achieve a full octet by losing electrons.
• Nonmetals, like the halogens, achieve an octet by gaining electrons.

This rule is the driving force behind the formation of ionic bonds. For example, when an alkaline earth metal (with two valence electrons) loses these electrons, it forms a cation. Meanwhile, a halogen (with seven valence electrons) gains these electrons to form an anion. Both achieve a full octet: the metal by losing electrons and the halogen by gaining electrons.