Question

What is the general formula for an ionic compound formed by elements in the following groups? Explain your reasoning and provide an example for each (name and formula). a. Group 1 with group 7 b. Group 2 with group 7 c. Group 1 with group 6 d. Group 2 with group 6

Short answer

The general formulas for ionic compounds formed by elements from the specified groups are as follows: a. Group 1 with group 7: AB (e.g. NaCl) b. Group 2 with group 7: AB₂ (e.g. MgCl₂) c. Group 1 with group 6: A₂B (e.g. K₂O) d. Group 2 with group 6: AB (e.g. CaO) These formulas are derived by considering the valency (or charge) of the elements in the respective groups and combining them in a manner that the overall charge of the compound is neutral.

Step-by-step solution

a. Group 1 with group 7

Elements in Group 1 have an oxidation state (charge) of +1 because they tend to lose one electron to achieve a stable electron configuration. Elements in Group 7, on the other hand, have an oxidation state of -1 because they tend to gain one electron to achieve a stable electron configuration. When elements from Group 1 and Group 7 form a compound, they will combine in a 1:1 ratio to balance their charges, and the general formula will be AB, where A is the Group 1 element and B is the Group 7 element. Example: Sodium (Na) from Group 1 reacting with Chlorine (Cl) from Group 7 forms Sodium Chloride (NaCl).

b. Group 2 with group 7

Elements in Group 2 have an oxidation state of +2 because they tend to lose two electrons to achieve a stable electron configuration. As mentioned earlier, Group 7 elements have an oxidation state of -1. When elements from Group 2 and Group 7 combine, they will require a 1:2 ratio to balance their charges, and the general formula will be AB₂, where A is the Group 2 element and B is the Group 7 element. Example: Magnesium (Mg) from Group 2 reacting with Chlorine (Cl) from Group 7 forms Magnesium Chloride (MgCl₂).

c. Group 1 with group 6

Group 6 elements have an oxidation state of -2 because they tend to gain two electrons to achieve a stable electron configuration. Group 1 elements have a +1 oxidation state, as mentioned earlier. When elements from Group 1 and Group 6 combine, they will require a 2:1 ratio to balance their charges, and the general formula will be A₂B, where A is the Group 1 element and B is the Group 6 element. Example: Potassium (K) from Group 1 reacting with Oxygen (O) from Group 6 forms Potassium Oxide (K₂O).

d. Group 2 with group 6

Elements in Group 2 have an oxidation state of +2, as mentioned earlier, while Group 6 elements have an oxidation state of -2. When elements from Group 2 and Group 6 combine, they will require a 1:1 ratio to balance their charges, and the general formula will be AB, where A is the Group 2 element and B is the Group 6 element. Example: Calcium (Ca) from Group 2 reacting with Oxygen (O) from Group 6 forms Calcium Oxide (CaO).

Key concepts

Group 1 elements

Group 1 elements are known as alkali metals. These elements include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). They are located in the first column of the periodic table.
Alkali metals have one electron in their outermost shell, making them highly reactive. They tend to lose this electron easily to achieve a full outer shell, resulting in a positive charge of +1.

• Alkali metals are soft and can be cut with a knife.
• They have low melting points compared to most other metals.
• Alkali metals react vigorously with water, producing hydrogen gas and a strong alkaline solution.

This single valence electron provides them the ability to form ionic compounds with non-metals from other groups, such as Group 7 elements, by transferring the electron to achieve stability.

Group 2 elements

Group 2 elements are known as alkaline earth metals. The group is comprised of: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). Located in the second column of the periodic table, they are slightly less reactive than Group 1 elements.
Each of these metals has two electrons in their outermost shell. They tend to lose both electrons to achieve a stable electronic configuration, giving them a charge of +2.

• These elements are harder than Group 1 metals.
• They have higher melting points.
• Group 2 elements react with water, but not as vigorously as Group 1 counterparts, often requiring heat to start the reaction.

Because they can lose two electrons, these elements form ionic compounds with elements from Group 6 and 7 in particular stoichiometries.

Group 6 elements

Group 6 elements belong to a group known as the chalcogens, which include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). Interestingly, the most commonly known member is oxygen.
These elements have six electrons in their outer shells and aim to gain two additional electrons to complete their valence shells, which gives them a combined charge of -2.

• They are typically non-metals, although polonium is a metalloid.
• Oxygen is a vital element for respiration and is widely abundant.
• These elements generally form compounds with metals, gaining electrons to form anions.

Group 6 elements easily form ionic compounds by accepting electrons from Group 1 or Group 2 metals, balancing the charges effectively.

Group 7 elements

Group 7 elements, also known as halogens, include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These are found in the seventeenth column of the periodic table.
Halogens have seven electrons in their outermost shell and tend to gain one electron to complete their valence shell, thus acquiring a -1 charge.

• They are highly reactive non-metals, particularly with alkali and alkaline earth metals.
• F, Cl, and Br are gases at room temperature, while I is a solid, and At is a metalloid.
• Halogens are used in disinfection and bleaching processes due to their strong oxidative properties.

Because of their electron configuration, Group 7 elements easily accept electrons from metals in Groups 1 and 2 to form stable ionic compounds.