Problem 174 - Introductory Chemistry Atoms First

Question

Circle the correct choice to indicate how many electrons each element must gain or lose to form an octet: \(\begin{array}{llll}\text { (a) } \mathrm{Mg} & \text { gains, loses } & 1,2,3 & \text { electrons }\end{array}\) \(\begin{array}{lll}\text { (b) Se gains, loses } 1,2,3 & \text { electrons }\end{array}\) (c) Al gains, loses \(1,2,3\) electrons \(\begin{array}{llll}\text { (d) Sr gains, loses } & 1,2,3 & \text { electrons }\end{array}\) (e) Br gains, loses \(1,2,3\) electrons (f) P gains, loses \(1,2,3\) electrons

Step-by-Step Solution

Answer

(a) Mg loses 2 electrons (b) Se gains 2 electrons (c) Al loses 3 electrons (d) Sr loses 2 electrons (e) Br gains 1 electron (f) P gains 3 electrons

Step 1: Identify the number of valence electrons for each element

Use the periodic table to find the group number for each element, which corresponds to the number of valence electrons. (a) Mg - Group 2, so 2 valence electrons (b) Se - Group 6, so 6 valence electrons (c) Al - Group 13, so 3 valence electrons (d) Sr - Group 2, so 2 valence electrons (e) Br - Group 17, so 7 valence electrons (f) P - Group 15, so 5 valence electrons

Step 2: Determine if the elements will gain or lose electrons to achieve an octet

Elements in groups 1 - 3 usually lose electrons to achieve an octet, while elements in groups 15 - 17 usually gain electrons to achieve an octet. (a) Mg - Loses 2 electrons (b) Se - Gains 2 electrons (c) Al - Loses 3 electrons (d) Sr - Loses 2 electrons (e) Br - Gains 1 electron (f) P - Gains 3 electrons

Step 3: Circle the correct answers

Based on our analysis, the correct answers are: (a) Mg loses 2 electrons (b) Se gains 2 electrons (c) Al loses 3 electrons (d) Sr loses 2 electrons (e) Br gains 1 electron (f) P gains 3 electrons

Key Concepts

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. These electrons play a crucial role in chemical bonding. They're essentially the "outer representatives" of an element, determining how an element interacts with others during chemical reactions.
Let's take a look at a practical example from our exercise. Magnesium (Mg) is in Group 2, meaning it has 2 valence electrons. Similarly, Selenium (Se) in Group 6 has 6 valence electrons. The group number is a quick way to determine valence electrons for elements in the main groups of the periodic table.

• Group 1 elements have 1 valence electron.
• Group 2 elements have 2 valence electrons.
• Group 13 elements have 3 valence electrons, and so on up to Group 18, which have 8 valence electrons (except Helium which has 2).

Understanding valence electrons is crucial for predicting how an element will bond with others. It sets the stage for the next key concept: electron gain or loss.

Element Groups

Element groups are columns on the periodic table that categorize elements based on similar chemical properties. The elements in each group share the same number of valence electrons, which defines their reactivity and bonding behavior.
For instance, elements in Group 2, like Magnesium (Mg) and Strontium (Sr), are called alkaline earth metals. They typically have 2 valence electrons. These electrons determine their tendency to lose electrons in reactions, striving to achieve a stable electron configuration.
Elements found in Groups 15 to 17 are often nonmetals, and they tend to gain electrons. For instance, Bromine (Br) in Group 17 needs just one more electron to achieve an octet, thus it strives to gain one electron in chemical reactions.
Understanding the groups helps in predicting the behavior of elements, such as whether they will gain or lose electrons to fulfill the octet rule.

Electron Gain or Loss

Atoms strive to achieve a full set of valence electrons, commonly known as an octet, to enhance stability. This is where the concept of electron gain or loss comes into play.
Metals, especially those in Groups 1 to 3, typically lose electrons. For example, Magnesium (Mg) loses its 2 valence electrons to form a stable octet, resulting in a positively charged ion.
On the other hand, nonmetals, usually found in Groups 15 to 17, have nearly complete outer shells and tend to gain electrons. Selenium (Se) will gain 2 electrons to complete its octet, while Bromine (Br) needs only one more electron to reach stability.

• Metals lose electrons, forming cations (positive charge).
• Nonmetals gain electrons, forming anions (negative charge).

Recognizing whether an element gains or loses electrons not only explains its chemical reactivity but also determines the type of bonds it will form.