Question

Give the number of valence electrons in \(\mathrm{N},\) As \(, \mathrm{Br}\) , and Se. Predict the number of covalent bonds needed for each of these elements to satisfy the octet rule.

Short answer

N and As have 5 valence electrons each and need to form 3 covalent bonds to satisfy the octet rule. Br has 7 valence electrons and needs to form 1 covalent bond, while Se has 6 valence electrons and needs to form 2 covalent bonds.

Step-by-step solution

Determine the group number for each element

In the periodic table, N belongs to Group 15, As also belongs to Group 15, Br belongs to Group 17, and Se belongs to Group 16.

Find the number of valence electrons for each element

Since the group number represents the number of valence electrons for each element, we can directly find the valence electrons for N, As, Br, and Se. - N (Group 15): 5 valence electrons - As (Group 15): 5 valence electrons - Br (Group 17): 7 valence electrons - Se (Group 16): 6 valence electrons

Use the octet rule to predict the number of covalent bonds

According to the octet rule, atoms gain, lose, or share electrons to achieve a stable electron configuration, which means having 8 electrons in the outermost shell. We will now predict the number of covalent bonds needed for N, As, Br, and Se to satisfy the octet rule: - N: It has 5 valence electrons and needs 3 more to complete its octet (8 electrons). Hence, N needs to form 3 covalent bonds. - As: Like N, it also has 5 valence electrons and requires 3 more to complete its octet. So, As also needs to form 3 covalent bonds. - Br: It has 7 valence electrons and needs only 1 more to complete its octet. Therefore, Br needs to form 1 covalent bond. - Se: With 6 valence electrons, Se needs 2 more to fulfil the octet rule. Thus, Se needs to form 2 covalent bonds. In conclusion, N and As need to form 3 covalent bonds each, Br needs to form 1 covalent bond and Se needs to form 2 covalent bonds to satisfy the octet rule.

Key concepts

Covalent Bonds

Covalent bonds are a type of chemical bond where atoms share pairs of electrons. This sharing occurs because atoms want to achieve a full outer shell of electrons, usually consisting of eight electrons, according to the octet rule.
When atoms share electrons, they create a stable electronic arrangement.
This stable configuration is similar to that of the noble gases, which are known for their lack of reactivity due to having full valence shells. Atoms involved in a covalent bond can share multiple electron pairs, forming single, double, or even triple bonds.

• Single bonds involve sharing one pair of electrons.
• Double bonds share two pairs of electrons.
• Triple bonds involve three pairs of electrons.

To satisfy the octet rule,

• an element like nitrogen (N) forms three covalent bonds because it has five valence electrons and needs three more to make eight.
• Similarly, arsenic (As), which is in the same group as nitrogen, also forms three bonds.
• Bromine (Br), with seven valence electrons, forms one covalent bond.
• Selenium (Se), with six valence electrons, needs two more electrons and forms two covalent bonds.

In summary, the number and type of covalent bonds depend on how many electrons an atom needs to complete its octet.

Valence Electrons

Valence electrons are the electrons found in an atom's outermost shell. These electrons play a crucial role in chemical bonding because they are the ones that interact in chemical reactions.
For easy recall, you can often predict the number of valence electrons based on an element's group number on the periodic table. For example, nitrogen (N) and arsenic (As) both belong to Group 15 of the periodic table.

• This indicates that they have 5 valence electrons each.

Bromine (Br) is a Group 17 element, thus, it possesses 7 valence electrons, just one short of a full shell.
Selenium (Se), found in Group 16, has 6 valence electrons. During chemical reactions, valence electrons are either shared between atoms to form covalent bonds or transferred between atoms to form ionic bonds.
These interactions allow atoms to achieve more stable electron configurations, often fulfilling the octet rule.
By understanding valence electrons, you can predict how many covalent bonds an atom will form to reach a stable state.

Periodic Table Groups

The periodic table is not just a map of elements but an organized system that groups elements with similar properties together. Each column on the periodic table is known as a group.
Elements in the same group often share similar chemical properties because they have the same number of valence electrons. For instance, all elements in Group 15, including nitrogen (N) and arsenic (As), have five valence electrons.
This similarity explains why they form the same number of covalent bonds to satisfy the octet rule.

• Group 17 elements like bromine (Br) have seven valence electrons.
• Group 16 elements like selenium (Se) have six valence electrons.

The group number is a handy tool for predicting the number of valence electrons and, consequently, the bonding patterns of elements.
Understanding periodic table groups helps you quickly determine the chemical behavior and reactivity of elements, especially how they bond to achieve a complete outer electron shell.