Question 1

What is a Lewis structure?

Accepted Answer

A Lewis structure (also called Lewis dot structure or electron dot structure) is a diagram that shows the bonding between atoms of a molecule and the lone pairs of electrons that may exist. It uses dots to represent valence electrons and lines to represent covalent bonds (each line = 2 electrons). Lewis structures help visualize molecular structure, predict molecular geometry, and understand chemical bonding and reactivity.

Question 2

How do you draw a Lewis structure step by step?

Accepted Answer

Step 1: Count total valence electrons (group number for each atom, adjust for charge). Step 2: Identify the central atom (least electronegative, usually not H or F). Step 3: Connect atoms with single bonds (2 electrons each). Step 4: Complete octets for outer atoms with remaining electrons. Step 5: Place remaining electrons on central atom. Step 6: If central atom doesn't have octet, form multiple bonds. Step 7: Check that all electrons are used and atoms satisfy the octet rule (except H wants 2).

Question 3

What is the octet rule?

Accepted Answer

The octet rule states that atoms tend to form bonds until they are surrounded by eight valence electrons, achieving a stable noble gas electron configuration. This applies to most main group elements. Exceptions include: Hydrogen (wants 2 electrons - duet rule), Boron (can have 6), and elements in period 3 and beyond (can exceed 8 with d-orbitals). The octet rule helps predict bonding patterns and Lewis structure stability.

Question 4

How do you count valence electrons for Lewis structures?

Accepted Answer

Valence electrons equal the group number for main group elements: Group 1 = 1 electron, Group 2 = 2, Groups 13-18 have 3-8 electrons respectively. For molecules: sum all atoms' valence electrons. For ions: subtract electrons for positive charge (cation), add electrons for negative charge (anion). Example: SO₄²⁻ has 6(S) + 4×6(O) + 2(charge) = 32 valence electrons total.

Question 5

What is VSEPR theory and how does it relate to Lewis structures?

Accepted Answer

VSEPR (Valence Shell Electron Pair Repulsion) theory predicts molecular geometry based on electron pairs around the central atom. After drawing the Lewis structure, count bonding and lone pairs around the central atom. These pairs repel each other and arrange to minimize repulsion: 2 pairs = linear, 3 pairs = trigonal planar, 4 pairs = tetrahedral. Lone pairs affect the final molecular shape differently than bonding pairs, causing bent or pyramidal geometries.

Question 6

How do you determine the central atom in a Lewis structure?

Accepted Answer

The central atom is typically: 1) The least electronegative atom (excluding hydrogen). 2) The atom that can form the most bonds. 3) Usually the first atom in a formula (except H). 4) Never hydrogen or fluorine. For example: in CO₂, carbon is central (less electronegative than oxygen). In NH₃, nitrogen is central. In H₂O, oxygen is central. Carbon is often central in organic molecules due to its ability to form four bonds.

Question 7

What are bonding pairs and lone pairs?

Accepted Answer

Bonding pairs are pairs of electrons shared between two atoms, forming covalent bonds. One pair = single bond, two pairs = double bond, three pairs = triple bond. Lone pairs (non-bonding pairs) are electron pairs that belong to one atom and aren't shared. For example, in H₂O: oxygen has 2 bonding pairs (connecting to each H) and 2 lone pairs. Lone pairs affect molecular geometry and reactivity but don't bond atoms together.

Question 8

How do you handle multiple bonds in Lewis structures?

Accepted Answer

Multiple bonds form when atoms need to share more than one pair of electrons to satisfy the octet rule. After placing single bonds and lone pairs, if the central atom lacks an octet, form double or triple bonds with neighboring atoms. Example: CO₂ has double bonds: O=C=O (each = is 4 electrons). O₂ has a double bond: O=O. N₂ has a triple bond: N≡N. Multiple bonds are shorter and stronger than single bonds.

Question 9

What is formal charge and why is it important?

Accepted Answer

Formal charge helps determine the most stable Lewis structure. Formula: FC = V - N - B/2, where V = valence electrons of free atom, N = non-bonding electrons, B = bonding electrons. The best Lewis structure has: 1) Lowest formal charges. 2) Negative formal charges on most electronegative atoms. 3) Like charges separated. For example, in CO₂, the structure with double bonds has all formal charges = 0, making it most stable.

Question 10

What are common exceptions to the octet rule?

Accepted Answer

Exceptions to the octet rule include: 1) Incomplete octets: Boron (BF₃ has 6 electrons around B), Beryllium (BeH₂ has 4). 2) Expanded octets: Elements in period 3+ can exceed 8 using d-orbitals (PCl₅ has 10, SF₆ has 12). 3) Odd-electron molecules: NO has 11 total electrons (radical). 4) Hydrogen: follows duet rule (wants 2 electrons). These exceptions are important for understanding reactivity and bonding in many compounds.

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