CHM1 CH.8

How do you determine the number of valence electrons for an atom?

Count the electrons in the outermost s and p orbitals (for main group elements, it's equal to the group number).

How do you determine the number of valence electrons in a molecule or ion?

Add up the valence electrons for all atoms, adjusting for any charges (add for anions, subtract for cations).

What is electronegativity?

The ability of an atom to attract electrons in a chemical bond.

What is the trend of electronegativity in the periodic table?

Increases across a period, decreases down a group; fluorine is the most electronegative element.

What is the difference between an ionic and a covalent bond?

Ionic bonds involve electron transfer (metal + nonmetal), while covalent bonds involve electron sharing (nonmetals).

What type of bond forms when the electronegativity difference is > 1.7?

Ionic bond.

What type of bond forms when the electronegativity difference is between 0.4 and 1.7?

Polar covalent bond.

What type of bond forms when the electronegativity difference is < 0.4?

Nonpolar covalent bond.

How do you draw a Lewis structure?

Count valence electrons, connect atoms with single bonds, complete octets, and adjust with multiple bonds or lone pairs as needed.

When do you use double or triple bonds in Lewis structures?

When atoms do not have complete octets with single bonds, especially C, N, O, and S.

What are isoelectronic species?

Atoms or ions that have the same number of electrons and same electron configuration.

When does resonance occur in a molecule?

When there is more than one valid Lewis structure where electron placement (not atom placement) differs.

How do you recognize valid resonance structures?

Atoms stay in the same place; only electron pairs (bonds or lone pairs) are rearranged.

How do you calculate the formal charge on an atom?

Formal charge = valence electrons − (nonbonding electrons + ½ bonding electrons).

What is the electroneutrality principle?

The most stable Lewis structure minimizes formal charges and places negative charges on the most electronegative atoms.

What is an incomplete octet and which elements allow it?

When an atom has fewer than 8 electrons; common in Be (4 e⁻), B and Al (6 e⁻).

What is an expanded octet and which elements allow it?

When an atom has more than 8 electrons; elements in period 3 or beyond (e.g., P, S, Xe) can do this.

What is a free radical?

A molecule or ion with an odd number of electrons, resulting in one unpaired electron.

How do you determine if a molecule is polar or nonpolar?

Check for polar bonds and overall symmetry—if the dipoles cancel, it's nonpolar; if they don't, it's polar.

What is the relationship between bond order, bond length, and bond energy?

Higher bond order = shorter bond = stronger (higher bond energy).

What is the electron-pair geometry for 2 electron domains?

Linear (180°).

What is the electron-pair geometry for 3 electron domains?

Trigonal planar (120°).

What is the electron-pair geometry for 4 electron domains?

Tetrahedral (109.5°).

What is the electron-pair geometry for 5 electron domains?

Trigonal bipyramidal (90° and 120°).

What is the electron-pair geometry for 6 electron domains?

Octahedral (90°).

What is the molecular geometry of a molecule with 3 bonding pairs and 1 lone pair?

Trigonal pyramidal.

What is the molecular geometry of a molecule with 2 bonding pairs and 2 lone pairs?

Bent.

Why are bond angles in molecules with lone pairs smaller than the ideal?

Lone pairs repel more strongly than bonding pairs, pushing bonds closer together.~