Electrons bonding and structure

Describe how electrons are arranged in an atom according to quantum shells, sub-shells and orbitals.

• Electrons are arranged in shells, which contain sub-shells (s, p, d, f).

• Sub-shells contain orbitals that can each hold 2 electrons with opposite spins.

• s holds 1 orbital, p holds 3, d holds 5.

• Electrons fill lowest energy sub-shells first

Explain why the 4s sub-shell fills before the 3d sub-shell.

YThe 4s sub-shell has a lower energy level than 3d, so electrons enter 4s first.

Explain why nitrogen can form a triple bond.

• Each nitrogen has five outer electrons, so each shares three electrons to complete the octet.

• Gives a triple shared pair between atoms.

Predict and explain the shape of ammonia (NH₃).

• 4 electron pairs around N (3 bonding, 1 lone).

• Lone pair repels more than bonding pairs.

• Shape = trigonal pyramidal, 107°.

Predict and explain the shape and bond angle of BF₃.

• 3 bonding pairs, no lone pairs around B.

• Electron pairs repel equally.

• Shape: Trigonal planar, 120°.

Explain why carbon dioxide is non-polar even though it contains polar bonds.

• Molecule is linear.

• Bond dipoles act in opposite directions and cancel out → overall non-polar.

Explain why ice is less dense than liquid water.

• Hydrogen bonds hold molecules in an open lattice.

• Molecules are further apart → lower density.