Video narration
Why do atoms sometimes hand electrons over, and sometimes share them? Think of classroom counters: passing one along resembles transfer; holding them together resembles sharing-but it doesn't explain shape. In the simple ionic model, sodium gives one electron to chlorine, making NaCl in a one-to-one ratio. Aluminum gives one electron to each of three chlorine atoms, giving AlCl3; two sodium atoms each give one to oxygen, giving Na2O. That electron-transfer picture is simplified, especially for AlCl3. But bonding isn't always transfer. In a covalent bond, atoms share pairs of electrons. Bond type tells you whether electrons are transferred or shared. VSEPR, short for valence-shell electron-pair repulsion, says electron groups repel and arrange themselves to spread apart; that arrangement gives a covalent molecule its shape. So atoms transfer electrons to form oppositely charged ions, or share them in covalent bonds; unequal sharing makes a bond polar.