▪ An atom or ion with no unpaired electrons is not attracted by a magnet. ▪ If an orbital is completely filled: ▪ It has two electrons, one with an +½ spin and – ½ spin. ▪ The two electron’s spins cancel out. ▪ The net spin is zero.
The Level of Paramagnetism Can Be Measured with a Gouy Balance
Learning Check ▪ Which of these species is paramagnetic: F – , Zn 2+ , and Ti? Answer: Ti
Most Species are Diamagnetic ▪ Stable substances seldom contain free atoms. ▪ Instead, atoms are bonded together in molecules, resulting in the pairing of electrons and the cancellation of spin. ▪ Paramagnetism is observed primarily among salts of the transition and rare-earth metals, whose cations have partially filled d and f orbitals.
Excited States ▪ A ground-state configuration is the most stable arrangement of electrons, so an atom or ion will usually have this configuration. ▪ When an atom absorbs energy, however, it can reach an excited state with a new electron configuration. ▪ Ex. Sodium, Na ▪ Ground-state configuration: ▪ [Ne] 3 s 1 ▪ Possible Excited-state configurations: ▪ [Ne] 3 p 1 or [Ne] 4 s 1
Excited State Species Have Many Applications ▪ Excited atoms are unstable and spontaneously return to the ground-state configuration, giving up their excess energy in the process. ▪ Example ▪ Excited Na gives off photons as it returns to ground state in sodium vapor lamps. ▪ Excited Sr 2+ ions emit red light in highway flares as they relax to ground state. ▪ Excited metals used in fireworks emit light as they return to ground state.
Underlying Patterns of the Periodic Table ▪ Atomic orbitals become larger and less stable as principal quantum number n increases (and Z eff increases slowly. ▪ Orbitals expand, stability decreases. ▪ n is the most important factor in determining orbital size and stability within a column.
Underlying Patterns of the Periodic Table ▪ Any given atomic orbital becomes smaller and more stable as atomic number Z increases while n is fixed. ▪ Orbitals shrink, stability increases.
Periodic Trends in Atomic Radius ▪ The size of an orbital increases: ▪ From top to bottom in a column on the periodic table. ▪ n increases from top to bottom. ▪ From right to left in the periodic table. ▪ Z eff decreases from right to left .
Trends in Atomic Radii Li Na K Cs Rb
Learning Check ▪ For each of the following pairs, predict which atom is larger and why: Si or Cl, S or Se, and Mo or Ag. Answers: Si is larger than Cl. Se is larger than S. Mo and Ag are approximately the same size because of the offsetting effects of screening and Z eff .