Historical clue formulations

• Periodic table figs. — 2017-04-02
• 5 for B and 6 for C — 2016-01-27
• Periodic law figs. — 2012-06-02
• B and O figures: Abbr. — 2009-02-27
• 7 for N, 8 for O, etc. — 2006-04-27