Write electron configurations in noble gas shorthand, identify the number of valence electrons for a main-group element, and draw its Lewis dot symbol.
Full electron configurations get long for larger atoms, so chemists use noble gas shorthand: write the symbol of the noble gas that comes right before the element (in brackets), then continue the configuration from there. Since chlorine (17 electrons) comes right after neon (10 electrons) on the table, chlorine's shorthand configuration is [Ne] 3s^2 3p^5 -- the bracket silently represents the first 10 electrons, and only the remaining 7 are written out.
Valence electrons are the electrons in an atom's outermost occupied energy level, and they are the only electrons involved in bonding -- everything about how an element reacts comes down to its valence electron count. For main-group elements, the valence electron count matches a simple pattern based on group number: group 1 has 1 valence electron, group 2 has 2, and groups 13 through 18 have 3 through 8 valence electrons respectively (helium, in group 18, is the one exception, with only 2).
A Lewis dot symbol shows an element's valence electrons as dots placed around its symbol, one on each side first, then paired up once all four sides have one dot. Oxygen, with 6 valence electrons, is drawn with two lone pairs of dots and two single dots around the letter O -- this simple picture is the foundation for drawing entire molecules once bonding begins.
Mastery looks like: They write correct noble gas shorthand configurations, state valence electron counts for main-group elements, and draw accurate Lewis dot symbols.
Common stumbles: Choosing the wrong noble gas (one that comes after, not before, the element); miscounting valence electrons for groups 13-18 by forgetting to convert the modern group number correctly.