What is the shorthand notation for a ground-state boron (B) atom?
Start by writing the complete electron configuration of boron,
This means that the electron configuration of a neutral boron atom must account for a total of
#"B: " 1s^2 2s^2 2p^1#
Now, the noble gas that comes immediately before boron in the periodic table is located in period 1, group 18, and it's called helium,
The complete electron configuration of a helium atom looks like this
#"He: " color(blue)(1s^2)#
Notice that the complete electron configuration of boron includes the complete electron configuration of helium. You can thus replace this in the electron configuration of boron to get its noble gas shorthand notation
#"B: " color(blue)(1s^2) 2s^2 2p^1" "#will now become #" " color(green)(|bar(ul(color(white)(a/a)color(black)("B: " ["He"] 2s^2 2p^1)color(white)(a/a)|)))#
Notice that the brackets and the symbol of the noble gas are used to symbolize its electron configuration.