long way? no idea.
short way. maybe this:
from the periodic table, you get to know the number of protons = number of electrons.
e.g Na has 11 protons = 11 electrons
so the configuration is 2.8.1
or you can just see it from the table.
e.g Na is in group 1 and period 3. and so the configuration is 2.8.1
and from that configuration, we know its electron configuration is
1s2 2s2 2p6 3s1.
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