The electron configuration for chlorine, Cl is [Ne]3s23p5. How many valence electrons does a Cl atom have?
Write the symbol for the ion it forms to achieve a noble-gas configuration.
Seven, it needs to gain one more to achieve noble gas configuration. So if Chlorine gains another electron it will become negative so the symbol would be Cl with a little negative one (-1) or sometimes when the charge is negative one you can just write the negative sign (-) on the upper right side of it (like a power).
Hope this helps.
To find the valence electrons, find the highest energy level (in this case, 3), and count how many electrons are present in each orbital.
There are 2 in the 3s orbital.
There are 5 in the 3p orbital.
5+2 = 7.
You could also do this just by looking at the periodic table. Chlorine is in column 7 (immediately before the noble gases). This is the halogen group, and all of them have 7 valence electrons (hence the column: 7).
As for the symbol, remember that an atom wants 8 electrons in its outer ring if possible. To do this it will more likely gain 1 than lose 7, so it will be a Chlorine ion with an extra electron. Electrons have a -1 charge each, so we write Cl with a -1 superscript
chlorine is a halogen or in group 7A meaning it has 7 valence electrons. a noble gas is in group 8A meaning it has 8 valence electrons thus satisfying the octet rule which states that an atom is stable when it achieves a noble gas configuration (full octet). so because chlorine only needs to gain one valence electron to achieve pseudo-noble gas configuration it would have to attain one additional electron giving it a -1 charge.
Cl has 7 valence electrons