Question

how to calculate atomic weight of a chlorine

Answer 1

Atomic weight of chlorine is 35

atomic no. 17 atomic weight 17*2+1=35

Always remember that atomic weight is always more than or equal to the double of atomic no.

Till the first 20 elements , you have to plus 1 with the double of all the odd atomic no. and they will result in the atomic weight. e. g.
Sodium has atomic no. 11 and its atomic weight will be 11*2+1 =23
however there are some exceptions...
First : beryllium.. atomic no. 4 atomic weight 9
second : nitrogen... atomic no. 7 atomic weight 14
third : argon ..atomic no. 18 atomic weight 40

Answer 2

to calculate the atomic weight of chlorine

we know that,

n = A - Z

where n = number of neutrons

A = atomic weight

Z = atomic number

we also know

neutrons = 18

atomic number = 17

now if we change this equation

we get,

A = Z + n

A = 17 + 18

which gives us

A = 35

but the actual weight of chlorine is 35.5

it is taken because chlorine has 3 isotopes

Cl - 37

Cl - 36

Cl - 35 ...

and the stable one is Cl - 35

it is taken as 35.5