Question

valancy niklana ka baad kasa pata chala ga positive charge laga ga ya negative

Answer 1

for this uu have to learn that which elements are electro+ve and electro-ve...... U have to learn them... That's the only option

Answer 2

Firstly the valency can be taken out by writing the electronic configuration of a certain metal or non metal .
For eg : Calcium is a metal and it has valency +2...therefore we have to write its electronicconfiguration i.e.
2,8,8,2 to become stable it has to lose 2 electrons so..its valency is 2

Chlorine is a non metal with atomic no.17 and it has -1 valency
Its electronic configuration is 2,8,7 it has to gain one more electron to become stable...therefore its valency is -1

Metals always have positive valency coz they have 1 ,2,3 electrons in their valence shell ...as theg hve to lose them nd hence they require positive charge
Non Metals have 5,6,7 electrons in their valence shell so they have to gain electron to become stable nd hence they require negative charge .

Hope it helps u...but u have to practice this alot...otherwise u will not be able to understand it...☺