1) So, the first step in finding the formula of a certain compound is to look at the periodic table and see what group they are in. The group they are in corresponds to their charge :
Group 1 has charges of 1+
Group 2 has 2+
Group 3 has 3+
Group 4 has +/-4
Group 5 has -3
Group 6 has -2
Group 7 has -1
Group 8 doesn’t form ions because they don’t react.
Do you notice a pattern? Group 4 is like the middle and both sides.
For example, lets look at the first question:
Na (Sodium) is in group 1. This means that its ions have a 1+ charge. And Cl (Chlorine) in in group 7, meaning it has a 1- charge.
2) In the next step we try to cancel out the charges by finding the lowest common multiple. But, in this case, there is no need because 1+ and 1- cancel out to have an overall charge of 0 when they are in a compound.
3) Lastly, we put the two elements together. If we multiplied it to cancel the charges in the previous step, then we need to put the number we multiplied it by in front of it.
Lets do another example:
Magnesium is in group 2, therefore it forms a 2+ ion.
Oxygen is in group 6, therefore it forms a 2- ion.
Both charges cancel out and we are left with MgO.
The next example involved the 2nd step.
Lithium is in group 1, so it forms 1+ ions.
Oxygen is in group 6, so it forms 2- ions.
In order to cancel out the charges, we have to multiply lithium by to to get Li2 with a 2+ charge.
Now the charges cancel and we can put them together to get Li2O.
Hope this helped! If you have any more questions please ask :))