Module 2.3 Writing Chemical Formulas

Question 1

What is the chemical formula for

Calcium Sulfide

Answer 1

First write element names (Ca) & (S)

Then using periodic table find the charge of the metal (Ca)

Ca is group 2 (+2 charge)

Find group number of non metal and - 8 for its charge (S) in group 6; - 8 = -2 charge

If the charges for both are the same then they equal one (both have a charge of 2)

= CaS

Question 2

A metal always has a ? charge based on its ? number on the periodic table

Answer 2

Positive Charge

Group Number

Question 3

What is the chemical formula for

Sodium Phosphide

Answer 3

First write element names (Na) & (P)

Then find charge of metal from periodic table Na group 1 so +1 charge

The find group of non metal and - 8.
P group is 5. - 8 = (-3) charge

If charges are not equal switch the positive and negative charges to the opposite element

Na⁺¹P⁻³ becomes Na₃P

Question 4

If the compound ends with (ide) it is a ?

Answer 4

Binary Compound

Question 5

What is the chemical formula for

Calcium Phosphate

Answer 5

Name the element and polyatomic group
(CaPO₄)

Find charge for metal on periodic table
Ca is group 2 so +2 charge

We know PO₄ has a -3 charge

If the charges are different switch the charge placement

Ca⁺²PO₄⁻³ becomes Ca₃(PO₄)₂

Make sure to use parentheses