To tell if Na2SO4 (Sodium sulfate) is ionic or covalent (also called molecular) we look at the Periodic Table that and see that Na is a metal and SO4 is a group of non-metals. Here the group of nonmetals make up a polyatomic ion. When we have a metal and a group of non-metals the compound is usually considered ionic.

Because we have a metal and non-metals in Na2SO4 there will be a difference in electronegativity between the metal and group of nonmetals. This difference results in an electron(s) being transferred from the metal (lower electronegativity) to the non-metal (higher electronegativity). The results in the metal becoming a positive ion and the group of non-metals a negative polyatomic ion. The two opposite charges are attracted and form the ionic bond between ions in Sodium sulfate.
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Helpful Resources

Metals, Non-Metals on the P- Table: https://youtu.be/OoooStZQHdA
Ionic, Covalent, & Polar Covalent: https://youtu.be/OHFGXfWB_r4
Electronegativity for each element: https://en.wikipedia.org/wiki/Electronegativity
Memorizing Polyatomic Ions: https://youtu.be/vepxhM_bZqk
Finding Ionic Charge: https://youtu.be/N4N1Njh7nCo
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Because we have a combination of a metal and non-metal Na2SO4 (Sodium sulfate) is considered an ionic compound.

In general, ionic compounds:

- form crystals.
- have high melting points and boiling points.
- are hard and brittle.
- conduct electricity when dissolved in water.
- as solids do not conduct electricity.


For more chemistry help, see http://www.Breslyn.org.