In this video we'll write the correct formula for Phosphorus tribromide (PBr3). To write the formula for Phosphorus tribromide we’ll use the Periodic Table and follow some simple rules.

When we have a non-metal and a non-metal we have a molecular compound (sometimes called covalent). Molecular compounds are some of the simplest to name.

Keys for Writing the Formulas for Molecular Compounds:
- Write the element symbol for both elements.
- Place a subscript after each element according to its prefix.

Note: Don’t write the subscript '1'.
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Prefixes
mono- 1
di- 2
tri- 3
tetra- 4
penta- 5
hexa- 6
hepta- 7
octa- 8
nona- 9
deca- 10
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Caution: We only write "mono" for the second element in a molecular compound. So CO would be carbon monoxide. Monocarbon monooxide is incorrect.

---Formula Writing Resources---
• Writing Formulas for Molecular compounds: https://youtu.be/ElhicLT-pCc
• Introduction to Formula Writing: https://youtube.com/playlist?list=PLZR1BGWBaZ1zR3-Dy1RyHA-sfVbhrvBHv

For a complete tutorial on naming and formula writing for compounds, like Phosphorus tribromide and more, visit:

http://www.breslyn.org/chemistry/naming

Drawing/writing done in InkScape (http://www.InkScape.org). Screen capture done with Camtasia Studio 4.0. Done on a Dell Dimension laptop computer with a Wacom digital tablet (Bamboo).