# Difference between “physical quantities” and “units”?

My textbook, Atkins' Physical Chemistry, 11th edition, by Atkins, de Paula, and Keeler, says the following:

Although it is good practice to use only SI units, there will be occasions where accepted practice is so deeply rooted that physical quantities are expressed using other, non-SI units. By international convention, all physical quantities are represented by oblique (sloping) letters (for instance, $$m$$ for mass); units are given in roman (upright) letters (for instance m for metre).

I'm confused about what the difference here is between "physical quantities" and "units"?

I would appreciate it if people would please take the time to clarify this.

• I think that passage explains it pretty well, but to give another example energy is a physical quantity that can be expressed using different units like Joules and calories. Physical quantities describe some property of an object, units are used to express a particular amount of that property. – Tyberius Feb 18 at 13:40
• @Tyberius So how does the different uses of $m$ and m in the textbook excerpt relate to this? – The Pointer Feb 18 at 13:42
• That's basically just telling you how to read expressions they write in the book. If you see $F=ma$, you know that m is referring to mass. If you see $1600$ m, it's talking about that many meters, not that number times mass. – Tyberius Feb 18 at 13:44
• @Tyberius Oh, so that's what they were referring to. It wasn't even clear to me until you used the $F = ma$ example. I think this could have been expressed more clearly; they managed to make such a simple point into something much vaguer than reasonable. Anyway, thank you for the clarification. – The Pointer Feb 18 at 13:45