Why , some units are represented in Uppercase?

we often notice that some of the units we commonly use are in lowercase but some are in uppercase. why is it so?     

              SI units which are named after scientist are represented using UPPERCASE letters and conventionally created ones are represented in lowercase

Eg:
⬛Unit of force -Newton is represented by N
⬛Unit of frequency-Hertz is represented by Hz
⬛Unit of capacitance-Farad is represented by F

▪Unit of diatance-meter   represented by m
▪Unit of weight-gram represented by g
▪Unit of time-second  represented by s

For more info about units, click here