what is the difference between isolated thunderstorms and scattered thunderstorms?

Answer 1

Sometimes weather presenters describe thunderstorms, rain etc as being isolated or scattered.

Isolated means just the odd one here or there whereas scattered would indicate several dotted over a wide area.

There's no difference between the thunderstorms themselves, only in the liklihood of them occuring.

Answer 2

As Defined by the National Weather Service,

20% slight chance isolated
30-50% chance scattered

http://www.crh.noaa.gov/lot/severe/wxterms.php

Also
Isolated
A National Weather Service convective precipitation descriptor for a 10% chance of measurable precipitation (0.01"). Isolated is used interchangeably with few. See Precipitation Probability (PoP).

Isolated Storm
An individual cell or a group of cells that are identifiable and separate from other cells in a geographic area.

Answer 3

Isolated would only be in certain particular area, scattered are in a wider coverage area.