Re: why were more elements discovered in 1800-1900

Harriet,

Interesting question. You can find a list of when all the elements were discovered here. I'll be referring to the chart on that page for the rest of this answer, so click on it now.

Taking a quick look through the 1800-1900 category (the 19th Century) in the list on that page, we can see that many elements were discovered through what was then cutting-edge technology. Chemists like Humphrey Davy discovered many elements through the use of a voltaic pile, which was an early form of the battery.
This sort of technology wasn't available to scientists before the 1800s because it had not been thought up or invented by then. Other elements were discovered when a scientist looked at the spectroscopic emission lines for an element by using technology that wasn't available before the 1800s.

As we can see, the technology used to discover elements in the 1800s-1900s had just been invented and was being improved as discoveries took place. The same trend is obvious in the other timeline categories on that chart - the 20th century saw the production of the first human-made element due to technological advances.

To recap, more elements were discovered from 1800-1900 because of newly emerging technologies.