I hope this question is not too basic.

I recently edited a question and was accused of correcting a phrase that was grammatically correct to a phrase that is now wrong.

The context is:

I have Sodium and Potassium.

This type of element (i.e. elements considered to be metals)...

I changed the second sentence to

This type of elements...

because I thought there are two elements in this context.

Which of the above usage is correct? And Why?

Thanks in advance!

EDIT:

To make my question more obvious, I've made up some more complete examples:

He prepared some French fries and insomnia cookies, assuming that this type of (crispy) snacks would fit my appetite.

I am recently addicted to Triumph of Death and Tessellation; when I listen to this type of (metal) songs, I feel alive.

Is there a trusted source which claims the use of "this type of things" to be ungrammatical?

UPDATE:

Thank you all for your input. But I am more confused now.

While most of the comments seem to agree upon the same rule, some of the answers have different opinions on this. (hence downvoted?)

Could this be a preference then, where the majority of native speakers prefer not to use "this type of things"?