“This element” vs “these elements”

Question

I wrote:

 Many non-ferrous metallic alloys contain this element: bronze, brass, etc.

But teacher said:

 Many non-ferrous metallic alloys contain these elements: bronze, brass, etc.

But why?

Context is:

Copper is a ductile, malleable metal with a very high thermal and electrical conductivity. Many non-ferrous metallic alloys contain this element: bronze, brass, etc.

Answer 1

1. Many non-ferrous metallic alloys contain this element: bronze, brass, etc.

2. Many non-ferrous metallic alloys contain these elements: bronze, brass, etc.

The two sentences have different meanings altogether.

1. Many non-ferrous metallic alloys contain this element: bronze, brass, etc.

This implies that bronze, brass etc contain the element copper. This sentence fits very well in the context provided.

Why?

Copper is a ductile, malleable metal with a very high thermal and electrical conductivity. Many non-ferrous metallic alloys contain this element: bronze, brass, etc.

"This" makes a clear link to "copper" and therefore, relates to the context as a whole.

But -

2. Many non-ferrous metallic alloys contain these elements: bronze, brass, etc.

suggests that many non-ferrous metallic alloys contain bronze, brass, etc. This does not make sense. As mentioned by Edwin Ashworth below,

Calling bronze and brass 'elements' in a chemistry lesson is inexcusable.

They're alloys.

In conclusion, your teacher is wrong.

Answer 2

The antecedent of 'this' (or 'these') seems to be ambiguous, as it may refer to copper in the previous sentence, or the alloys that follow. It appears your teacher assumed it meant the alloys that follow. If you rearrange your sentence it should be clear that "this element" refers to copper mentioned in the earlier sentence: "Many non-ferrous metallic alloys, such as bronze, brass, etc., contain this element."