Leave it as it is. The punctuation is fine. : )

The period placement is fine. Might want to check the two spaces that should follow it.

Thank you!! :)

Putting two spaces after a period is no longer necessary, at least in MLA style. I’m not 100% sure whether APA still requires it.

I have been told by every typography and writing teacher I’ve ever had not to put two spaces after a sentence. My dad used to encourage two spaces, but that was back when he had spent more time typing on typewriters than on computers. It is no longer considered correct punctuation.

@NuGoonie23 Your punctuation is fine, don’t worry about it! :)

I did not know that, but now I do. Thank you @Seelix & @ParaParaYukiko. :)

Edit: Never mind, I’m just going to shut up now.

@Fyrius ?

Oh, don’t mind me. I just misread your question as a general request for punctuation placement help. And then I misread it as a transcription of a spoken quotation. And then by the time I realised I had read it wrong twice I didn’t really have anything useful to say any more.
Next time I will shut my clumsy big pie hole and read the question properly first. Probably.

You just wanted to know whether referencing a source as ”(Walker, n.d.)” is correct, right?

Yes.
Sorry to have confused you. Guess my wording is misleading. Though I can’t tell if you’re being sarcastic or if you’re really that hard on yourself.

The period after the parens is correct.

You should omit the extra “that” and insert a comma:

It was then noted that if other elements were exposed to these neutrons, that new radioactive elements emerged . . .

@NuGoonie23
No, I’m serious. I made a mistake and I’m owning up to it.