Given the task definition, example input & output, solve the new input case.
Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.
Example: Sentence1: pesticides cause pollution. 
Sentence2: pollution can harm animals.
Output: pollution.
The word "pollution" is common to Sentence1 and Sentence2. So, it's a good answer.

New input case for you: Sentence1: if a neutral atom loses an electron then an atom with a negative charge will be formed. 
Sentence2: When an atom or molecule is no longer neutral it is called an ion .
Output:
neutral