Scientists use as chemical symbols.

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letters
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Letters

Explanation:

Chemical symbols are used to standardize the 'language of chemistry' and to identify elements and atoms in a chemical formula easily. Chemical symbols consist of one or two letters, most often derived from the name of the element.

1. a)c-14 has two more protons

2. d)they have different numbers of protons, neutrons, and valence electrons; they are all gases at room temperature.

3. c) iron and nickel.