A mystery atom has 17 protons, 17 electrons, and 18 neutrons. Which element is it? What is the mass of an atom with seven protons, seven neutrons, and eight electrons?

Because there exist nuclei with with 7 protons and MORE OR LESS than 7 neutrons; that is nitrogen possesses a number of ISOTOPES.

Explanation:

If a nucleus possesses 7 protons, 7 fundamental positive particles, it is by definition a nitrogen atom. The most abundant isotope (as they are called) is  

14

N

, however, there exist isotopes where the nitrogen nucleus contains more than 7 neutrons. If there are 8 neutrons, fundamental neutral particles, we have the isotope  

15

N

. This isotope is still nitrogen (why? because it has 7 protons) but it masses more than the regular isotope.) and its chemistry is still the same. The atomic mass of nitrogen is thus the weighted average of the individual isotope.

the above statement/question is (true);

sir frederic attributed inaccuracies to schemas. memory reconstruction is one part actual events and one part personal schema, which can result in inaccuracies when memories are reconstructed.