Define atomic number and isotopes. Determine if two given atomic notations represent isotopes. Calculate the number of neutrons in a given ion.

1. a) i)

• Atomic number: The number of protons in the nucleus of an atom. It defines the element.
• Isotopy: The phenomenon where atoms of the same element have the same number of protons but different numbers of neutrons, resulting in different mass numbers.

1. a) ii)
$^{12}_{6}\text{X}$ has 6 protons (atomic number = 6).
$^{14}_{7}\text{Y}$ has 7 protons (atomic number = 7).
For two atoms to be isotopes, they must have the same atomic number (same number of protons). Since $\text{X}$ and $\text{Y}$ have different atomic numbers, they are different elements and therefore not isotopes.

1. a) iii)
For $^{56}_{26}\text{Fe}^{2-}$:
Atomic mass (A) = 56
Atomic number (Z) = 26 (number of protons)
Number of neutrons = Atomic mass (A) - Atomic number (Z)
Number of neutrons $= 56 - 26 = 30$
There are $\boxed{\text{30 neutrons}}$ in $^{56}_{26}\text{Fe}^{2-}$.

1. b) i)
For the reaction $\text{N}_2