Exercise 1 solution

Reading the Hydrogen example box ("Hydrogen 1 H 1.0080", Periodic table article):
  atomic number   = 1
  symbol          = H
  atomic weight   = 1.0080
  protons         = 1 (the atomic number is the number of protons, Chapter 2)
  electrons       = 1 (neutral atom: electrons = protons)

The bracketed [209]:
  The Periodic table article says a bracketed value is the mass number of the
  most stable isotope of that element (no standard average is given).
  With Z = 84 (given for this exercise; check against a periodic table):
  neutrons = A - Z = 209 - 84 = 125

The subtraction was checked with Python.