The answer to the riddle is potassium.
Here's why:
- "I am shiny." – Potassium, like other metals, has a shiny appearance.
- "I form +1 ions." – Potassium typically loses one electron to form a +1 ion (K+).
- "I am the most reactive metal in Period 4." – Potassium (K) is indeed the most reactive metal in Period 4 of the periodic table.
Therefore, the correct answer is potassium.