Element with Z=1.

**The reasoning** Z stands for **atomic number** — the number of protons in an element's nucleus. It's like an element's unique ID card in the periodic table. Z = 1 means **one proton**. Only **Hydrogen (H)** has one proton. It's the very first element on the periodic table. Think of it this way: the periodic table is arranged by increasing atomic number, so Z=1 is position 1, which is Hydrogen. **Why the wrong options tempt you** - **He (Helium)** has Z=2 (two protons) — it's the second element - **Li (Lithium)** has Z=3 (three protons) — third element - **Be (Beryllium)** has Z=4 (four protons) — fourth element The trap is mixing up the symbols or forgetting that Z literally counts the position on the periodic table. **Quick takeaway** **Z = atomic number = protons count = position on periodic table.** Z=1 is always Hydrogen — the simplest atom in the universe with just one proton!