It is unknown who placed the clue in CERN or how it was put there. However, CERN is a research facility that is open to any scientist around the world but it is a secret Ekaterina stronghold which holds this clue.
Hydrogen is an element that is number 1 on the periodic table. Hydrogen is one of the few elements that is a gas at room temperature. It is the lightest element and is commonly used in balloons. Hydrogen is also very flammable.