Two of the three atoms in a molecule of water \(\left(\mathrm{H}_{2} \mathrm{O}\right)\) are hydrogen. Why are Earth's oceans not fusing hydrogen into helium and setting Earth ablaze?

Nuclear fusion is an amazing process that powers stars, like our Sun. For fusion to happen, certain very specific conditions need to be met. First, extremely high temperatures are necessary. We're talking about millions of degrees Celsius. When the temperature is that high, hydrogen nuclei can get close enough to each other to overcome their natural repulsion. This allows them to combine, or 'fuse', into helium.
Secondly, fusion requires incredibly high pressures. These high pressures ensure that the hydrogen nuclei are densely packed, making collisions between them more likely. This pressure is easily found in the core of stars, but not on Earth. In summary, for nuclear fusion, you need both:

• Temperatures in the millions of degrees.
• Extremely high pressures.

Without both these conditions, fusion simply can't occur.

A water molecule, represented as \(\text{H}_2\text{O}\), consists of two hydrogen atoms and one oxygen atom. The hydrogen atoms are bonded to the oxygen atom through what chemists call covalent bonds. These bonds are strong and involve the sharing of electrons. This is very important because:

• In a water molecule, the hydrogen atoms are 'locked' into place.
• There is no room for the nuclei to move freely around, collide, or fuse.

This means that the normal structure of water has none of the conditions needed for fusion to occur. The hydrogen in water is happy where it is, bonded to oxygen, and doesn't have the energy required to break free and fuse.

On Earth, the conditions are just not right for nuclear fusion to happen. The Earth's environment is quite different from the inside of a star. Here are some key points to understand why:

• The temperatures on Earth's surface and in its oceans are far too low. We need millions of degrees, but even the hottest places on Earth are only a few thousand degrees at most.
• The pressure on Earth is also not sufficient. The immense gravitational force in a star's core creates tremendous pressure, which we simply don't have here.

So, in short, while Earth has plenty of hydrogen, the environmental conditions just don't meet the extreme requirements needed for fusion. This is why our oceans don't turn into gigantic fusion reactors, and hydrogen in water stays safe and stable.