Why doesn’t a nuclear bomb create a chain reaction that destroys the entire planet?

Because real life is not Hollywood plus 4 reasons.

Fission vs. Fusion:

Nuclear bombs work on the principle of nuclear fission – splitting heavy atoms like uranium or plutonium.

This releases energy, sure, but to destroy the entire planet?

Not enough oomph.

What you'd need is a fusion reaction, the kind that fuels stars. That involves lighter atoms like hydrogen fusing, and it's way more powerful.

Think of fission as a firecracker, fusion as the sun. We're nowhere near making a fusion bomb as big as our planet.

Even in a fission bomb, the chain reaction doesn't run wild forever.

The explosion itself scatters the nuclear fuel, disrupting the critical mass needed to sustain the reaction.

It's like trying to keep a bonfire going by throwing the logs across the field.

The colossal energy released by a nuke mostly disperses as heat, light, and a shockwave.

Earth is just way too big to absorb all that and go kablooey.

Yes, it'll cause horrific damage within a certain radius, but it won't vaporize the whole planet.

Some folks worry about setting the atmosphere ablaze with a nuclear explosion.

But remember, our atmosphere is mostly nitrogen, which isn't exactly flammable.

Plus, the energy from the bomb, though intense, spreads outwards and gets diluted as it goes.

It can't keep conditions for a runaway chain reaction in the air.

A large-scale nuclear exchange would be catastrophic on an unimaginable scale, but even that wouldn't be an Earth-ending scenario.

For that, you'd need a planet-cracking asteroid or some seriously ticked-off aliens.