: which part of group 1 metal elements makes it highly reactive with water that it can cause explosions?
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They have a large atomic radii and low ionization energies, and one electron, meaning they offer their one electron easily.
All chemical reactions are about the outer electron layers of the various atoms either giving up electrons, accepting more electrons, or sharing electrons.
There is energy in the electron orbitals; the electrons are “held” there by electromagnetic attraction to the nucleus (the protons in the nucleus), and at those distances the electromagnetic forces can be pretty high, so there’s energy taken in or given off when an electron changes orbitals or is given up or accepted.
With magnets you can change that energy into motion (such as in a motor), or as electricity passes through a wire it can heat up. With chemical reaction the energy can be electrical (such as in a battery that generates electricity) or more typically it’s heat.
And although a single electron gives very little energy at our “everyday” scales, there are typically some [10^23 atoms](https://en.wikipedia.org/wiki/Avogadro_constant) in a spoon-full of material, so the energy of 10^23 electrons can add up to enough heat to start a fire or even cause an explosion.
So what you get is a fire when the carbon and hydrogen in wood give/share electrons with the oxygen in air, and when metal sodium or potassium atoms give electrons to the OH- group of atoms in water. By comparison, iron reacts with OH- and oxygen too, just a lot more slowly and without so much heat, to form rust.
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