To balance nuclear equations recognize that the top number is the mass and the bottom number the atomic number (# of protons). The total mass on both sides must be the same and the total # of protons on both sides must be the same.

On the left you have a total mass of 231 and a total of 91 protons.

On the right you have an alpha particle which is Helium with a mass of 4 and 2 protons.

91 -2 = 89 protons for the decay product which is element Ac, actinium.

231 - 4 = 227.

So the decay products are helium 2 4 and Actinium with a mass of 227.

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