Why is energy required to remove an electron from an atom?

Electrons in an atom are held there by the force of attraction exerted by the positive charge of the nucleus on the negative charge of the electron. Energy is required to remove an electron from atom to overcome this attractive force.

it is to be noted that an alpha particle is an helium nucleus. so it contains 2 protons 2 neutrons and has a mass number of 4. when alpha decay particles are ejected from the nucleus it gets transformed into an atom which has atomic number less than 2 and a mass number less than 4.

an equation that could represent alpha decay is radium 226 decay of alpha particle forms radon 222.

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a) the particles of the chair are strongly held together.