Mass

The Mass of an Atom is closely related to the Number of Protons and Neutrons the Atom has. An Atom with a larger amount of Protons and/or Neutrons will be heavier than an Atom with less. The Atomic Mass of an element also has to do with Protons/neutrons. The Atomic Mass stands for the Total number of Protons/neutrons an element has. The Atomic number stands for the number of Electrons or Protons an element has. (They should be equal)

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Protons: 92

Neutrons:146

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Uranium has an Atomic Mass of 238.029

(Dude don't you just love Nuclear explosions? They occur when you split a Uranium atom)