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Atomic Structure
Atoms consist of positively charged protons and chargeless neutrons in a central nucleus; negatively charged electrons orbit this positive nucleus, giving the atom an overall neutral charge. In textbooks, atoms might appear as individual entities that remain unaffected by their surroundings. In reality, atoms regularly lose and gain electrons depending on the bonds they form with surrounding atoms. When atoms gain or lose electrons, they become charged particles -- ions -- yet they remain the same element. The proton fusion that takes place in the sun uses hydrogen nuclei denuded of electrons. Although they lack electrons, ionic hydrogen nuclei remain hydrogen.
Atomic Mass
Atomic numbers denote the number of protons an element has, but the closely related concept of atomic mass tells scientists how much the atom's whole nucleus weighs. As nuclei typically contain two types of particles, protons and neutrons, the atomic mass is the sum of the weights of both particles. Most elements have an equal number of protons and neutrons in their stable form, but other forms of the element have fewer or more neutrons than protons in their nuclei. These isotopes --- atoms with unequal numbers of protons and neutrons --- weigh more or less than standard atoms of their kind, but they remain atoms of the same element. Atomic numbers do not change depending on the number of neutrons in the atom.
Atomic Mass Versus Atomic Number
A neutral, stable isotope of an element that contains equal numbers of neutrons and protons in its nucleus has an atomic mass exactly double that of its atomic number. In nature, the occurrence of various isotopes means that a given sample of an element typically has an atomic mass that only approximates twice its atomic number. Because atomic mass can vary depending on the quantity of a given isotope in a sample and atomic number cannot; an element's atomic number is its most unchangeable characteristic.
Radioactive Decay
Atoms can lose or gain electrons and neutrons, yet remain essentially unchanged. Should they lose a proton, elements can change their fundamental natures, becoming entirely different substances. Radioactive isotopes that lose protons via natural decay become new elements. For example, uranium-238 decays into thorium-234 as it loses a pair of protons in the form of an alpha particle. The byproducts of such nuclear fission reactions may themselves be radioactive and lose more protons, turning into different elements.