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The Nucleus
At the center of the atom is the nucleus, which is made of positively-charged particles called protons, and neutrally-charged particles called neutrons. These particles are bound together by nuclear force. The nucleus makes up the bulk of an atom's mass. The atomic number (or proton number) of an element is determined by the number of protons in its nucleus. For example, hydrogen has a single proton, so it has an atomic number of 1.
Isotopes
While many atoms contain the same number of protons and neutrons, the number of neutrons can vary with the element retaining its atomic number. The variations among the number of neutrons produce different isotopes of an element, some of which are radioactive. For example, carbon has an atomic number of 6, which means it always has 6 protons in its nucleus; it can contain more than 6 neutrons, however, producing carbon-13, which is a stable isotope, and carbon-14, a radioactive isotope.
Electrons
The nucleus of the atom is surrounded by electrons, which are negatively charged particles. These rotate around the nucleus, and have much smaller mass than protons. Electrons exist in valence shells, meaning that only a certain number of electrons can fill each level, increasing with distance from the nucleus. For example, the innermost valence shell can only hold two electrons, while the third can hold a maximum of 10.
Electrons and Bonding
The number of electrons in an atom's outermost shell determine how an atom bonds with other atoms. When the outermost electron shell of an atom is full, it is more stable and less likely to react with other elements. For example, the noble gases (such as argon and helium) have full outermost shells, so they are less likely to react than an element like hydrogen, which has only one electron in its outermost shell and bonds with other atoms for stability.