Atoms are composed of three differently charged particles: the positively charged proton, the negatively charged electron and the neutral neutron. The charges of the proton and electron are equal in magnitude but opposite in direction. Protons and neutrons are held together within the nucleus of an atom by the strong force. The electrons within the electron cloud surrounding the nucleus are held to the atom by the much weaker electromagnetic force.
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This one's simple: electrons have a negative charge, protons have a positive charge and neutrons — as the name implies — are neutral.
Elements are differentiated from each other by the number of protons within their nucleus. For example, carbon atoms have six protons in their nucleus. Atoms with seven protons are nitrogen atoms. The number of protons for each element is known as the atomic number and does not change in chemical reactions. In other words, the elements at the beginning of a reaction -- known as the reactants -- are the same elements at the end of a reaction -- known as the products.
Although elements have a specific number of protons, atoms of the same element may have different numbers of neutrons and are termed isotopes. For example, hydrogen has three isotopes, each with a single proton. Protium is an isotope of hydrogen with zero neutrons, deuterium has one neutron, and tritium has two neutrons. Although the number of neutrons may differ between isotopes, the isotopes all behave in a chemically similar manner.
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Electrons are not bound as tightly to the atom as protons and neutrons. This allows electrons to be lost, gained or even shared between atoms. Atoms that lose an electron become ions with a +1 charge, since there is now one more proton than electrons. Atoms that gain an electron have one more electron than protons and become a -1 ion. Chemical bonds that hold atoms together to form compounds result from these changes in the number and arrangement of electrons.
The mass of an atom is determined by the number of protons and neutrons in the nucleus. Electrons have such a small fraction of mass compared to protons and neutrons that they are generally disregarded when determining an atom's mass. The sum of protons and neutrons is known as the atomic mass and is different for each isotope. For instance, hydrogen's isotope protium has one proton and an atomic mass of one. Deuterium with one proton and one neutron has an atomic mass of two.
Chemical reactions involve many, many atoms, and in nature, these atoms are a mixture of isotopes. The atomic weight for an element is the atomic mass of an element weighted for the percentage of isotopes found in a sample. Most hydrogen atoms are protium isotopes with an atomic mass of one. However, a small percentage of these atoms are deuterium with an atomic mass of two and tritium with an atomic mass of three. Thus, a sample of hydrogen atoms would have an atomic weight of 1.008 due to the small amount of these heavier isotopes slightly increasing the average atomic mass. Note that the percentage of isotopes may vary between samples but are generally very similar.