Atoms can join together with chemical bonds to form molecules. Molecules composed of two or more different elements are known as compounds. Properties of the resulting compound, such as melting point and electrical conductivity, depend on the attractive forces between the atoms and the resulting bonds that form.

According to the atomic model, electrons orbit the nucleus at specific energy levels, or shells. Electrons in the outermost shell, known as valence electrons, are involved in chemical bonding. Atoms are more stable when their outer shell is filled; they therefore tend to lose, gain, or share electrons to complete their outer shells. Three common types of chemical bonds—covalent, ionic, and metallic—result from the way in which valence electrons shift to fill the outer shells.

A single atom is held together by the attraction between the protons in the nucleus and the orbiting electrons. When two atoms approach each other, each nucleus also attracts the other atom's electrons. In a covalent bond, both atoms "fight for" the other atom's electrons, though neither one succeeds. Forces hold the atoms in a position where they are effectively stuck together and bonded through their shared valence electrons.

In an ionic bond, electrons transfer from one atom to another. Some elements have outer shells that are almost empty; these elements tend to lose their electrons. Other elements have outer shells that are mostly full; these elements tend to hold on to their electrons and attract electrons from other atoms. When an atom gains electrons, a negatively charged atom—a negative ion—is created. Conversely, when an atom loses electrons, a positive ion is created. The oppositely charged ions attract one another, creating an ionic bond.

In a metallic bond, one or more of the outer electrons of the atoms are "delocalized." In other words, the electrons are free to move around all the atoms involved, surrounding the atoms with a cloud of valence electrons. The strong attraction between the electron cloud and the positively charged atoms binds the atoms together, creating a metallic bond.

The physical properties of compounds are related to the nature of their chemical bonds. For instance, both ionic and metallic compounds tend to have high melting points—in the case of ionic bonds this is due to the strong force between positive and negative ions; with metallic bonds it is because of the strong attraction between the electron cloud and positively charged atoms. On the other hand, because the forces between molecules in covalent compounds are relatively weak, covalent compounds generally have low melting points.

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