What are the two main types of chemical bonds

Explanation:

Ionic bonds are formed by the electrostatic attraction between a positive ion (named cation) and a negative ion (named anion).

A cation is formed when an atom donates one or more electrons from its valence shell, while an anion is formed when an atom accepts one or more electrons.

The most classical example of a ionic bond is that of sodium chloride: sodium (Na) is a metal with one valence electron, and chlorine (Cl) is a nonmetal with 7 valence electrons. Then, sodium releases its valence electron (forming the cation Na⁺), which is accepted by chlorine (forming the anion chloride, Cl⁻). These two ions with opposite charge are bonded by the attractive electrostatic force.

Covalent bonds are the result of sharing valence  electrons. In this case, the atoms do not exchange (donate/accept) electrons but share them; meaning that the shared electrons belong to the outermost energy level of the two atoms, now bonded by the covalent bond.

Furthermore, the covalent bond may be polar or non-polar. A polar covalent bond is formed when the two atoms have different electronegativities, because in such case one atom pulls the electrons harder than the other, resulting in an uneven distribution of the charge, i.e. forming a dipole.

c- cold periods in earth's history during which glaciers covered much of the surface.

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