The two major types of bonds are and . Electrons are either or to fill the last shells of atoms that ions. The number of an ion is either positive or negative. When we combine ions we can only combine with ions. When atoms share electrons they form bonds. The electrons involved with bonding, in the last shell that are shared are called electrons. Three examples of macromolecules are , and . The four main atoms involved in carbon chemistry are .

chemical bonds and covalent bonds

Explanation: covalent bonds are in which the valence electrons density is shared between two atoms and  chemical  bonds are the connections between atoms in a molecule . electrons  can be either free [not attached to any atom] or bound to nucleus of an atom. atom or molecule is a number of an ion which is either positive or negative. when we combine ion we can only combine elements with ionic bonds.  when atoms join together , they form chemical bonds without transferring any electrons but they rather share them. copper sulphide  are the last shell that are shared with bonding . proteins, carbohydrates, lipids are the example of macromolecules .  nucleic acids, lipids, proteins, carbohydrates are the main atoms involved in carbon chemistry.

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answer by yourhope:

how do models compare to real-world objects?

d. most models represent a real-world object in a simpler way.

: )

the answer is b & c i believe : )