Nursing Preparation Study Guide

Nursing Preparation Study Guide

Molecule Molecules are two or more atoms joined through a chemical bond to form chemicals. Molecules differ from atoms in that molecules can be further broken down into smaller pieces and into elements. In fact, the 18 th -century definition of an atomwas “a recognizable structure that can no longer be broken down into smaller bits”. Atoms are joined into molecules in two main ways: through covalent bonds and ionic bonds. Covalent bonds are the primary type of chemical bond that forms molecules. They occur when atoms with only partially-filled valence electron shells, an atom’s outermost electron shell, come together to share electrons. Hydrogen atoms, for instance, each have only one electron, while their valence shell is capable of holding two. When two hydrogen atoms come together, each share the other’s electron, using it to occupy its valence shell’s free space, thus forming the H2 molecule, hydrogen gas. Not all covalent bonds are the same. Different atoms have different levels of positive charge coming from in their nuclei, and although under normal circumstances, the negative charge of the atom’s electrons balances that out (keeping the atom electrically neutral), the chemical bonding process has a way of exploiting this situation. If we look at the H2 molecule again, everyday experience tells us that it has a strong tendency to seek out and bond with oxygen (O) molecules forming H2O, or water. There are two main reasons for this. The first comes from the regular old covalent bonds that are already holding H2 together. If bonded to another atom, hydrogen gains the ability to form a new valence shell that can hold six electrons. Since oxygen is the only molecule naturally to have six electrons in its valence shell, it is the most eager to bond with hydrogen. However, oxygen also has eight protons in its nucleus compared to the total of two in the H2 molecule. This means that as the atoms come closer and prepare to bond, the electrons fromboth atoms are pulled closer to the oxygen molecule and farther from the hydrogen. An atom’s proclivity to pull electrons towards itself is called its electronegativity, which creates covalent bonds that are polar. Because of this connection, these polar covalent bonds are the strongest molecular bond, which is why molecules like water are so prevalent in our solar system (and also likely throughout the galaxy). An interesting aspect of polar covalent bonds is the hydrogen bond. When a hydrogen atom bonds with another electronegative atom, the newly created molecule develops an intense polar attraction to all other electronegative atoms. This attraction works almost like a magnet – one end of the molecule exhibits a positive charge (due to the effects of the polar covalent bonds pulling all the electrons towards one end of the molecule) and the other end exhibits a negative charge. This phenomenon is responsible for, among other things, the way water molecules stick to one another so readily. This is the reason why a glass of water can be filled to a millimeter or so above the rim before it spills. Hydrogen bonds are also responsible for how hydrophilic and hydrophobic molecules react to being mixed with water. Hydrophilic molecules are molecules like NaCl (salt) that exhibit their own strong charge for reasons we will discuss in a moment. The charged salt molecules mix eagerly with the charged water molecules due to the extra pull of the hydrogen bond. Conversely, hydrophobic molecules, such as oil, will not mix with water because they are neutrally charged and do not like

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