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We will review the importance of some of the sub-atomic properties in chemistry that are relevant in biological systems. This becomes especially important when we think about how water fits into the biological picture.
We have reviewed some disparate fields of biology in order to get an idea of the breadth of the discipline. Biology encompasses living systems and what affects them, from biogeography, to organ systems within our bodies, to the sub-atomic properties of the elements that make up living tissues.
Electrons do not orbit the nucleus like planets. Instead, electrons can be anywhere in a cloud-like region around the nucleus. The “cloud” symbolizes where electrons are likely to be found. An electron’s movement is related to its energy level, or the specific amount of energy it has. Electrons of different energy levels are likely to be found in different places, lower energy electrons close to the nucleus and higher energy electrons farther away from the nucleus.
An atom’s valence electrons are those electrons that have the highest energy level and are held most loosely, furthest from the nucleus of an atom. The number of valence electrons in an atom of an element determines many properties of that element, including the ways in which the atom can bond with other atoms. Atoms with five, six, or seven valance electrons become more stable when this number increases to eight. Similarly, atoms with one, two, or three valance electrons can lose electrons and become more stable. When these two types of atoms combine, electrons are transferred from one atom to another, making the atoms more stable and able to bond tightly together in an ionic bond. The chemical bond formed when two atoms share electrons is called a covalent bond. A strong, stable bond is formed when the total number of shared valance electrons equals eight. Covalent bonds usually form between atoms of non-metals, whereas ionic bonds usually form between a metal and a non-metal.
Valance electrons are found in
The correct answer is B. An atom’s valence electrons are those electrons that have the highest energy level and are held most loosely, furthest from the nucleus of an atom. One of the most important features of an element’s reactivity is how many valence electrons (or space for donations) are available to form bonds with other atoms.
Covalent bonds hold the hydrogen and oxygen atoms together in a water molecule. In covalent reactions, the atoms have an electrical charge (for example, positive or negative), but they do not give or take electrons. Instead, the atoms share electrons, which essentially cause one of the atoms to have a more positive charge and the other to have a more negative charge. In water, the oxygen is a bit more negative, and the hydrogen atoms are a bit more positive. The opposite charges cause the hydrogen atoms of one water molecule to be pulled toward the oxygen atom of other nearby water molecules, creating a strong non-covalent bond to hold the water together. This strong bond, which is called a hydrogen bond, makes it difficult to boil or freeze water, which is important because a living organism contains so much water. The covalent bond strength means that water is a stable molecule; it is very hard to pull the oxygen and hydrogen of a water molecule apart into it’s constituent, the atoms. Although water contains covalent bonds, the characteristics of boiling and freezing water are a result of its network of hydrogen bonds, not its covalent bonds.