Ross & Wilson Anatomy and Physiology in Health and Illness Read Online Free PDF Page B
Author: Anne Waugh
with one or more other atoms to achieve stability. This will be described more fully in the section discussing molecules and compounds.
Isotopes
These are atoms of an element in which there is a different number of neutrons in the nucleus . This does not affect the electrical activity of these atoms because neutrons carry no electrical charge, but it does affect their atomic weight. For example, there are three forms of the hydrogen atom. The most common form has one proton in the nucleus and one orbiting electron. Another form ( deuterium ) has one proton and one neutron in the nucleus. A third form ( tritium ) has one proton and two neutrons in the nucleus and one orbiting electron. These three forms of hydrogen are called isotopes ( Fig. 2.3 ).
Figure 2.3 The isotopes of hydrogen.
Because the atomic weight of an element is actually an average atomic weight calculated using all its atoms, the true atomic weight of hydrogen is 1.008, although for most practical purposes it can be taken as 1.
Chlorine has an atomic weight of 35.5, because it contains two isotopes, one with an atomic weight of 35 (with 18 neutrons in the nucleus) and the other 37 (with 20 neutrons in the nucleus). Because the proportion of these two forms is not equal, the average atomic weight is 35.5.
Molecules and compounds
It was mentioned earlier that the atoms of each element have a specific number of electrons around the nucleus. When the number of electrons in the outer shell of an element is either the maximum number ( Fig. 2.1 ), or a stable proportion of this fraction, the element is described as inert or chemically unreactive, i.e. it will not easily combine with other elements to form compounds. These elements are the inert gases – helium, neon, argon, krypton, xenon and radon.
Molecules consist of two or more atoms that are chemically combined. The atoms may be of the same element, e.g. a molecule of atmospheric oxygen (O 2 ) contains two oxygen atoms. Most molecules, however, contain two or more different elements, e.g. a water molecule (H 2 O) consists of two hydrogen atoms and an oxygen atom. As mentioned earlier, when two or more elements combine, the resulting molecule is referred to as a compound.
Compounds that contain the elements carbon and hydrogen are classified as organic , and all others as inorganic . Living tissues are based on organic compounds, but the body requires inorganic compounds too.
Covalent and ionic bonds
The vast array of chemical processes on which life is based is completely dependent upon the way atoms come together, bind and break apart. For example, the simple water molecule is a crucial foundation of all life on Earth. If water was a less stable compound, and the atoms came apart easily, human biology could never have evolved. On the other hand, the body is dependent upon the breaking down of various molecules (e.g. sugars, fats) to release energy for cellular activities. When atoms are joined together, they form a chemical bond that is generally one of two types: covalent or ionic .
Covalent bonds are formed when atoms share their electrons with each other. Most molecules are held together with this type of bond; it forms a strong and stable link between its constituent atoms. A water molecule is built using covalent bonds. Hydrogen has one electron in its outer shell, but the optimum number for this shell is two. Oxygen has six electrons in its outer shell, but the optimum number for this shell is eight. Therefore, if one oxygen atom and two hydrogen atoms combine, each hydrogen atom will share its electron with the oxygen atom, giving the oxygen atom a total of eight outer electrons, making it stable. The oxygen atom shares one of its electrons with each of the two hydrogen atoms, so that each hydrogen atom has two electrons in its outer shell, and they too are stable ( Fig. 2.4 ).
Figure 2.4 A water molecule, showing the covalent bonds between hydrogen (yellow) and oxygen (green).
Ionic
Isotopes
These are atoms of an element in which there is a different number of neutrons in the nucleus . This does not affect the electrical activity of these atoms because neutrons carry no electrical charge, but it does affect their atomic weight. For example, there are three forms of the hydrogen atom. The most common form has one proton in the nucleus and one orbiting electron. Another form ( deuterium ) has one proton and one neutron in the nucleus. A third form ( tritium ) has one proton and two neutrons in the nucleus and one orbiting electron. These three forms of hydrogen are called isotopes ( Fig. 2.3 ).
Figure 2.3 The isotopes of hydrogen.
Because the atomic weight of an element is actually an average atomic weight calculated using all its atoms, the true atomic weight of hydrogen is 1.008, although for most practical purposes it can be taken as 1.
Chlorine has an atomic weight of 35.5, because it contains two isotopes, one with an atomic weight of 35 (with 18 neutrons in the nucleus) and the other 37 (with 20 neutrons in the nucleus). Because the proportion of these two forms is not equal, the average atomic weight is 35.5.
Molecules and compounds
It was mentioned earlier that the atoms of each element have a specific number of electrons around the nucleus. When the number of electrons in the outer shell of an element is either the maximum number ( Fig. 2.1 ), or a stable proportion of this fraction, the element is described as inert or chemically unreactive, i.e. it will not easily combine with other elements to form compounds. These elements are the inert gases – helium, neon, argon, krypton, xenon and radon.
Molecules consist of two or more atoms that are chemically combined. The atoms may be of the same element, e.g. a molecule of atmospheric oxygen (O 2 ) contains two oxygen atoms. Most molecules, however, contain two or more different elements, e.g. a water molecule (H 2 O) consists of two hydrogen atoms and an oxygen atom. As mentioned earlier, when two or more elements combine, the resulting molecule is referred to as a compound.
Compounds that contain the elements carbon and hydrogen are classified as organic , and all others as inorganic . Living tissues are based on organic compounds, but the body requires inorganic compounds too.
Covalent and ionic bonds
The vast array of chemical processes on which life is based is completely dependent upon the way atoms come together, bind and break apart. For example, the simple water molecule is a crucial foundation of all life on Earth. If water was a less stable compound, and the atoms came apart easily, human biology could never have evolved. On the other hand, the body is dependent upon the breaking down of various molecules (e.g. sugars, fats) to release energy for cellular activities. When atoms are joined together, they form a chemical bond that is generally one of two types: covalent or ionic .
Covalent bonds are formed when atoms share their electrons with each other. Most molecules are held together with this type of bond; it forms a strong and stable link between its constituent atoms. A water molecule is built using covalent bonds. Hydrogen has one electron in its outer shell, but the optimum number for this shell is two. Oxygen has six electrons in its outer shell, but the optimum number for this shell is eight. Therefore, if one oxygen atom and two hydrogen atoms combine, each hydrogen atom will share its electron with the oxygen atom, giving the oxygen atom a total of eight outer electrons, making it stable. The oxygen atom shares one of its electrons with each of the two hydrogen atoms, so that each hydrogen atom has two electrons in its outer shell, and they too are stable ( Fig. 2.4 ).
Figure 2.4 A water molecule, showing the covalent bonds between hydrogen (yellow) and oxygen (green).
Ionic
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