《医学生理学 Medical Physiology》参考教材资料（PDF电子书籍）Mader Understanding Human Anatomy and Physiology 5th edition

Mader. Understanding Companies, 2004 Physiology, Fifth Edition h Chemistry of Life Cholesterol is just one of many types of chapter outline learning objectives After you have studied this chapter, you should be able to 2⊥ Basic Chemistry (p. 2.3 Molecules of Life(p. 24) 2.6 Proteins(p. 28) a Describe how an atom is organized, and tell List the four classes of m olecules in I State the major functions of proteins, and tell I Define radioactive isotope, and describe how reaction and a hydrolysis reaction Name the individual subunits that comprise 7 Nucleic Acids (p. 31) treatment of disease rbohydrates, lipids, proteins, and nucleic Describe the structure and function of DNA Distinguish between an ionic bond and a cids Explain the importance of ATP in the body. 2.2 Water, Acids, and Bases (p. 22) Give some examples of different types of Medical Focus a Describe the characteristics of water and functions of water in the human body. cellsydrates and their specific functions in Nutrition Labels(p. 30) Explain the difference between an acid and a base with examples. 2.5 Lipids (p 2( rstand the pH scale. I Describe the composition of a neutral fat, and give examples of how lipids function in the 7

Mader: Understanding Human Anatomy & Physiology, Fifth Edition I. Human Organization 2. Chemistry of Life © The McGraw−Hill Companies, 2004 chapter Chemistry of Life Cholesterol crystals photographed in polarized light. Cholesterol is just one of many types of organic molecules. chapter outline & learning objectives After you have studied this chapter, you should be able to: 2.1 Basic Chemistry (p. 18) ■ Describe how an atom is organized, and tell why atoms interact. ■ Define radioactive isotope, and describe how they can be used in the diagnosis and treatment of disease. ■ Distinguish between an ionic bond and a covalent bond. 2.2 Water, Acids, and Bases (p. 22) ■ Describe the characteristics of water and three functions of water in the human body. ■ Explain the difference between an acid and a base with examples. ■ Use and understand the pH scale. 2.3 Molecules of Life (p. 24) ■ List the four classes of macromolecules in cells, and distinguish between a dehydration reaction and a hydrolysis reaction. ■ Name the individual subunits that comprise carbohydrates, lipids, proteins, and nucleic acids. 2.4 Carbohydrates (p. 24) ■ Give some examples of different types of carbohydrates and their specific functions in cells. 2.5 Lipids (p. 26) ■ Describe the composition of a neutral fat, and give examples of how lipids function in the body. 2.6 Proteins (p. 28) ■ State the major functions of proteins, and tell how globular proteins are organized. 2.7 Nucleic Acids (p. 31) ■ Describe the structure and function of DNA and RNA in cells. ■ Explain the importance of ATP in the body. Medical Focus Nutrition Labels (p. 30) 17

Mader. Understanding T L Human Organization 2 Chemistry of Life T ② The McG ysiology, Fifth Edition 2.1 Basic Chemistr Figure 2.1 Elements and atoms. a. The atomic symbol, number, and weight are given for common elements in the bod Matter is anything that takes up space and has weight; it can b. The structure of carbon shows that an atom contains the be a solid, a liquid, or a gas. Therefore, not only are we hu- subatomic particles called protons (p)and neutrons(n)in the mans matter, but so are the water we drink and the air we nucleus(colored pink)and electrons(colored blue)in shells about the nucleus Elements and Atoms Common Elements in Living Things All matter is composed of basic substances called elements It's quite remarkable that there are only 92 naturally occurring Element Symbol Number WeightComment elements. It is even more surprising that over 90% of the hu- These man body is composed of just four elements: carbon, nitro- make gen, oxygen, and hydrogen oxygen Every element has a name and a symbol; for example, car- phosphorusP bon has been assigned the atomic symbol C( Fig. 2.1a). Some sulfur molecules of the symbols we use for elements are derived from Latin. For example, the symbol for sodium is Na because natrium in Latin means sodium as dissolved Elements are composed of tiny particles called atoms. The calciu same name is given to both an element and its atoms Atoms p= prot An atom is the smallest unit of an element that still retains the O=el chemical and physical properties of the element. Although it is possible to split an atom by physical means, an atom is the smallest unit to enter into chemical reactions. For our pur- poses, it is satisfactory to think of each atom as having a cen- tral nucleus and pathways about the nucleus called shells. The subatomic particles called protons and neutrons are located in the nucleus, and electrons orbit about the nucleus in the shells (Fig. 2.1b). Most of an atom is empty space. If we could draw an atom the size of a football stadium, the nucleus Carbon would be like a gumball in the center of the field, and the elec trons would be tiny specks whirling about in the upper atomic number. Protons carry a positive(+)charge, and electrons have a negative(-)charge. The atomic number of an atom tells you with only one shell are stable when this shell contains two how many protons, and therefore how many electrons, an electrons. atom has when it is electrically neutral. For example, the The subatomic particles are so light that their weight is atomic number of carbon is six; therefore, when carbon is indicated by special designations called atomic mass units neutral, it has six protons and six electrons. How many elec- Protons and neutrons each have a weight of one atomic mass trons are in each shell of an atom? The inner shell is the low- unit, and electrons have almost no mass. Therefore, the est energy level and can hold only two electrons; after that, atomic weight of an atom generally tells you the number of each shell, for the atoms noted in Figure 2.la, can hold up to protons plus the number of neutrons. How could you cal eight electrons. Using this information, we can calculate that culate that carbon( C)has six neutrons? Carbons atomic carbon has two shells and that the outer shell has four weight is 12, and you know from its atomic number that it electrons has six protons. Therefore, carbon has six neutrons(Fig The number of electrons in the outer shell determines the 2.1b) chemical properties of an atom, including how readily it en- Also, as shown in Figure 2. 1b, the atomic number of an ters into chemical reactions. As we shall see, an atom is most atom is often written as a subscript to the lower left of the stable when the outer shell has eight electrons.(Hydrogen, atomic symbol. The atomic weight is often written as a supe with only one shell, is an exception to this statement. Atoms script to the upper left of the atomic symbol 18Part I Human Organization

Mader: Understanding Human Anatomy & Physiology, Fifth Edition I. Human Organization 2. Chemistry of Life © The McGraw−Hill Companies, 2004 2.1 Basic Chemistry Matter is anything that takes up space and has weight; it can be a solid, a liquid, or a gas. Therefore, not only are we hu￾mans matter, but so are the water we drink and the air we breathe. Elements and Atoms All matter is composed of basic substances called elements. It’s quite remarkable that there are only 92 naturally occurring elements. It is even more surprising that over 90% of the hu￾man body is composed of just four elements: carbon, nitro￾gen, oxygen, and hydrogen. Every element has a name and a symbol; for example, car￾bon has been assigned the atomic symbol C (Fig. 2.1a). Some of the symbols we use for elements are derived from Latin. For example, the symbol for sodium is Na because natrium in Latin means sodium. Elements are composed of tiny particles called atoms. The same name is given to both an element and its atoms. Atoms An atom is the smallest unit of an element that still retains the chemical and physical properties of the element. Although it is possible to split an atom by physical means, an atom is the smallest unit to enter into chemical reactions. For our pur￾poses, it is satisfactory to think of each atom as having a cen￾tral nucleus and pathways about the nucleus called shells. The subatomic particles called protons and neutrons are located in the nucleus, and electrons orbit about the nucleus in the shells (Fig. 2.1b). Most of an atom is empty space. If we could draw an atom the size of a football stadium, the nucleus would be like a gumball in the center of the field, and the elec￾trons would be tiny specks whirling about in the upper stands. Protons carry a positive (
) charge, and electrons have a negative () charge. The atomic number of an atom tells you how many protons, and therefore how many electrons, an atom has when it is electrically neutral. For example, the atomic number of carbon is six; therefore, when carbon is neutral, it has six protons and six electrons. How many elec￾trons are in each shell of an atom? The inner shell is the low￾est energy level and can hold only two electrons; after that, each shell, for the atoms noted in Figure 2.1a, can hold up to eight electrons. Using this information, we can calculate that carbon has two shells and that the outer shell has four electrons. The number of electrons in the outer shell determines the chemical properties of an atom, including how readily it en￾ters into chemical reactions. As we shall see, an atom is most stable when the outer shell has eight electrons. (Hydrogen, with only one shell, is an exception to this statement. Atoms with only one shell are stable when this shell contains two electrons.) The subatomic particles are so light that their weight is indicated by special designations called atomic mass units. Protons and neutrons each have a weight of one atomic mass unit, and electrons have almost no mass. Therefore, the atomic weight of an atom generally tells you the number of protons plus the number of neutrons. How could you cal￾culate that carbon (C) has six neutrons? Carbon’s atomic weight is 12, and you know from its atomic number that it has six protons. Therefore, carbon has six neutrons (Fig. 2.1b). Also, as shown in Figure 2.1b, the atomic number of an atom is often written as a subscript to the lower left of the atomic symbol. The atomic weight is often written as a super￾script to the upper left of the atomic symbol. 18 Part I Human Organization a. sodium magnesium chlorine potassium calcium Na Mg Cl K Ca 11 12 17 19 20 23 24 35 39 40 These elements occur mainly as dissolved salts. hydrogen carbon nitrogen oxygen phosphorus sulfur H C N O P S 1 6 7 8 15 16 1 12 14 16 31 32 These elements make up most biological molecules. Element Comment Atomic Number Atomic Weight Atomic Symbol Common Elements in Living Things Carbon atomic weight atomic number 6p 6n p = protons n = neutrons = electrons 6 12C b. Figure 2.1 Elements and atoms. a. The atomic symbol, number, and weight are given for common elements in the body. b. The structure of carbon shows that an atom contains the subatomic particles called protons (p) and neutrons (n) in the nucleus (colored pink) and electrons (colored blue) in shells about the nucleus.

Mader. Understanding T L Human Organization 2 Chemistry of Life T ② The McG ysiology, Fifth Edition Isotopes ing a minute amount of radioactive iodine(I), the tracer be comes concentrated in the thyroid, which takes it up to make Isotopes of the same type of atom differ in the number of the hormone thyroxine. (No other organ takes up 3"1.)A sub- neutrons and therefore in weight. For example, the element sequent image of the thyroid indicates whether it is healthy in structure and function( Fig. 2. 2). Positron-emission tomogra- phy(PEt) is a way to determine the comparative activity of tis- sues. Radioactively labeled glucose emits a subatomic pa known as a positron. When labeled glucose is injected into the Carbon 12 has six neutrons, carbon 13 has seven neutrons, body. The radiation given off is detected by sensors and ana- topes of carbon, carbon 14 is unstable and breaks down over which tissues took up glucose anld color image that shows and carbon 14 has eight neutrons. Unlike the other two iso- lyzed by a computer. The result time.As carbon 14 decays, it releases various types of energy (Fig. 2.3). A PET scan of the brain can help diagnose a brain tu- in the form of rays and subatomic particles, and therefore it is mor, Alzheimer disease, epilepsy, or stroke. a radioactive isotope. The radiation given off by radioactive isotopes can be detected in various ways. You may be familiar with the use of a Geiger counter to detect radiation. High Levels of radiation Radioactive substances in the environment can harm cells Low Levels of radiation damage DNA, and cause cancer. The release of radioactive particles following a nuclear power plant accident can have The importance of chemistry to biology and medicine is far-reaching and long-lasting effects on human health. The nowhere more evident than in the many uses of radioactive harmful effects of radiation can also be put to good use, how- isotopes. A radioactive isotope behaves the same as do the sta- ever. Radiation from radioactive isotopes has been used for ble isotopes of an element. This means that you can put a many years to sterilize medical and dental products. Now the small amount of radioactive isotope in a sample, and it be- possibility exists that it can be used to sterilize the U.S. mail to comes a tracer by which to detect molecular changes Specific tracers are used in imaging the body s organs and free it of possible pathogene kill cells is often applied to can- tissues. For example, after a patient drinks a solution contain cer cells. Radioisotopes can be introduced into the body in a way that allows radiation to destroy only the cancerous cells, Figure 2.2 Use of radiation to aid a diagnosis. After the with little risk to the rest of the body administration of radioactive iodine, a scan of the thyroid reveals pathology. The missing portion of the gland is cancerous and therefore failed to take up the iodine. Figure 2.3 Use of radiation to study the brain. After the administration of radioactively labeled glucose, a PET scan reveals which portions of the brain are most active. thyroid a Patient entering trachea b Scan of thyro Chapter 2 Chemistry of Life 19

Mader: Understanding Human Anatomy & Physiology, Fifth Edition I. Human Organization 2. Chemistry of Life © The McGraw−Hill Companies, 2004 Isotopes Isotopes of the same type of atom differ in the number of neutrons and therefore in weight. For example, the element carbon has three common isotopes: Carbon 12 has six neutrons, carbon 13 has seven neutrons, and carbon 14 has eight neutrons. Unlike the other two iso￾topes of carbon, carbon 14 is unstable and breaks down over time. As carbon 14 decays, it releases various types of energy in the form of rays and subatomic particles, and therefore it is a radioactive isotope. The radiation given off by radioactive isotopes can be detected in various ways. You may be familiar with the use of a Geiger counter to detect radiation. Low Levels of Radiation The importance of chemistry to biology and medicine is nowhere more evident than in the many uses of radioactive isotopes. A radioactive isotope behaves the same as do the sta￾ble isotopes of an element. This means that you can put a small amount of radioactive isotope in a sample, and it be￾comes a tracer by which to detect molecular changes. Specific tracers are used in imaging the body’s organs and tissues. For example, after a patient drinks a solution contain- 6 12C 6 13C 6 14C* *radioactive ing a minute amount of radioactive iodine (131I), the tracer be￾comes concentrated in the thyroid, which takes it up to make the hormone thyroxine. (No other organ takes up 131I.) A sub￾sequent image of the thyroid indicates whether it is healthy in structure and function (Fig. 2.2). Positron-emission tomogra￾phy (PET) is a way to determine the comparative activity of tis￾sues. Radioactively labeled glucose emits a subatomic particle known as a positron. When labeled glucose is injected into the body. The radiation given off is detected by sensors and ana￾lyzed by a computer. The result is a color image that shows which tissues took up glucose and are metabolically active (Fig. 2.3). A PET scan of the brain can help diagnose a brain tu￾mor, Alzheimer disease, epilepsy, or stroke. High Levels of Radiation Radioactive substances in the environment can harm cells, damage DNA, and cause cancer. The release of radioactive particles following a nuclear power plant accident can have far-reaching and long-lasting effects on human health. The harmful effects of radiation can also be put to good use, how￾ever. Radiation from radioactive isotopes has been used for many years to sterilize medical and dental products. Now the possibility exists that it can be used to sterilize the U.S. mail to free it of possible pathogens, such as anthrax spores. The ability of radiation to kill cells is often applied to can￾cer cells. Radioisotopes can be introduced into the body in a way that allows radiation to destroy only the cancerous cells, with little risk to the rest of the body. Chapter 2 Chemistry of Life 19 thyroid gland trachea (windpipe) Figure 2.2 Use of radiation to aid a diagnosis. After the administration of radioactive iodine, a scan of the thyroid reveals pathology. The missing portion of the gland is cancerous and therefore failed to take up the iodine. Figure 2.3 Use of radiation to study the brain. After the administration of radioactively labeled glucose, a PET scan reveals which portions of the brain are most active. a. Drawing of thyroid a. Patient entering PET scanner b. Scan of thyroid b. PET scan

Mader. Understanding T L Human Organization 2 Chemistry of Life T ② The McG ysiology, Fifth Edition Molecules and Compounds once the reaction is finished and sodium loses one electron to chlorine, its outer shell will have eight electrons. Similarly, a Atoms often bond with each other to form a chemical unit chlorine atom, which has seven electrons already, needs only to called a molecule. A molecule can contain atoms of the same kind, as when an oxygen atom joins with another oxy- acquire one more electron to have a stable outer shell gen atom to form oxygen gas. Or the atoms can be different, (-)charge. When the reaction between sodium and chlorine as when an oxygen atom joins with two hydrogen atoms to finished, the sodium ion carries a positive charge because it form water. When the atoms are different, a compound now has one more proton than electrons, and the chloride ion carries a negative charge because it now has one fewer proton co, Two types of bonds join atoms: the ionic bond and the than electrons. The attraction between oppositely charged alent bond. The first type of bond can be associated with sodium ions and chloride ions forms an ionic bond. The re- norganic molecules, which constitute nonliving matter, and sulting compound, sodium chloride, is table salt, which we use the second type can be associated with organic molecules, to enliven the taste of foods. Salts characteristically form an which are unique to living things ionic lattice that dissociates in water(Fig. 2.4b) In contrast to sodium, why would calcium, with two elec- lonic bonds trons in the outer shell, react with two chlorine atoms? Be cause whereas calcium needs to lose two electrons, each chlo- Recall that atoms with more than one shell are most stable rine, with seven electrons already, requires only one more when the outer shell contains eight electrons. Sometimes dur- electron to have a stable outer shell. The resulting salt( CaCl2) ing a reaction, atoms give up or take on an electron(s)in or- is called calcium chloride der to achieve a stable outer shell Figure 2. 4 depicts a reaction between a sodium(Na)atom health Too much sodium in the blood can contribute to hy and a chlo outer shell, reacts with a single chlorine atom. Why? Because pertension(high blood pressure); not enough calcium leads to Figure 2.4 lonic reaction. a. During the formation of sodium chloride, an electron is transferred from the sodium atom to the chlorine tom. At the completion of the reaction, each atom has eight electrons in the outer shell, but each also carries a charge as shown. b In a sodium chloride crystal, bonding between ions creates a three-dimensional lattice in which each Na* ion is surrounded by six Cl- ions, and each CI" is surrounded by six Na- a. sodium atom(Na) chlorine atom(cn) sodium ion(Nat) chloride ion(Cn) sodium chloride(Nacl) 20 Part I Human Organization

Mader: Understanding Human Anatomy & Physiology, Fifth Edition I. Human Organization 2. Chemistry of Life © The McGraw−Hill Companies, 2004 Molecules and Compounds Atoms often bond with each other to form a chemical unit called a molecule. A molecule can contain atoms of the same kind, as when an oxygen atom joins with another oxy￾gen atom to form oxygen gas. Or the atoms can be different, as when an oxygen atom joins with two hydrogen atoms to form water. When the atoms are different, a compound results. Two types of bonds join atoms: the ionic bond and the covalent bond. The first type of bond can be associated with inorganic molecules, which constitute nonliving matter, and the second type can be associated with organic molecules, which are unique to living things. Ionic Bonds Recall that atoms with more than one shell are most stable when the outer shell contains eight electrons. Sometimes dur￾ing a reaction, atoms give up or take on an electron(s) in or￾der to achieve a stable outer shell. Figure 2.4 depicts a reaction between a sodium (Na) atom and a chlorine (Cl) atom. Sodium, with one electron in the outer shell, reacts with a single chlorine atom. Why? Because once the reaction is finished and sodium loses one electron to chlorine, its outer shell will have eight electrons. Similarly, a chlorine atom, which has seven electrons already, needs only to acquire one more electron to have a stable outer shell. Ions are particles that carry either a positive (
) or negative () charge. When the reaction between sodium and chlorine is finished, the sodium ion carries a positive charge because it now has one more proton than electrons, and the chloride ion carries a negative charge because it now has one fewer proton than electrons. The attraction between oppositely charged sodium ions and chloride ions forms an ionic bond. The re￾sulting compound, sodium chloride, is table salt, which we use to enliven the taste of foods. Salts characteristically form an ionic lattice that dissociates in water (Fig. 2.4b). In contrast to sodium, why would calcium, with two elec￾trons in the outer shell, react with two chlorine atoms? Be￾cause whereas calcium needs to lose two electrons, each chlo￾rine, with seven electrons already, requires only one more electron to have a stable outer shell. The resulting salt (CaCl2) is called calcium chloride. The balance of various ions in the body is important to our health. Too much sodium in the blood can contribute to hy￾pertension (high blood pressure); not enough calcium leads to 20 Part I Human Organization 1 mm sodium atom (Na) chlorine atom (Cl) sodium chloride (NaCl) sodium ion (Na+) chloride ion (Cl−) Na+ Cl− a. b. Na Cl Na Cl + – + Figure 2.4 Ionic reaction. a. During the formation of sodium chloride, an electron is transferred from the sodium atom to the chlorine atom. At the completion of the reaction, each atom has eight electrons in the outer shell, but each also carries a charge as shown. b. In a sodium chloride crystal, bonding between ions creates a three-dimensional lattice in which each Na
ion is surrounded by six Cl ions, and each Cl is surrounded by six Na
.

Mader. Understanding T L Human Organization 2 Chemistry of Life T ② The McG ysiology, Fifth Edition rickets(a bowing of the legs )in children; too much or too lit Double and Triple Bonds Besides a single bond, in which tle potassium results in arrhythmia(heartbeat irregularities). atoms share only a pair of electrons, a double or a triple bond Bicarbonate, hydrogen, and hydroxide ions are all involved in can form. In a double bond, atoms share two pairs of ele maintaining the acid-base balance of the body(see page 22) trons, and in a triple bond, atoms share three pairs of elec- trons between them. For example, in Figure 2.5, each nitrogen Covalent Bonds atom(N)requires three electrons to achieve a total of eight electrons in the outer shell. notice that six electrons are a result of other reactions, atoms share electrons in cova- placed in the outer overlapping shells in the diagram and that lent bonds instead of losing or gaining them. The overlap three straight lines are in the structural formula for nitrogen ping outermost shells in Figure 2. 5 indicate that the atoms are gas(N2) sharing electrons. Just as two hands participate in a hand- What would be the structural and molecular formulas for shake, each atom contributes one electron to the pair that is carbon dioxide? Carbon, with four electrons in the outer shared. These electrons spend part of their time in the outer shell, requires four more electrons to complete its outer shell hell of each atom; therefore, they are counted as belonging to Each oxygen, with six electrons in the outer shell, needs only both bonded atoms two electrons to complete its outer shell. Therefore, carbon In contrast to the diagrams in Figure 2.5, structural formulas formulas are as followg. ons with each oxygen atom, and the Covalent bonds can be represented in a number of ways ares two pairs of ele use straight lines to show the covalent bonds between the Structural formula: O-C=O atoms. Each line represents a pair of shared electrons Molec ular formulas indicate only the number of each type of atom Molecular formula: CO2 making up a molecule. A comparison follows tructural formula: Cl-Cl Molecular formula: CIz Figure 2.5 Covalent reactions. After a covalent reaction, each atom will have filled its outer shell by sharing electrons To determine this, it is necessary to count the shared electrons as belonging to both bonded atoms. Oxygen and nitrogen are most stable with eight electrons in the outer shell. Hydrogen is most stable with two electrons in the outer shell. .⑨ oxygen water什H2O) Chapter 2 Chemistry of Life21

Mader: Understanding Human Anatomy & Physiology, Fifth Edition I. Human Organization 2. Chemistry of Life © The McGraw−Hill Companies, 2004 rickets (a bowing of the legs) in children; too much or too lit￾tle potassium results in arrhythmia (heartbeat irregularities). Bicarbonate, hydrogen, and hydroxide ions are all involved in maintaining the acid-base balance of the body (see page 22). Covalent Bonds As a result of other reactions, atoms share electrons in cova￾lent bonds instead of losing or gaining them. The overlap￾ping outermost shells in Figure 2.5 indicate that the atoms are sharing electrons. Just as two hands participate in a hand￾shake, each atom contributes one electron to the pair that is shared. These electrons spend part of their time in the outer shell of each atom; therefore, they are counted as belonging to both bonded atoms. Covalent bonds can be represented in a number of ways. In contrast to the diagrams in Figure 2.5, structural formulas use straight lines to show the covalent bonds between the atoms. Each line represents a pair of shared electrons. Molec￾ular formulas indicate only the number of each type of atom making up a molecule. A comparison follows: Structural formula: Cl—Cl Molecular formula: Cl2 Double and Triple Bonds Besides a single bond, in which atoms share only a pair of electrons, a double or a triple bond can form. In a double bond, atoms share two pairs of elec￾trons, and in a triple bond, atoms share three pairs of elec￾trons between them. For example, in Figure 2.5, each nitrogen atom (N) requires three electrons to achieve a total of eight electrons in the outer shell. Notice that six electrons are placed in the outer overlapping shells in the diagram and that three straight lines are in the structural formula for nitrogen gas (N2). What would be the structural and molecular formulas for carbon dioxide? Carbon, with four electrons in the outer shell, requires four more electrons to complete its outer shell. Each oxygen, with six electrons in the outer shell, needs only two electrons to complete its outer shell. Therefore, carbon shares two pairs of electrons with each oxygen atom, and the formulas are as follows: Structural formula: O——C——O Molecular formula: CO2 Chapter 2 Chemistry of Life 21 nitrogen nitrogen nitrogen gas (N2) oxygen 2 hydrogen water (H2O) N O 2 H N N N H H O + + 7p 7n 7p 7n 8p 8n 1p 1p 1p 8p 8n 7p 7n 7p 7n 1p Figure 2.5 Covalent reactions. After a covalent reaction, each atom will have filled its outer shell by sharing electrons. To determine this, it is necessary to count the shared electrons as belonging to both bonded atoms. Oxygen and nitrogen are most stable with eight electrons in the outer shell. Hydrogen is most stable with two electrons in the outer shell