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1B Focus on the Human Body: The Elements of Life

1B Focus on the Human Body: The Elements of Life

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ELEMENTS







35



FIGURE 2.1



The Periodic Table of the Elements



1A

1

1

H



1



8A

18

2

He



4A

14



5A

15



6A

16



7A

17



4.0026



3

Li



2A

2

4

Be



3A

13

5

B



6

C



7

N



8

O



9

F



10

Ne



6.941



9.0122



10.811



12.011



14.0067



15.9994



18.9984



20.1797



11

Na



12

Mg



22.9898



1.0079



2



3



2B

12



13

Al



14

Si



15

P



16

S



17

Cl



18

Ar



10



1B

11



26.9815



28.0855



30.9738



32.066



35.453



39.948



27

Co



28

Ni



29

Cu



30

Zn



31

Ga



32

Ge



33

As



34

Se



35

Br



36

Kr



55.845



58.9332



58.693



63.546



65.41



69.723



72.64



74.9216



78.96



79.904



83.80



43

Tc



44

Ru



45

Rh



46

Pd



47

Ag



48

Cd



49

In



50

Sn



51

Sb



52

Te



53

I



54

Xe



95.94



(98)



101.07



102.9055



106.42



114.82



118.710



121.760



127.60



126.9045



131.29



73

Ta



74

W



75

Re



76

Os



77

Ir



78

Pt



79

Au



80

Hg



81

Tl



82

Pb



83

Bi



84

Po



85

At



86

Rn



178.49



180.9479



183.84



186.207



190.2



192.22



195.08



196.9665



200.59



204.3833



207.2



208.9804



(209)



(210)



(222)



24.3050



3B

3



4B

4



5B

5



6B

6



7B

7



19

K



20

Ca



21

Sc



22

Ti



23

V



24

Cr



39.0983



40.078



44.9559



47.88



50.9415



37

Rb



38

Sr



39

Y



40

Zr



85.4678



87.62



88.9059



55

Cs



56

Ba



57

La



4



5



6



132.9054 137.327 138.9055



7



8



8B

9



25

Mn



26

Fe



51.9961



54.9380



41

Nb



42

Mo



91.224



92.9064



72

Hf



107.8682 112.411



4



88

Ra



89

Ac



104

Rf



105

Db



106

Sg



107

Bh



108

Hs



109

Mt



110

Ds



111

Rg



112





114





116





(223)



(226)



(227)



(267)



(268)



(271)



(272)



(270)



(276)



(281)



(280)



(285)



(289)



(293)



58

Ce



59

Pr



60

Nd



61

Pm



62

Sm



63

Eu



64

Gd



65

Tb



66

Dy



67

Ho



68

Er



69

Tm



70

Yb



71

Lu



144.24



(145)



150.36



151.964



157.25



158.9253



162.50



164.9303



167.26



168.9342



173.04



174.967



92

U



93

Np



94

Pu



95

Am



96

Cm



97

Bk



98

Cf



99

Es



100

Fm



101

Md



102

No



103

Lr



(237)



(244)



(243)



(247)



(247)



(251)



(252)



(257)



(258)



(259)



(262)



140.115 140.9076



7



90

Th



91

Pa



232.0381 231.03588 238.0289



metal



metalloid



nonmetal



silver (Ag)



silicon (Si)



carbon (C)



sodium (Na)



arsenic (As)



sulfur (S)



mercury (Hg)



boron (B)



bromine (Br)



2



3



87

Fr



6



1



5



6



7



6



7



• Metals like silver, sodium, and mercury are shiny substances that conduct heat and electricity.

• Metalloids like silicon, arsenic, and boron have properties intermediate between metals and nonmetals.

• Nonmetals like carbon, sulfur, and bromine are poor conductors of heat and electricity.



smi26573_ch02.indd 35



11/13/08 4:27:13 PM



36



ATOMS AND THE PERIODIC TABLE







FIGURE 2.2



The Elements of Life



Building-Block Elements

Oxygen (O)

Carbon (C)

Hydrogen (H)

Nitrogen (N)

These four elements compose

almost 96% of the mass of the

human body.

Major Minerals

Potassium (K), sodium (Na), and

chlorine (Cl) are present in body

fluids.

Magnesium (Mg) and sulfur (S) are

found in proteins.

Calcium (Ca) and phosphorus (P)

are present in teeth and bones.

Each major mineral is present in

0.1–2% by mass. At least 100 mg of

each mineral is needed in the daily

diet.

Trace Elements

Arsenic (As)

Boron (B)

Chromium (Cr)

Cobalt (Co)

Copper (Cu)

Fluorine (F)

Iodine (I)



Iron (Fe)

Manganese (Mn)

Molybdenum (Mo)

Nickel (Ni)

Selenium (Se)

Silicon (Si)

Zinc (Zn)



Each trace element is present in

less than 0.1% by mass. A small

quantity (15 mg or less) of each

element is needed in the daily diet.



2.1C



COMPOUNDS



In Section 1.3 we learned that a compound is a pure substance formed by chemically combining two or more elements together. Element symbols are used to write chemical formulas for

compounds.

• A chemical formula uses element symbols to show the identity of the elements forming

a compound and subscripts to show the ratio of atoms (the building blocks of matter)

contained in the compound.



For example, table salt is formed from sodium (Na) and chlorine (Cl) in a ratio of 1:1, so its

formula is NaCl. Water, on the other hand, is formed from two hydrogen atoms for each oxygen

atom, so its formula is H2O. The subscript “1” is understood when no subscript is written. Other

examples of chemical formulas are shown below.

H2O

water



smi26573_ch02.indd 36



2 H atoms for

each O atom



CO2



2 O atoms for

each C atom



carbon dioxide

(dry ice)



3 C atoms

C3H8



8 H atoms



propane



11/13/08 4:27:17 PM



STRUCTURE OF THE ATOM



37







FIGURE 2.3



Common Element Colors Used in Molecular Art



C



H



O



N



F



Cl



Br



I



S



P



As we learned in Section 1.2, molecular art will often be used to illustrate the composition and

state of elements and compounds. Color-coded spheres, shown in Figure 2.3, are used to identify

the common elements that form compounds.

For example, a red sphere is used for the element oxygen and gray is used for the element hydrogen, so H2O is represented as a red sphere joined to two gray spheres. Sometimes the spheres will

be connected by “sticks” to generate a ball-and-stick representation for a compound. At other

times, the spheres will be drawn close together to form a space-filling representation. No matter

how the spheres are depicted, H2O always consists of one red sphere for the oxygen atom and two

gray spheres for the two hydrogen atoms.



)0ESBXOJOB

TQBDFGJMMJOH

SFQSFTFOUBUJPO



PYZHFO

)0



â



XBUFS

IZESPHFO

CBMMBOETUJDL

SFQSFTFOUBUJPO



PROBLEM 2.6



Identify the elements in each chemical formula, and give the number of atoms of each element.

a. NaCN (sodium cyanide)

b. H2S (hydrogen sulfide)



PROBLEM 2.7



XBUFSJOUIFHBTQIBTF



c. C2H6 (ethane)

d. SnF2 (stannous fluoride)



e. CO (carbon monoxide)

f. C3H8O3 (glycerol)



Identify the elements used in each example of molecular art.



a.



b.



c.



2.2 STRUCTURE OF THE ATOM

All matter is composed of the same basic building blocks called atoms. An atom is much too

small to be seen even by the most powerful light microscopes. The period at the end of this sentence holds about 1 × 108 atoms, and a human cheek cell contains about 1 × 1016 atoms. An atom

is composed of three subatomic particles.

• A proton, symbolized by p, has a positive (+) charge.

• An electron, symbolized by e–, has a negative (–) charge.

• A neutron, symbolized by n, has no charge.



Protons and neutrons have approximately the same, exceedingly small mass, as shown in Table

2.3. The mass of an electron is much less, 1/1,836 the mass of a proton. These subatomic particles are not evenly distributed in the volume of an atom. There are two main components of an

atom.



smi26573_ch02.indd 37



11/13/08 4:27:18 PM



38



ATOMS AND THE PERIODIC TABLE



TABLE 2.3



Summary: The Properties of the Three Subatomic Particles



Subatomic Particle



Charge



Mass (g)



Mass (amu)



Proton



+1



1.6726 ì 1024



1



Neutron



0



1.6749 ì 1024



1



Electron



1



9.1093 ì 1028



Negligible



The nucleus is a dense core that contains the protons and neutrons. Most of the mass of

an atom resides in the nucleus.

• The electron cloud is composed of electrons that move rapidly in the almost empty space

surrounding the nucleus. The electron cloud comprises most of the volume of an atom.

10−10 m



nucleus



neutron

proton

+

+



electron cloud



+

+



10−15 m



main components of an atom



While the diameter of an atom is about 10–10 m, the diameter of a nucleus is only about 10–15 m.

For a macroscopic analogy, if the nucleus were the size of a baseball, an atom would be the size

of Yankee Stadium!

The charged particles of an atom can either attract or repel each other.

• Opposite charges attract while like charges repel each other.



Thus, two electrons or two protons repel each other, while a proton and an electron attract each other.

+



+



Positive charges repel.











Negative charges repel.



+







Opposite charges attract.



Since the mass of an individual atom is so small (on the order of 10–24 g), chemists use a standard mass unit, the atomic mass unit, which defines the mass of individual atoms relative to a

standard mass.

• One atomic mass unit (amu) equals one-twelfth the mass of a carbon atom that has six

protons and six neutrons; 1 amu = 1.661 ë 10–24 g.



Using this scale, one proton has a mass of 1.0073 amu, a value typically rounded to 1 amu. One

neutron has a mass of 1.0087 amu, a value also typically rounded to 1 amu. The mass of an electron is so small that it is ignored.

Every atom of a given type of element always has the same number of protons in the

nucleus, a value called the atomic number, symbolized by Z. Conversely, two different elements

have different atomic numbers.



smi26573_ch02.indd 38



11/13/08 4:27:19 PM



STRUCTURE OF THE ATOM



39



• The atomic number (Z) = the number of protons in the nucleus of an atom.



Thus, the element hydrogen has one proton in its nucleus, so its atomic number is one. Lithium

has three protons in its nucleus, so its atomic number is three. The periodic table is arranged in

order of increasing atomic number beginning at the upper left-hand corner. The atomic number

appears just above the element symbol for each entry in the table.

Since a neutral atom has no overall charge:

• Z = the number of protons in the nucleus = the number of electrons.



Thus, the atomic number tells us both the number of protons in the nucleus and the number of

electrons in the electron cloud of a neutral atom.

atomic number



3 electrons



3



Li

lithium

element symbol



4 neutrons

3 protons



SAMPLE PROBLEM 2.1

ANALYSIS

SOLUTION



PROBLEM 2.8



+



+



+



Identify the element that has an atomic number of 19, and give the number of protons and

electrons in the neutral atom.

The atomic number is unique to an element and tells the number of protons in the nucleus and

the number of electrons in the electron cloud of a neutral atom.

According to the periodic table, the element potassium has atomic number 19. A neutral

potassium atom has 19 protons and 19 electrons.

Identify the element with each atomic number, and give the number of protons and electrons in

the neutral atom: (a) 2; (b) 11; (c) 20; (d) 47; (e) 78.



Both protons and neutrons contribute to the mass of an atom. The mass number, symbolized by

A, is the sum of the number of protons and neutrons.

• Mass number (A) = the number of protons (Z) + the number of neutrons.



For example, a fluorine atom with nine protons and 10 neutrons in the nucleus has a mass number

of 19. Figure 2.4 lists the atomic number, mass number, and number of subatomic particles in

the four building-block elements—hydrogen, carbon, nitrogen, and oxygen—found in a wide

variety of compounds including caffeine (chemical formula C8H10N4O2), the bitter-tasting mild

stimulant in coffee, tea, and cola beverages.



smi26573_ch02.indd 39



11/13/08 4:27:20 PM



40



ATOMS AND THE PERIODIC TABLE







FIGURE 2.4



Atomic Composition of the Four Building-Block Elements



caffeine

(C8H10N4O2)



Element

Atomic number

Mass number

Number of protons

Number of electrons

Number of neutrons



SAMPLE PROBLEM 2.2



1



6



7



8



H



C



N



O



Hydrogen

1

1

1

1

0



Carbon

6

12

6

6

6



Nitrogen

7

14

7

7

7



Oxygen

8

16

8

8

8



How many protons, neutrons, and electrons are contained in an atom of argon, which has an

atomic number of 18 and a mass number of 40?



ANALYSIS



• In a neutral atom, the atomic number (Z) = the number of protons = the number of electrons.

• The mass number (A) = the number of protons + the number of neutrons.



SOLUTION



The atomic number of 18 means that argon has 18 protons and 18 electrons. To find the number

of neutrons, subtract the atomic number (Z) from the mass number (A).

number of neutrons = mass number – atomic number

=

40



18

=

22 neutrons



PROBLEM 2.9



How many protons, neutrons, and electrons are contained in each atom with the given atomic

number and mass number?

a. Z = 17, A = 35



b. Z = 14, A = 28



c. Z = 92, A = 238



PROBLEM 2.10



What element has an atomic number of 53 and contains 74 neutrons? How many electrons does

this atom contain? What is its mass number?



PROBLEM 2.11



What is the mass number of an atom that contains

a. 42 protons, 42 electrons, and 53 neutrons?



b. 24 protons, 24 electrons, and 28 neutrons?



2.3 ISOTOPES

Two atoms of the same element always have the same number of protons, but the number of

neutrons can vary.

• Isotopes are atoms of the same element having a different number of neutrons.



smi26573_ch02.indd 40



11/13/08 4:27:22 PM



ISOTOPES



41



2.3A



ISOTOPES, ATOMIC NUMBER, AND MASS NUMBER



Most elements in nature exist as a mixture of isotopes. For example, all atoms of the element chlorine contain 17 protons in the nucleus, but some of these atoms have 18 neutrons in the nucleus

and some have 20 neutrons. Thus, chlorine has two isotopes with different mass numbers, 35 and

37. These isotopes are often referred to as chlorine-35 (or Cl-35) and chlorine-37 (or Cl-37).

Isotopes are also written using the element symbol with the atomic number written as a subscript

and the mass number written as a superscript, both to the left.

Two isotopes of the element chlorine

mass number



35

17Cl



atomic number



37

17Cl



chlorine-35



chlorine-37



The element hydrogen has three isotopes. Most hydrogen atoms have one proton and no neutrons, giving them a mass number of one. About 1% of hydrogen atoms have one proton and one

neutron, giving them a mass number of two. This isotope is called deuterium, and it is often

symbolized as D. An even smaller number of hydrogen atoms contain one proton and two neutrons, giving them a mass number of three. This isotope is called tritium, symbolized as T.



SAMPLE PROBLEM 2.3



2

1H



3

1H



hydrogen



deuterium



tritium



+



+



1 proton



1 proton

1 neutron



1 proton

2 neutrons



118

50Sn



b.



195

78Pt



• The superscript gives the mass number and the subscript gives the atomic number for each

element.

• The atomic number = the number of protons = the number of electrons.

• The mass number = the number of protons + the number of neutrons.



SOLUTION



smi26573_ch02.indd 41



+



For each atom give the following information: [1] the atomic number; [2] the mass number;

[3] the number of protons; [4] the number of neutrons; [5] the number of electrons.

a.



ANALYSIS



1

1H



Atomic

Number



Mass

Number



Number of

Protons



Number of

Neutrons



Number of

Electrons



a.



118

50Sn



50



118



50



118 – 50 = 68



50



b.



195

78Pt



78



195



78



195 – 78 = 117



78



11/13/08 4:27:22 PM



42



ATOMS AND THE PERIODIC TABLE



PROBLEM 2.12



For each atom give the following information: [1] the atomic number; [2] the mass number;

[3] the number of protons; [4] the number of neutrons; [5] the number of electrons.

a.



13

6C



b.



121

51Sb



Determine the number of neutrons in each isotope: (a) carbon-14; (b) 81Br.



SAMPLE PROBLEM 2.4

ANALYSIS



• The identity of the element tells us the atomic number.

• The number of neutrons = mass number (A) – atomic number (Z).



SOLUTION



a. Carbon’s atomic number (Z) is 6. Carbon-14 has a mass number (A) of 14.

number of neutrons = A – Z

= 14 – 6 = 8 neutrons

b. Bromine’s atomic number is 35 and the mass number of the given isotope is 81.

number of neutrons = A – Z

= 81 – 35 = 46 neutrons



PROBLEM 2.13



Magnesium has three isotopes that contain 12, 13, and 14 neutrons. For each isotope give the

following information: (a) the number of protons; (b) the number of electrons; (c) the atomic

number; (d) the mass number. Write the element symbol of each isotope using a superscript and

subscript for mass number and atomic number, respectively.



2.3B ATOMIC WEIGHT

Some elements like fluorine occur naturally as a single isotope. More commonly, an element is

a mixture of isotopes, and it is useful to know the average mass, called the atomic weight (or

atomic mass), of the atoms in a sample.

• The atomic weight is the weighted average of the mass of the naturally occurring

isotopes of a particular element reported in atomic mass units.



The atomic weights of the elements appear in the alphabetical list of elements on the inside front

cover. The atomic weight is also given under the element symbol in the periodic table on the

inside front cover.

6



C

12.01



atomic number

element symbol

atomic weight



carbon



To determine the atomic weight of an element, two quantities must be known: the mass of each

isotope in atomic mass units, and the frequency with which each isotope occurs.



HOW TO

EXAMPLE

Step [1]



Determine the Atomic Weight of an Element

What is the atomic weight of the element chlorine?

List each isotope, along with its mass in atomic mass units (amu) and the percentage that each isotope occurs in nature.

• Chlorine has two isotopes—Cl-35 and Cl-37.

• To solve the problem, the masses and abundances of the isotopes must be known.



smi26573_ch02.indd 42



Mass (amu)



Isotopic Abundance



Cl-35



34.97



75.78% = 0.7578



Cl-37



36.97



24.22% = 0.2422



11/13/08 4:27:23 PM



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