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1 2 At the right end of the row, the
H Abundance in cells He energy level has a full comple-
3 4 5 6 7 8 9 10 ment of electrons.
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Li Be High Low Trace Rare or none B C N O F Ne The elements in a vertical
11 12 13 14 15 16 17 18
Na Mg Al Si P S Cl Ar column of the periodic table are
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 called a group or family. Mem-
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr bers of a group all have the same
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 number of electrons in their
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
55 56 57-71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 outermost level. For example,
Cs Ba La-Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn carbon (C) and lead (Pb) both
87 88 89-103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 have four electrons in their out-
Fr Ra Ac-Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og
ermost level. The number of
electrons in the outermost level
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 determines in large part how
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu elements interact with other
89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 elements to form a diversity of
Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
molecules, as we will explore in

FIGURE 1.4 The periodic table of the elements the next section.
Elements are arranged by increasing number of protons, the atomic number. The atomic number is All living organisms are
shown above the element. The elements in a column share similar chemical properties. made up of atoms that can be
combined to make molecules.
the elements , shown in FIGURE 1.4 describes valence elec- The four elements common to every organism on the planet
,
trons and other properties of elements. In the periodic table, the are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).
elements are indicated by their chemical symbols and arranged Although organisms use elements in the first five rows of the
in order of increasing atomic number. For example, the second periodic table, these most highly used elements belong to rows
row of the periodic table begins with lithium (Li), which has 1 and 2. Phosphorus, a member of the third row, is also present
3 protons and ends with neon (Ne), which has 10 protons. in large amounts in organic molecules.
For the second and third horizontal rows in the periodic
table, elements in the same row have the same number of
energy levels. Moving across a row, each element has one more ✓ Concept Check
proton and one more electron than the preceding element. 3. Describe the components of an atom.
Let’s take a look at the second row of elements, those that

span from lithium (Li) to neon (Ne), shown in FIGURE 1.5 . 4. Describe how the rows and columns of the periodic

table of the elements are organized.
All of these elements have two energy levels. The innermost 5. Identify what the superscripts of N and N signify.
15
14

level of all of these elements is full. Only the outermost energy 6. Calculate how many additional electrons would be
level has a varied number of electrons, starting with one for needed to fill the outer energy level of C.
lithium and progressively adding one electron to the outer 7. Identify the location of the electrons around an
shell of the elements as we move from left to right across the atom that have the most energy associated with them.
row, ending with eight electrons in neon’s outer energy level.

Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon

FIGURE 1.5 Number of electrons across row 2 of the periodic table

Moving from left to right, each atom contains one more electron than the last, from lithium to
neon. Neon has a full complement of eight electrons in its outer energy level.
32 UNIT 1 CHEMISTRY OF LIFE

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