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Part 5:The Periodic Table

Vocabulary

Mendeleyev

You will not usually see the elements and their symbols listed in alphabetical order. Instead, you will see them in a brightly colored grid or table (something like a calendar) that has nothing to do with alphabetical order at all. This familiar table is called "The Periodic Table of the Elements". "Periodic" means "repeating," and like the calendar the periodic table has some things in it that repeat over and over.

How did Mendeleyev know the weights of atoms when no one had ever weighed a single atom? A chemist named Avogadro came up with a way to weigh atoms indirectly. He found that if you put two gases in two equal containers, and made sure that the gases were at the same temperature and under the same pressure, then they must have the same number of atoms. All you had to do was compare the weights of the gases. Then you could tell which atom was heavier.

In the late 1800s, a Russian chemist named Dmitri Mendeleyev who was writing a textbook decided to take the known elements (there were only 63 at that time) and organize them. First, as other scientists had done, he arranged them in order by the weight of their atoms. Instead of leaving them as a simple list, he put them in a table or grid, and he placed them so that they had a pattern of repeating properties. It turns out that arranging the atoms in order of their weights is the same as arranging them in order of the atomic number, or the number of protons in the nucleus of an atom of that element. Hydrogen is the first element, with one proton, so its atomic number is 1. Helium is the second, with two protons, so its atomic number is 2. He left gaps where there seemed as if there ought to be elements, even though they had not been discovered yet.

Within his lifetime, those gaps were filled by newly discovered elements. There are no gaps left in the Periodic Table now, so if any new elements were discovered they would have to go at the end. This means they would be very heavy. The heaviest elements can only be created in the laboratory and only in very small amounts. Their atoms are so full of protons that they fall apart!

The heaviest recent element is probably number 116, which was made in 1999. It doesn't have a name yet. Scientists thought they had made element 118 in 1999, but two years later they said they were wrong.

What It Looks Like

People who study chemistry become very familiar with the Periodic Table. The one below contains only the basic facts: the symbols for the elements and their atomic numbers.

H
1

Periodic Table of the Elements

 He
2
Li
3
Be
4
B
5 		C
6 		N
7 		O
8 		F
9 		Ne
10
Na
11 		Mg
12

Al
13

 Si
14 		P
15 		S
16 		Cl
17 		Ar
18
K
19 		Ca
20

Sc
21

Ti
22 		V
23 		Cr
24 		Mn
25 		Fe
26 		Co
27 		Ni
28 		Cu
29 		Zn
30 		Ga
31 		Ge
32

As
33

Se
34 		Br
35 		Kr
36
Rb
37 		Sr
38 		Y
39 		Zr
40 		Nb
41 		Mo
42 		Tc
43 		Ru
44 		Rh
45 		Pd
46 		Ag
47 		Cd
48 		In
49 		Sn
50 		Sb
51 		Te
52

I
53

Xe
54
Cs
55

Ba
56

La
57

Hf
72

Ta
73 		W
74 		Re
75 		Os
76 		Ir
77

Pt
78

Au
79 		Hg
80 		Tl
81 		Pb
82 		Bi
83 		Po
84 		At
85 		Rn
86
Fr
87 		Ra
88 		Ac
89 		Rf
104		 Db
105 		Sg
106 		Bh
107 		Hs
108 		Mt
109
110
58-71 Lanthanum series Ce
58

Pr
59

Nd
60 		Pm
61 		Sm
62 		Eu
63 		Gd
64 		Tb
65 		Dy
66 		Ho
67 		Er
68 		Tm
69 		Yb
70 		Lu
71

90-103 Actinium series

 Th
90 		Pa
91 		U
92 		Np
93 		Pu
94 		Am
95 		Cm
96 		Bk
97 		Cf
98 		Es
99 		Fm
100 		Md
101 		No
102 		Lr
103

Where an element is located on the Periodic Table has something to do with the way it behaves. The elements in the far right column (He to Rn) are called "noble gases," for instance, because they are all found in nature as gases and because they do not like to combine with other elements.

Summary

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This page last modified August 15, 2002

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