|
To date, there are around 120 known types of atoms or elements. Of these, about 90 elements can be found in nature. All matters are made of these elements. The rest, usually those of heavier ones (from uranium onwards) no longer
exist or are found only in traces as intermediate decay products alongside with other radioisotopes. These heavier elements, however, can be produced
in a nuclear reactor.
All known elements are arranged systematically in a table called the Periodic Table of the Elements. They are arranged according to the number of protons in atomic nuclei and grouped according to their chemical properties. Below shows a typical table arrangement:
|
1A |
2A |
3B |
4B |
5B |
6B |
7B |
8 |
8 |
8 |
1B |
2B |
3A |
4A |
5A |
6A |
7A |
O |
1 |
1
H |
|
2
He |
2 |
3
Li |
4
Be |
|
5
B |
6
C |
7
N |
8
O |
9
F |
10
Ne |
3 |
11
Na |
12
Mg |
|
13
Al |
14
Si |
15
P |
16
S |
17
Cl |
18
Ar |
4 |
19
K |
20
Ca |
21
Sc |
22
Ti |
23
V |
24
Cr |
25
Mn |
26
Fe |
27
Co |
28
Ni |
29
Cu |
30
Zn |
31
Ga |
32
Ge |
33
As |
34
Se |
35
Br |
36
Kr |
5 |
37
Rb |
38
Sr |
39
Y |
40
Zr |
41
Nb |
42
Mo |
43
Tc |
44
Ru |
45
Rh |
46
Pd |
47
Ag |
48
Cd |
49
In |
50
Sn |
51
Sb |
52
Te |
53
I |
54
Xe |
6 |
55
Cs |
56
Ba |
+ |
72
Hf |
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 |
7 |
87
Fr |
88
Ra |
* |
104
Rf |
105
Db |
106
Sg |
107
Bh |
108
Hs |
109
Mt |
110
Ds |
+ Lanthanide: |
57
La |
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 |
|
* Actinide: |
89
Ac |
90
Th |
91
Pa |
92
U |
93
Np |
94
Pu |
95
Am |
96
Cm |
97
Bk |
98
Cf |
99
Es |
100
Fm |
101
Md |
102
No |
103
Lr |
|
Click here to go to the main Table of the Elements site.
Atomic number - The number of proton in the nucleus of an atom. This is the number that is usually shown as a whole number in the periodic table, as shown above. The number is unique that identifies a particular atom. Remember, isotopes of an atom differ by the number of neutron in the nucleus. The atomic number also indicates equal number electrons in a neutral atom.
Atomic mass - This is the total number of proton and neutron in an atom. However, since an element may have several isotopes, the relative mass is more important which takes into consideration the natural proportion of isotopes present in nature and compare the weighted average mass with the other elements. For example, the relative atomic mass of hydrogen is 1.00795 where the fraction is due to the presence of small amount of deuterium (1 proton 1 neutron) in nature.
Standard reference of carbon-12 - By knowing the relative mass we are able to select amounts of elements needed, say for a chemical reaction, and calculate the expected amount of product yields. However, it is necessary to have a reference base against which to compare the relative mass. Since 1961, the agreed-upon reference is the isotope of carbon with 12 protons (carbon-12), which is defined to have a mass of exactly 12.000 atomic mass unit (amu). The number so chosen is arbitrary. However, it turns out that the lightest element, hydrogen, has a mass being almost 12 times less than that of carbon-12 or a
sensible starting mass value of about 1 amu.
|
