Chemical Elements
A chemical element (usually just called an element) is a class of atoms with a specific number of protons in their nuclei (plural of nucleus in Latin). Each element has its own name and is usually listed according to its atomic number. Isotopes of an element have different numbers of neutrons. Often the average atomic weight of an element is also stated. This number takes into account the percentages of isotopes, the masses of the particles, and nuclear effects. The average atomic weight is approximately the number of protons and neutrons of the most common isotope of the element.
Questions you may have include:
- What determines average atomic weight?
- What are all the elements?
Atomic number
The elements are listed according to their atomic number. The atomic number is designated by the number of protons in the nucleus. For example, Hydrogen has one proton, Helium has two protons, Oxygen has eight protons, and so on.
Since the number of electrons equals the number of protons in an electrically stable atom, the atomic number determines many of the chemical characteristics of the element. This is shown in the Periodic Table.
Average atomic weight
The atomic weight of an atom was originally defined as a sum of its protons and neutrons. The unit of measurement is the atomic mass unit (amu or u).
Mass defect
Later, it was found that some mass is lost to binding energy required to hold the nucleus together. This is called the mass defect and is the principle behind nuclear energy, according the to the famous equation E = mc2.
Thus the atomic weight of an individual atom is slightly different than the number of protons and neutrons.
Isotopes
An element has several different number of neutrons in its nucleus. Each is called an isotope of that element. For example, Oxygen typically has 8 protons and 8 neutrons in its nucleus, with an atomic weight of about 16 u. But there is a very small percentage of Oxygen atoms with 9 neutrons in their nuclei and atomic weight of approximately 17 u. There are even some atoms with 10 neutrons.
Thus for the element Oxygen, taking into account for the mass defect and averaging the atomic weight for all its isotopes, you get an average atomic weight of 15.9994 u for Oxygen.
Finding number of neutrons
Looking on the list of elements below, you will see that the Average Atomic Weight is not integer. You can find the number of neutrons in the most common and stable nucleus of an element by simply rounding off the atomic weight and subtracting the atomic number (number of protons).
For example, Magnesium (Mg) is number 12 and has an average atomic weight of 24.3050 u. This rounds off to 24. Thus the number of protons in the most common isotope of Magnesium is 24 - 12 = 12 neutrons.
List of elements
Following is a list of all the elements, according to atomic number.
Elements with the weight in [brackets] are so unstable that scientists have not been able to accurately measure the weight. All of the elements after Uranium (number 92) are artificial and unstable.
An artificial element is one that is so unstable that it does not occur in nature. High energy atomic collisions can manufacture such an element. It immediately decays into a stable element.
Atomic Number
Symbol
Name
Average Atomic Weight(u)
1
H
Hydrogen
1.00794
2
He
Helium
4.002602
3
Li
Lithium
6.941
4
Be
Beryllium
9.012182
5
B
Boron
10.811
6
C
Carbon
12.0107
7
N
Nitrogen
14.0067
8
O
Oxygen
15.9994
9
F
Fluorine
18.9984032
10
Ne
Neon
20.1797
11
Na
Sodium
22.989770
12
Mg
Magnesium
24.3050
13
Al
Aluminium
26.981538
14
Si
Silicon
28.0855
15
P
Phosphorus
30.973761
16
S
Sulfur
32.065
17
Cl
Chlorine
35.453
18
Ar
Argon
39.948
19
K
Potassium
39.0983
20
Ca
Calcium
40.078
21
Sc
Scandium
44.955910
22
Ti
Titanium
47.867
23
V
Vanadium
50.9415
24
Cr
Chromium
51.9961
25
Mn
Manganese
54.938049
26
Fe
Iron
55.845
27
Co
Cobalt
58.933200
28
Ni
Nickel
58.6934
29
Cu
Copper
63.546
30
Zn
Zinc
65.39
31
Ga
Gallium
69.723
32
Ge
Germanium
72.64
33
As
Arsenic
74.92160
34
Se
Selenium
78.96
35
Br
Bromine
79.904
36
Kr
Krypton
83.80
37
Rb
Rubidium
85.4678
38
Sr
Strontium
87.62
39
Y
Yttrium
88.90585
40
Zr
Zirconium
91.224
41
Nb
Niobium
92.90638
42
Mo
Molybdenum
95.94
43
Tc
Technetium
[98]
44
Ru
Ruthenium
101.07
45
Rh
Rhodium
102.90550
46
Pd
Palladium
106.42
47
Ag
Silver
107.8682
48
Cd
Cadmium
112.411
49
In
Indium
114.818
50
Sn
Tin
118.710
51
Sb
Antimony
121.760
52
Te
Tellurium
127.60
53
I
Iodine
126.90447
54
Xe
Xenon
131.293
55
Cs
Caesium
132.90545
56
Ba
Barium
137.327
57
La
Lanthanum
138.9055
58
Ce
Cerium
140.116
59
Pr
Praseodymium
140.90765
60
Nd
Neodymium
144.24
61
Pm
Promethium
[145]
62
Sm
Samarium
150.36
63
Eu
Europium
151.964
64
Gd
Gadolinium
157.25
65
Tb
Terbium
158.92534
66
Dy
Dysprosium
162.50
67
Ho
Holmium
164.93032
68
Er
Erbium
167.259
69
Tm
Thulium
168.93421
70
Yb
Ytterbium
173.04
71
Lu
Lutetium
174.967
72
Hf
Hafnium
178.49
73
Ta
Tantalum
180.9479
74
W
Tungsten
183.84
75
Re
Rhenium
186.207
76
Os
Osmium
190.23
77
Ir
Iridium
192.217
78
Pt
Platinum
195.078
79
Au
Gold
196.96655
80
Hg
Mercury
200.59
81
Tl
Thallium
204.3833
82
Pb
Lead
207.2
83
Bi
Bismuth
208.98038
84
Po
Polonium
[209]
85
At
Astatine
[210]
86
Rn
Radon
[222]
87
Fr
Francium
[223]
88
Ra
Radium
[226]
89
Ac
Actinium
[227]
90
Th
Thorium
232.0381
91
Pa
Protactinium
231.03588
92
U
Uranium
238.02891
93
Np
Neptunium
[237]
94
Pu
Plutonium
[244]
95
Am
Americium
[243]
96
Cm
Curium
[247]
97
Bk
Berkelium
[247]
98
Cf
Californium
[251]
99
Es
Einsteinium
[252]
100
Fm
Fermium
[257]
101
Md
Mendelevium
[258]
102
No
Nobelium
[259]
103
Lr
Lawrencium
[262]
104
Rf
Rutherfordium
[261]
105
Db
Dubnium
[262]
106
Sg
Seaborgium
[266]
107
Bh
Bohrium
[264]
108
Hs
Hassium
[277]
109
Mt
Meitnerium
[268]
110
Uun
Ununnilium
[281]
111
Uuu
Unununium
[272]
112
Uub
Ununbium
[285]
114
Uuq
Ununquadium
[289]
116
Uuh
Ununhexium
unknown
118
Uuo
Ununoctium
unknown
Summary
An element is a basic chemical unit. Elements have an atomic number and atomic weight assigned to them. There are 92 natural elements, plus some that have been artificially created. Artificial elements are highly unstable and usually exist for only a fraction of a second.
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