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Chemical Elements
by Ron Kurtus (revised 4 January 2016)
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 is atomic number?
- What determines average atomic weight?
- What are all the elements?
This lesson will answer those questions.
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.
(See Periodic Table for more information.)
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.
(See Atomic Weight in Physical Science for more information.)
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 |
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 |
Aluminum |
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 |
Cesium |
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] |
113 |
Uut |
ununtrium |
new - 2016 |
114 |
Uuq |
Ununquadium |
[289] |
115 |
Uup |
Ununpentium |
new - 2016 |
116 |
Uuh |
Ununhexium |
unknown |
117 |
Uus |
Ununseptium |
new - 2016 |
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.
Be observant
Resources and references
Websites
Chemical Elements - Site with details on Periodic Table
Books
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Students and researchers
The Web address of this page is:
www.school-for-champions.com/chemistry/
elements.htm
Please include it as a link on your website or as a reference in your report, document, or thesis.
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Chemical Elements