1
H
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 1
D
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Chemical Elements

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Chemical Calculators | Atomic Collider Simulation
 2
He
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 3
Li
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 4
Be
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 5
B
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 6
C
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 7
N
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 8
O
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 9
F
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 10
Ne
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 11
Na
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 12
Mg
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 13
Al
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 14
Si
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 15
P
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 16
S
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 17
Cl
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 18
Ar
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 19
K
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 20
Ca
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 21
Sc
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 22
Ti
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 23
V
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 24
Cr
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 25
Mn
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 26
Fe
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 27
Co
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 28
Ni
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 29
Cu
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 30
Zn
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 31
Ga
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 32
Ge
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 33
As
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 34
Se
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 35
Br
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 36
Kr
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 37
Rb
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 38
Sr
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 39
Y
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 40
Zr
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 41
Nb
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 42
Mo
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 43
Tc
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 44
Ru
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 45
Rh
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 46
Pd
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 47
Ag
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 48
Cd
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 49
In
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 50
Sn
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 51
Sb
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 52
Te
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 53
I
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 54
Xe
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 55
Cs
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 56
Ba
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 57-71
La-Lu

 72
Hf
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 73
Ta
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 74
W
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 75
Re
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 76
Os
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 77
Ir
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 78
Pt
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 79
Au
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 80
Hg
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 81
Tl
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 82
Pb
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 83
Bi
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 84
Po
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 85
At
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 86
Rn
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 87
Fr
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 88
Ra
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 89-103
Ac-Lr

 104
Rf

 105
Db

 106
Sg

 107
Bh

 108
Hs

 109
Mt

 110
Ds

 111
Rg

 112
Cn

 113
Nh

 114
Fl

 115
Mc

 116
Lv

 117
Ts

 118
Og

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 57
La
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 58
Ce
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 59
Pr
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 60
Nd
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 61
Pm
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 62
Sm
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 63
Eu
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 64
Gd
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 65
Tb
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 66
Dy
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 67
Ho
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 68
Er
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 69
Tm
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 70
Yb
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 71
Lu
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Islands of Stability
 89
Ac
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 90
Th
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 91
Pa
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 92
U
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 93
Np
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 94
Pu
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 95
Am
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 96
Cm
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 97
Bk
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 98
Cf
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 99
Es
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 100
Fm
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 101
Md
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 102
No
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 103
Lr

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The drawing is in the possession of the Oesper Collection, University of Cincinnati

Mendeleev's first draft of the periodic table, 2/17/1869

Knowledge: The Periodic Table


The periodic table of elements is one of the most famous concepts in science. It was developed mainly by Dmitri Mendeleev and Lothar Meyer around 1869. It is a table of all the elements of the world (and even some that don't occur in nature). World here explicitly means the universe, our Earth consists of the same substances as space. The elements are ordered in periods (from left to right) and in groups (top to bottom).

All matter around us, which we observe, is made from these elements. An element is a substance made of atoms, which all have the same amount of protons. The word atom derives from the Greek word for indivisible. If you take a certain amount of an element, e.g. a lump of gold, and divide it into two parts, you have two lumps which are still gold. You can divide them further and they will stay gold, but only up to a certain point. If you have a single gold atom, this also can be divided (in theory, but it is very difficult to do), but it won't be gold after that. So atoms are not indivisible by principle, they just become other atoms when divided.

Atoms for the very largest part consist of nothing (spatial that is). The rest is nucleus and shell. The nucleus consists of protons and neutrons, the shell of electrons. Neutrons are electrically neutral (hence their name), protons are positively charged. They are responsible for the negatively charged electrons circling around the nucleus, because plus and minus attract each other, like the oppositely charged poles of a magnet attract each other. The number and arrangement of the electrons determines the chemical properties of an element. Most important here are the electrons in the outermost shell, the valence electrons. Therefore different elements have different chemical properties, but elements of the same group have similar properties, because they have the same amount of valence electrons. The number of shells of an atom depends on the number of electrons, whereas the notion shell shouldn't be taken literally. Electrons don't make it easy for the imagination, they are not located at an exactly determinable place, but only are there with a certain probability.
Atoms aspire to fill their outermost shell with electrons. This is one factor that causes chemical reactions and, as a consequence, compounds. These are molecules, where certain atoms 'lend' electrons from other, fitting atoms. Atoms that have a number of electrons that differs from their number of protons and that therefore are charged are called ions. With less electrons they are cations, with more anions. Compounds of cations and anions are called salts. Noble gases have their outermost shell already filled up and therefore don't react chemically at all or only very reluctantly.
The most simple element is hydrogen: one proton and therefore one electron. After that comes helium with two of each. For further elements see above. Additionally there are also neutrons in the nucleus. These make it stable. For larger atoms, this stability is harder to achieve and more neutrons are needed. A certain element has a certain amount of protons, but the amount of neutrons can vary. Atoms with the same number of protons, but a different number of neutrons are called isotopes of an element. The number of neutrons is not arbitrary, if there are too few or too many, then the atom is radioactive, it decays after some time. Or it can't emerge at all. 1H, protium, is the only isotope, which gets by without any neutron. Already 2He can't emerge, it immediately transforms into 2H (deuterium). Thereby a proton is turned into a neutron. By the way, the superscripted numbers tell the total amount of neutrons and protons in the nucleus.
Many isotopes are radioactive. Atoms with an odd amount of protons by trend have fewer stable isotopes than those with an even amount. For some elements, which are 43, 61 and those from 83 upwards, no stable isotopes exist, they all decay more or less quickly to other, mostly smaller isotopes. Thereby energy is released, which has a form that is extremely harmful for living beings. The harmfulness of course depends on the amount of released energy. The weakest radioactive element is bismuth, 209Bi, where it is very hard to bring together an amount that could do damage. From the most notorious element of all, plutonium, some milligrams are enough to bring a healthy human being very nastily to death. Very strong radioactive elements however, like those with three-digit numbers, can only be produced with big efforts in very low amounts and quickly decay.

Some elements produce energy at a fission (the larger ones), some at a fusion (the smaller ones). The controlled fission of heavy elements, like in nuclear power plants, is technically easier, but has much more dangerous side effects. To name only some: the completely unsolved problem of safe and durable storage of nuclear waste, the emission of radioactive substances like 137caesium, ecological disasters like those at Chernobyl and Fukushima.
On the other hand, nuclear fusion would be operable with lower risks, because no big nuclear explosion can happen there, radioactivity is released only locally and the waste is only the harmless 4helium. The problem here is the enormous technical difficulty to manage a controlled fusion over a longer period of time. To get a clue, how much energy the fusion from hydrogen to helium can produce, just look at the Sun (but not with unprotected eyes, please). There is a simple and reasonable way to win energy from fusion: by solar cells or photovoltaics. But this is a different topic.
An end of fusion and decay is reached at 56iron. This isotope is, together with iron 58 and nickel 62, the energetic ideal state of matter, the nucleus with the highest binding energy per nuclear particle (binding energy is negative potential energy). To transform this into another nucleus, energy had to be used.

Most elements are metals, which at first sight may astonish, because they are rarely found in nature. The reason for this is that most metals like to react with other substances to form compounds, which don't look metallic any more. In pure metals and alloys of different metals the atoms form a spatial grid, in which the electrons can move freely. This is responsible for typical metallic properties like good conductivity, ductility and metallic luster. The metals are adjusted in the periodic table to the lower left. Simplified, an element is the more metallic, the heavier (by atomic mass) it is and the less valence electrons it has. In the display above, metals have a blue background.

The first element that occurred during the big bang was hydrogen, as single protons. In the first three minutes, the universe was so hot and dense, that this could fuse to become helium. After that, in the production of elements nothing happened for quite a long time. There were twelve times more hydrogen atoms than helium atoms by this time, or three times more hydrogen by mass, but no other atoms.
Only after millions of years later the first stars had formed, new elements were produced, in the inside of those stars at a temperature of millions of degrees. Stars at the end of their lives produce heavier elements like carbon. But the heavier elements, those beyond nickel, are primarily made in the explosions of the biggest stars, the supernovae. Countless of these explosions had to take place over billions of years, until there were enough heavy elements to allow such planets as Earth. We all are made from stardust, bred in exploding suns. In the universe, the average fraction of atoms heavier than helium is still below one percent. The reason, that on Earth hydrogen and helium aren't in the majority is that those light gases were ripped away from Earth in the early phase of the solar system by strong solar winds. Hydrogen can join other elements to form compounds (like H2O, water) and so is still here in quite large amounts, but helium doesn't have this ability and therefore is rare on Earth.


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