# Chemistry

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## Electron arrangement

### Write the electron arrangement of atoms belonging to periods 1 to 3

The electronic
arrangement describes the distribution of the electrons in the different shells, it is written respecting the following rules:

Each shell occupied by electrons is noted using its letter K, L, M etc.
The occupied shells are indicated in the kernel remoteness order (which is also an alphabetical order).
Each shell is noted in parenthesis with the number of electrons in superscript (sometimes parentheses are omitted).

Generally, to determine the electronic arrangement of an atom we proceed as follows:

Find the number of electrons of the atom (the periodic table atoms gives it atomic numbers which is equal to the number of electrons)
These electrons are distributed in shells according to a simple rule, they first occupy the lowest shells and they are placed first in the shell K. If it has a single electron it is noted (K)1 and if it is full it is noted (K)2.
If the atom has more than two electrons then the following electrons occupy the shell L noted (L) with at most 8 electrons.
If the atom has more than 10 electrons (2 + 8) then the remaining electrons are placed in the M shell… until all electrons are associated with a shell

Example 1: the boron atom

Its atomic number Z = 5 so it has 5 electrons
The first two electrons occupy the shell K which is noted (K) 2
There then remain 3 electrons, which occupy the L layer, are noted (L) 3
The electronic arrangement of the boron atom is therefore: (K) 2 (L) 3
Note: its outer layer is L.

Example 2: the sulfur atom

Its atomic number is Z = 16 so it has 16 electrons
The first two electrons are on the layer K noted (K) 2
There are 14 left, so the next 8 are on the L layer noted (L) 8
There remain 6 electrons on the layer M noted (M) 6
Therefore, we obtain an electronic structure (electronic configuration) which is noted (K) 2 (L) 8 (M) 6
Note: its outermost shell is M.

### Write the electron arrangement of atoms belonging to periods 4 to 7

The filling of these shells follows the same rule as the periods (lines) 1 to 3 of the periodic table. Electrons occupy first time the lowest energy levels in priority.

However, from the fourth period it is necessary to consider another particularity of the electronic shells: they are subdivided into sub-shells each one having its own energy level. If one of the sub-shells has an energy higher than one of the sub-shells of the upper shell therefore the filling is not necessarily made by the lowest shells to the highest shells. It becomes more relevant to describe the electronic distribution by writing the electronic configuration (which details the distribution on the sub-shells).

### Electron arrangement list

 Atom Z Electronic Structure ( electron configuration) Hydrogen 1 (K)1 Helium 2 (K)2 Lithium 3 (K)2(L)1 Beryllium 4 (K)2(L)2 Bore 5 (K)2(L)3 Carbon 6 (K)2(L)4 Azote 7 (K)2(L)5 Oxygen 8 (K)2(L)6 Fluor 9 (K)2(L)7 Neon 10 (K)2(L)8 Sodium 11 (K)2(L)8 (M)1 Magnesium 12 (K)2(L)8 (M)2 Aluminum 13 (K)2(L)8 (M)3 Silicon 14 (K)2(L)8 (M)4 Phosphor 15 (K)2(L)8 (M)5 Sulfur 16 (K)2(L)8 (M)6 Chlorine 17 (K)2(L)8 (M)7 Argon 18 (K)2(L)8 (M)8 Potassium 19 (K)2(L)8 (M)8(N)1 Calcium 20 (K)2(L)8 (M)8(N)2 Scandium 21 (K)2(L)8 (M)9(N)2 Titanium 22 (K)2(L)8 (M)10(N)2 Vanadium 23 (K)2(L)8 (M)11(N)2 Chrome 24 (K)2(L)8 (M)13(N)1 Manganese 25 (K)2(L)8 (M)13(N)2 Iron 26 (K)2(L)8 (M)14(N)2 Cobalt 27 (K)2(L)8 (M)15(N)2 Nickel 28 (K)2(L)8 (M)16(N)2 Copper 29 (K)2(L)8 (M)18(N)1 Zinc 30 (K)2(L)8 (M)18(N)2 Gallium 31 (K)2(L)8 (M)18(N)3 Germanium 32 (K)2(L)8 (M)18(N)4 Arsenic 33 (K)2(L)8 (M)18(N)5 Selenium 34 (K)2(L)8 (M)18(N)6 Brome 35 (K)2(L)8 (M)18(N)7 Krypton 36 (K)2(L)8 (M)18(N)8 Rubidium 37 (K)2(L)8 (M)18(N)8(O)1 Strontium 38 (K)2(L)8 (M)18(N)8(O)2 Yttrium 39 (K)2(L)8 (M)18(N)9(O)2 Zirconium 40 (K)2(L)8 (M)18(N)10(O)2 Niobium 41 (K)2(L)8 (M)18(N)12(O)1 Molybdenum 42 (K)2(L)8 (M)18(N)13(O)1 Technetium 43 (K)2(L)8 (M)18(N)13(O)2 Ruthenium 44 (K)2(L)8 (M)18(N)15(O)1 Rhodium 45 (K)2(L)8 (M)18(N)16(O)1 Palladium 46 (K)2(L)8 (M)18(N)18 Silver 47 (K)2(L)8 (M)18(N)18(O)1 Cadmium 48 (K)2(L)8 (M)18(N)18(O)2 Indium 49 (K)2(L)8 (M)18(N)18(O)3 Pewter 50 (K)2(L)8 (M)18(N)18(O)4 Antimony 51 (K)2(L)8 (M)18(N)18(O)5 Tellurium 52 (K)2(L)8 (M)18(N)18(O)6 Iodine 53 (K)2(L)8 (M)18(N)18(O)7 Xenon 54 (K)2(L)8 (M)18(N)18(O)8 Cesium 55 (K)2(L)8 (M)18(N)18(O)8(P)1 Barium 56 (K)2(L)8 (M)18(N)18(O)8(P)2 Lanthanum 57 (K)2(L)8 (M)18(N)18(O)9(P)2 Cerium 58 (K)2(L)8 (M)18(N)19(O)9(P)2 Praseodymium 59 (K)2(L)8 (M)18(N)21(O)8(P)2 Neodymium 60 (K)2(L)8 (M)18(N)22(O)8(P)2 Promethium 61 (K)2(L)8 (M)18(N)23(O)8(P)2 Samarium 62 (K)2(L)8 (M)18(N)24(O)8(P)2 Europium 63 (K)2(L)8 (M)18(N)25(O)8(P)2 Gadolinium 64 (K)2(L)8 (M)18(N)25(O)9(P)2 Terbium 65 (K)2(L)8 (M)18(N)27(O)8(P)2 Dysprosium 66 (K)2(L)8 (M)18(N)28(O)8(P)2 Holmium 67 (K)2(L)8 (M)18(N)29(O)8(P)2 Erbium 68 (K)2(L)8 (M)18(N)30(O)8(P)2 Thulium 69 (K)2(L)8 (M)18(N)31(O)8(P)2 Ytterbium 70 (K)2(L)8 (M)18(N)32(O)8(P)2 Lutetium 71 (K)2(L)8 (M)18(N)32(O)9(P)2 Hafnium 72 (K)2(L)8 (M)18(N)32(O)10(P)2 Tantalum 73 (K)2(L)8 (M)18(N)32(O)11(P)2 Tungsten 74 (K)2(L)8 (M)18(N)32(O)12(P)2 Rhenium 75 (K)2(L)8 (M)18(N)32(O)13(P)2 Osmium 76 (K)2(L)8 (M)18(N)32(O)14(P)2 Iridium 77 (K)2(L)8 (M)18(N)32(O)15(P)2 Platine 78 (K)2(L)8 (M)18(N)32(O)17(P)1 Gold 79 (K)2(L)8 (M)18(N)32(O)18(P)1 Mercury 80 (K)2(L)8 (M)18(N)32(O)18(P)2 Thallium 81 (K)2(L)8 (M)18(N)32(O)18(P)3 Plumb 82 (K)2(L)8 (M)18(N)32(O)18(P)4 Bismuth 83 (K)2(L)8 (M)18(N)32(O)18(P)5 Polonium 84 (K)2(L)8 (M)18(N)32(O)18(P)6 Astatine 85 (K)2(L)8 (M)18(N)32(O)18(P)7 Radon 86 (K)2(L)8 (M)18(N)32(O)18(P)8 Francium 87 (K)2(L)8 (M)18(N)32(O)18(P)8(Q)1 Radium 88 (K)2(L)8 (M)18(N)32(O)18(P)8(Q)2 Actinium 89 (K)2(L)8 (M)18(N)32(O)18(P)9(Q)2 Thorium 90 (K)2(L)8 (M)18(N)32(O)18(P)10(Q)2 Protactinium 91 (K)2(L)8 (M)18(N)32(O)20(P)9(Q)2 Uranium 92 (K)2(L)8 (M)18(N)32(O)21(P)9(Q)2 Neptunium 93 (K)2(L)8 (M)18(N)32(O)22(P)9(Q)2 Plutonium 94 (K)2(L)8 (M)18(N)32(O)24(P)8(Q)2 Americium 95 (K)2(L)8 (M)18(N)32(O)25(P)8(Q)2 Curium 96 (K)2(L)8 (M)18(N)32(O)25(P)9(Q)2 Berkelium 97 (K)2(L)8 (M)18(N)32(O)27(P)8(Q)2 Californium 98 (K)2(L)8 (M)18(N)32(O)28(P)8(Q)2 Einsteinium 99 (K)2(L)8 (M)18(N)32(O)29(P)8(Q)2 Fermium 100 (K)2(L)8 (M)18(N)32(O)30(P)8(Q)2 Mendelevium 101 (K)2(L)8 (M)18(N)32(O)31(P)8(Q)2 Nobelium 102 (K)2(L)8 (M)18(N)32(O)32(P)8(Q)2 Lawrencium 103 (K)2(L)8 (M)18(N)32(O)32(P)8(Q)3 Rutherfordium 104 (K)2(L)8 (M)18(N)32(O)32(P)10(Q)2 Dubnium 105 (K)2(L)8 (M)18(N)32(O)32(P)11(Q)2 Seaborgium 106 (K)2(L)8 (M)18(N)32(O)32(P)12(Q)2 Bohrium 107 (K)2(L)8 (M)18(N)32(O)32(P)13(Q)2 Hassium 108 (K)2(L)8 (M)18(N)32(O)32(P)14(Q)2 Meitnerium 109 (K)2(L)8 (M)18(N)32(O)32(P)15(Q)2 Darmstadtium 110 (K)2(L)8 (M)18(N)32(O)32(P)16(Q)2 Roentgenium 111 (K)2(L)8 (M)18(N)32(O)32(P)17(Q)2 Copernicium 112 (K)2(L)8 (M)18(N)32(O)32(P)18(Q)2 Nihonium 113 (K)2(L)8 (M)18(N)32(O)32(P)18(Q)3 Flerovium 114 (K)2(L)8 (M)18(N)32(O)32(P)18(Q)4 Moscovium 115 (K)2(L)8 (M)18(N)32(O)32(P)18(Q)5 Livermorium 116 (K)2(L)8 (M)18(N)32(O)32(P)18(Q)6 Tennessine 117 (K)2(L)8 (M)18(N)32(O)32(P)18(Q)7 Oganesson 118 (K)2(L)8 (M)18(N)32(O)32(P)18(Q)8

### Write the electronic arrangement of a monoatomic ion in steady state

By definition, a monoatomic ion is a chemical species having either an electron defect compared to the atom (in this case it is a positively charged cation) or an excess of electrons (in this case it is an anion negatively charged).

To write the electronic arrangement of an ion, it is sufficient to determine the number of electrons it owns:

Find the atomic number of the corresponding element in the periodic table which corresponds to the number of electrons of the atomic form.
Use the chemical formula to determine the electron difference to the atomic shape. An ion with a single positive charge has one less electron than the atom, an ion with two positive charges has two fewer electrons, one ion carrying a negative charge has an extra electron relative to the atom shape, an ion with two negative charges has two less electrons etc.
Then apply the same method to determine the electronic arrangement of an atom.
Note: Stable ions for each element can be predicted by the duet and octet rule.

Example 1, sodium ion Na +

• The sodium has as an atomic number Z = 11 therefore the sodium atom has 11 electrons
• The sodium ion carries a positive charge so it has one electron less than the atom, 11-1 = 10 electrons.
• The first two electrons occupy the K shell
• The next eight are on the L layer
• The electronic arrangement of the sodium ion is therefore (K)2(L)8

Example 2, the sulfide ion S2-

The element sulfur has for atomic number Z = 16 therefore the sulfur atom has 16 electrons
The sulfide ion has two negative charges so it has two more electrons than the atom, 16 + 2 = 18 electrons.
The first two electrons occupy the K shell
The next eight are on the L shell
The last 8 are on the shell M
Therefore, the electronic structure of the sodium ion is (K)2(L)8 (M)8

### Electron arrangement list of ions of the first three periods

Note: some of these ions (hydride, silicon, phosphide ...) do not exist in aqueous solution (they are not stable) but may be present in ionic solid.

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