The distance from the nucleus that the electron spins is called its energy shell, energy level, or orbital.
Each energy level can only hold a certain amount of electrons
i. The first shell (K level) can hold 2 electrons
ii. The second shell (L level) can hold 8 electrons
iii. The third level (M level) can hold 18 electrons
iv. The fourth level (N level) can hold 32 electrons
v. The fifth level (O level) can hold 50 electrons
vi. The sixth level (P level) can hold 72 electrons
Each energy level is completely filled before electrons fill the next level.
The number of electrons in the outermost level are called valence electrons.
The energy sequence of the first 24 subshells is given in the following table. Each cell represents a subshell with n and given by its row and column indices, respectively. The number in the cell is the subshell's position in the sequence. Empty cells represent sublevels that do not exist.
The first period
Hydrogen has its only electron in the 1s orbital - 1s1, and at helium the first level is completely full - 1s2.
The second period
Now we need to start filling the second level, and hence start the second period. Lithium's electron goes into the 2s orbital because that has a lower energy than the 2p orbitals. Lithium has an electronic structure of 1s22s1. Beryllium adds a second electron to this same level - 1s22s2.
Now the 2p levels start to fill. These levels all have the same energy, and so the electrons go in singly at first.
B
1s22s22px1
C
1s22s22px12py1
N
1s22s22px12py12pz1
Note: The orbitals where something new is happening are shown in bold type. You wouldn't normally write them any differently from the other orbitals.
The next electrons to go in will have to pair up with those already there.
O
1s22s22px22py12pz1
F
1s22s22px22py22pz1
Writing the electronic structure of an element from hydrogen to krypton
Use the Periodic Table to find the atomic number, and hence number of electrons.
Fill up orbitals in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p - until you run out of electrons. The 3d is the awkward one - remember that specially. Fill p and d orbitals singly as far as possible before pairing electrons up.
Remember that chromium and copper have electronic structures which break the pattern in the first row of the d-block.
Writing the electronic structure of big s- or p-block elements
Note: We are deliberately excluding the d-block elements apart from the first row that we've already looked at in detail. The pattern of awkward structures isn't the same in the other rows. This is a problem for degree level.
First work out the number of outer electrons. This is quite likely all you will be asked to do anyway.
The number of outer electrons is the same as the group number. (The noble gases are a bit of a problem here, because they are normally called group 0 rather then group 8. Helium has 2 outer electrons; the rest have 8.) All elements in group 3, for example, have 3 electrons in their outer level. Fit these electrons into s and p orbitals as necessary. Which level orbitals? Count the periods in the Periodic Table (not forgetting the one with H and He in it).
Iodine is in group 7 and so has 7 outer electrons. It is in the fifth period and so its electrons will be in 5s and 5p orbitals. Iodine has the outer structure 5s25px25py25pz1.
What about the inner electrons if you need to work them out as well? The 1, 2 and 3 levels will all be full, and so will the 4s, 4p and 4d. The 4f levels don't fill until after anything you will be asked about at A'level. Just forget about them! That gives the full structure: 1s22s22p63s23p63d104s24p64d105s25px25py25pz1.
When you've finished, count all the electrons to make sure that they come to the same as the atomic number. Don't forget to make this check - it's easy to miss an orbital out when it gets this complicated.
Barium is in group 2 and so has 2 outer electrons. It is in the sixth period. Barium has the outer structure 6s2.
Including all the inner levels: 1s22s22p63s23p63d104s24p64d105s25p66s2.
It would be easy to include 5d10 as well by mistake, but the d level always fills after the next s level - so 5d fills after 6s just as 3d fills after 4s. As long as you counted the number of electrons you could easily spot this mistake because you would have 10 too many.
An atom's nucleus is held together by the strong nuclear force.
If the numbers of neutrons and protons are very different, the nucleus can become unstable and undergo radioactive decay.
Some nuclei decay by emitting an alpha particle.
Other nuclei decay by ejecting a beta particle.
Transmutation is a process in which one element changes into another through radioactive decay.
Half-life is a measure of the decay rate of a nucleus.
It is the time needed for one half of the mass of a sample of a radioactive isotope to decay.
Radioactive isotopes are used in medicine and for the study of the environment.
Tracer elements with short half-lives are followed through living systems to study certain things.
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