Notes to a video lecture on http://www.unizor.com
From the atomic viewpoint, classical chemical bonding between atoms are interactions that do not change the composition of these atoms' nuclei.
Only electrons that surround nuclei interact among themselves, breaking some old connections and establishing new ones.
Chemical bonding can occur between atoms of simple elements, like a single atom of sodium Na and a single atom of chlorine Cl forming their compositions, a molecule of sodium chloride NaCl, which is a molecule of regular salt.
Scientists thought about why some elements bond with each other, if mixed in certain proportion and at some external conditions, while some others are inert, that is refuse to combine with other elements. So called noble gases, like helium, neon, argon or krypton are inert.
Let's examine the electron configuration of above mentioned noble gases.
Helium with atomic number 2 has the following electron configuration
Neon with atomic number 10 has the following electron configuration
Ne: 1s2 2s2 2p6 =
= [He] + 2s2 2p6
Argon with atomic number 18 has the following electron configuration
Ar: 1s2 2s2 2p6 3s2 3p6 =
= [Ne] + 3s2 3p6
Krypton with atomic number 36 has the following electron configuration
Kr: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 =
= [Ar] + 3d10 4s2 4p6
As you see, all subshells of noble gases are filled to capacity:
max(s) = 2
max(p) = 6
max(d) = 10
and this is very important for stability of atoms.
Those elements that do not have this characteristic of its subshells will be easier to get into chemical bonding that facilitates sharing electrons to complete top most subshells of all involved components.
Let's examine the chemical process between elements using the above example of producing sodium chloride from sodium and chlorine
Na + Cl → NaCl
First of all, let's analyze the electronic structure of both substances.
Sodium has atomic number 11, and its electrons are arranged as
Na: 1s2 2s2 2p6 3s1
Chlorine has atomic number 17, and its electrons are arranged as
Cl: 1s2 2s2 2p6 3s2 3p5
As you see, the first subshell s of the shell #1 (that is, subshell 1s), which can hold up to 2 electrons, is completely filled for both elements, and there can be no more subshells within this shell #1.
The first subshell s of the shell #2 (that is, subshell 2s) is also completely filled for both elements, but shell #2 can have two subshells, s and p, and subshell p has capacity for 6 electrons. As we see, both 2p subshells are filled to capacity in both elements.
The first subshell s of the shell #3 (that is, subshell 3s) has only 1 electron in an atom of sodium.
The atom of chlorine, on the other hand, has this subshell filled to capacity of 2 electrons, but the next subshell 3p has only 5 electrons out of maximum 6.
Now the sharing of electrons comes to play.
The top subshell of sodium 3s contains 1 electron out of maximum 2. If it loses this electron, all its subshells will be complete, but losing electron means becoming positively charged.
The top subshell of chlorine 3p contains 5 electrons out of maximum 6. If it captures one electron, all its subshells will be complete, but capturing an electron means becoming negatively charged.
The mechanism of bonding can be represented in such a case as follows.
1. Sodium atom frees the only electron from the subshell 3s, becoming a positive ion
Na → Na+ + e−
2. Chlorine atom captures this electron in its subshell 3p, becoming negative ion
Cl + e− → Cl−
3. Now all subshells are filled to capacity and both atoms are electrically charged with opposite charges, which makes them stick to each other, forming a molecule of salt.
Let's make a few conclusive statements about this process of forming molecules.
I. Atoms like their subshells to be completely filled up to a maximum of 4m−2, where m is the subshell number (subshell #1 is labeled s, subshell #2 is labeled p etc.)
II. Some atoms are ready to accept electrons to fill up their top subshell from other atoms that are willing to give up their electrons.
III. Some other atoms are ready to give up electrons from their incomplete top subshell, leaving with themselves only complete subshells, if there is a recipient atom of these extra electrons that needs them to complete its top subshell.
IV. The necessity for an atom to have its shells filled up is very strong. Atoms with incomplete top subshells are looking for partners to get into chemical combination. If there is a fit between two atoms to transfer electrons from an incomplete top subshell of one of them to incomplete top subshell of another that completes both, electrons are transferred.
V. After the transfer of electrons that completes both atoms, they become oppositely charged ions that are attracted to each other, which establishes a stable combination.
VI. The complex combinations of more than 2 atoms can also be created based on the same principle. The mechanism is a little more complex, but follows the same idea to complete top subshells of all participants.
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