Electronic Structure of the Atom

Electronic Structure of the Atom

The electronic configuration of an atom is notation that describe the orbitals in which the electrons occupy and total number of electron in each orbitals.

Niels Bohr proposed that electrons only exist in specific energy states when an electron absorb or emits a specific amount of energy it instantaneously moves from one orbital to another.

 

Energy Levels of Electrons

As you may remember from chemistry, an atom consists of electrons orbiting around a nucleus. However, the electrons cannot choose any orbit they wish. They are restricted to orbits with only certain energies. Electrons can jump from one energy level to another, but they can never have orbits with energies other than the allowed energy levels.

The Energy difference between two particular energy level is called the quantum of energy.

- Ground State: When all the electron of an atom are in their lowest possible energy levels.

- Excited State: When on or more of an atom's electrons are in energy levels other than the lowest available level.

Electron Configuration

The letters S,P,D,F are different types of orbital.

Example

For example, let's say you have an atom of lithium. Lithium's atomic number is 3. So a neutral atom of lithium has 3 protons and 3 electrons. We would need space for 3 electrons. Write:

There are 2 electrons in the 1s sublevel and 1 electron in the 2s sub-level. 2 + 1 = 3. That's it!

Now let's try fluorine, which has an atomic number of 9. A neutral atom of fluorine has 9 protons and 9 electrons. We need enough space for 9 electrons. The 1s orbital can hold 2 electrons and the 2s orbital can hold 2 more electrons. The five remaining electrons must go into the next orbital, the 2p orbital. The 2p orbital can hold up to 6, but we only have 5. So the following would be the correct electron configuration for a neutral atom of fluorine. 2 + 2 + 5 = 9.

Writing Electron Configuration for Ion

For a negative ion: Add electrons (equal to the charge) to the last unfilled sub-shell starting where the neutral atom left off.

For a positive ion: 1) Start with the neutral configuration, remove electrons from the outermost shell first. 2) If there 's are electrons on both the S and P orbitals of the outermost shell, the electrons in p orbitals are removed first.

Valance Electrons

The number of valence electrons is just how many electrons an atom has in its outer shell. It's easy to figure out if you've got a periodic table.

All the elements in each column have the same number of electrons in their outer shells. All the elements in the first column all have a single valence electron (H, Li, Na, K, etc.).

The second column elements all have 2 valence electrons (Be, Mg, Ca, Sr, etc.).
Skipping over the gap, go to the Group 3 elements, which all have 3 valence electrons (B, Al, Ga, etc.).

The elements in the next column (C, Si, Ge, etc.) all have 4 valence electrons.
The elements in the next column (N, P, As, etc.) all have, yes, you guessed it, 5 valence electrons.
O, S, Se, and the others in this column have 6 valence electrons.
The halogens in the next-to-last column (F, Cl, Br, etc.) have 7 valence electrons.

The noble gases in the right-most column (Ne, Ar, Kr, etc.) all have 8 electrons in their out except for He, which only has 2 electrons.

If an atom is an ion, you must include the charge also:
For a positive ion, for each charge subtract one electron, *for instance, Na⁺ has 1-1 = 0, BUT it has 8 valence electrons because it has the same electron configuration as Ne. Just as K⁺ has the same configuration as Ar. Therefore, the alkali metal ions with a single positive charge will have 8 valence electrons.

For a negative ion, add one electron for each charge, for instance, O^2- has 6+2 = 8 valence electrons