atom is:
1s 1
Hydrogen: 1s 1
It means that Hydrogen has only one electron on the first S orbital.
The electronic configuration of the Helium (He) atom is:
1s 2
Helium: 1s 2
It means that He has two electrons on the first S orbital.
For an He atom, n quantum number =1, l (small L) quantum number =0. There is no p orbital for l=0. S orbital is completely filled. Helium cannot have more than 2 electrons on the 1S orbital. As a result, It is a noble gas. Helium cannot form bound with any other elements.
The next element in the periodic table is Lithium (Li). Li starts the second period and has the order number of 3. It means that it has 3 protons and 3 electrons. Its electronic configuration is:
1s 2 2s 1
Lithium: 1s 2 2s 1
The next element, Beryllium (Be) has the order number of 4 and it has 2 electrons on the 1S orbital and 2 electrons on the 2S orbital. Two S orbitals are completely filled for Beryllium. You may wonder why Beryllium is not a noble element if it has completely filled its S orbitals? The answer is that on the second shell the completed number of electrons is 8 (2 S electrons and 6 P electrons). For Beryllium, the quantum number n=2 and the quantum number l=1. As a result, an additional p orbital appears for n=2. This orbital is not filled for Beryllium. The Beryllium electronic configuration is:
1s 2 2s 2
Beryllium: 1s 2 2s 2
The next element is Boron (B). It has 5 electrons: 2 electrons on the 1S orbital, 2 electrons on the 2S orbital and 1 electron on the 2P orbital. Filling of the P orbital starts from Boron. Boron electronic configuration is:
1s 2 2s 2 2p 1
Boron: 1s 2 2s 2 2p 1
Next 5 elements have the outmost electrons on the P orbital and the number of P electrons is incremented by one for each consequential element.
1s 2 2s 2 2p 2
Carbon: 1s 2 2s 2 2p 2
Nitrogen: 1s 2 2s 2 2p 3
Oxigen: 1s 2 2s 2 2p 4
Fluorine: 1s 2 2s 2 2p 5
Neon: 1s 2 2s 2 2p 6
Neon (Ne) has 6 P electrons. The P orbital is completely filled for Neon. As a result, Neon is a noble gas.
The next element is Sodium (Na). Na order number is 11. It has the same electronic configuration as Neon plus additionally, it has 1 electron on the 3S orbital. The Sodium electronic configuration can be written in short form as [Ne] 3s1 or in long form:
1s 2 2s 2 2p 63s 1
Sodium: 1s 2 2s 2 2p 63s 1
The next element is Magnesium (Mg). Mg has 2 electrons on the 3S orbital.
1s 2 2s 2 2p 63s 2
Magnesium: 1s 2 2s 2 2p 63s 2
The next element is Aluminum and it has 1 electron on the 3P orbital. Again, filing of the P orbital starts from Al and for the next 5 elements the number of P electrons is incremented by one for each consequential element. Argon (Ar) has complete set of 6 electrons on the 3P orbitals and it is noble gas.
1s 2 2s 2 2p 63s 2 3p 1
Aluminum: 1s 2 2s 2 2p 63s 2 3p 1
Silicon: 1s 2 2s 2 2p 63s 2 3p 2
Phosphorus: 1s 2 2s 2 2p 63s 2 3p 3
Sulfur: 1s 2 2s 2 2p 63s 2 3p 4
Chlorine: 1s22s 22p63s23p 5
Argon: 1s 2 2s 2 2p 63s 2 3p 6
For Argon the main quantum number n is 3, l is 2. Remember, we pointed out earlier when l=2 additional d orbital appeared. So we may expect that the next element, Potassium, will have the outmost electron on the 3d orbital. Actually, Potassium has one the outmost electron on the 4S orbital. The 4S orbital has a lower energy than the 3d orbital and as a result the 4S orbital is filled before the 3d orbital.
