Now select a longer period, say the fourth period, starting with the element in the 1A group. Again, go from left to right. What do you notice about the pattern of change in the electron configuration as you go from left to right?

The pattern of change in the electron configuration increases from left to right across the period.

Explanation:

In the periodic table, elements having the same number of electrons in the outermost shell of their atoms are placed over one another in vertical columns. Each of the vertical columns is known as a group or family. while each of the resulting horizontal rows is known as a period or row.

There are 18 vertical columns and seven periods in the periodic table. Each period begins with an atom having a valence electron and ends with an atom having a complete outer shell structure of an inert gas.

The fourth period of the periodic table (d-block) consists of the transition elements.

The electron distribution into the energy levels of sublevels of atoms  can be shown in any of the tree important electronic configuration.

Electronic configuration shows the sequence of filling electrons into the orbitals of the sublevels as guided by some principles.

The maximum number of electrons in the orbitals of sublevels are two for s-sublevel(one orbital) ; six for p-sublevel( three orbital); ten for d-sublevel ( five orbitals) and fourteen for f-sublevel( seven orbitals). This indicates that the maximum number of electrons in an orbital is two

In the Periodic table ,The sublevels with lower energies are filled  up before those with higher energies. One important thing about this principle is that the sublevels do not fill up in numerical order. The pattern of filling is as follows:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f etc.

In the electron configuration of potassium, I observe that after the 3p orbital is filled, the next electron enters the 4s orbital instead of the 3d orbital because 3d is at a higher energy than 4s. This pattern shows that orbitals fill up according to the Aufbau principle. As I move on right, the number of electrons increases by one for each element until vanadium is reached. The electron configuration of chromium shows all Half-filled orbitals in the 4s and 3d energy levels. Again, as I move right, an exception occurs at copper. Copper’s electron configuration shows completely filled 4s and 3d orbitals. This is an exception to the Aufbau principle to attain more stable electron configurations, whenever possible. Half-filled electron configurations are more stable than other configurations and completely filled electron configurations are the most stable.

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