This is called Hund's Rule: "Half fill before you Full fill" and again this rule was established based on energy calculations that indicated that this was the way atoms actually distributed their electrons into the orbitals. One of the really cool things about electron configurations is their relationship to the periodic table. Basically the periodic table was constructed so that elements with similar electron configurations would be aligned into the same groups columns.
The periodic table shown above demonstrates how the configuration of each element was aligned so that the last orbital filled is the same except for the shell. The reason this was done is that the configuration of an element gives the element its properties and similar configurations yield similar properties. Let's go through some of the Periodic Properties that are influenced directly by the electron configuration:.
The size of atoms increases going down in the periodic table. This should be intuitive since with each row of the table you are adding a shell n. What is not as intuitive is why the size decreases from left to right. But again the construction of the electron configuration gives us the answer. What are you doing as you go across the periodic table? Answer, adding protons to the nucleus and adding electrons to the valence shell of the element.
What is not changing as you cross a period? Answer, the inner shell electrons. So think of it this way, the inner shell electrons are a shield against the pull of the nucleus.
As you cross a period and increase the number of protons in the nucleus you increase its pull but since you are only adding electrons to the new shell the shield is not increasing but remains the same all the way across. This means the pull on the electrons being added to the valence shell is increasing steadily all the way across.
What happens if you pull harder on the electrons? Well, they come closer to the nucleus and the size of the atom decreases. Electronegativity may be the most important of the periodic properties you can learn and understand since so many other properties are depend on its value.
Electronegativity is an atoms ability to pull electrons towards itself. Electronegativity is generally expressed by the Pauling Scale and the values were determined experimentally. The table below shows the scale values for the elements.
The important aspect is that we realize that knowing electron configurations helps us determine the valence electrons on an atom. This is important because valence electrons contribute to the unique chemistry of each atom. Hund's Rule. Hund's rule : every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin. These orbitals have different shapes e. It means there are 2- electrons in first energy level s-subshell and 2-electrons 2 nd energy level s- sub shell and 2-electrons in 2 nd energy level p-sub shell.
In chemistry, the valence or valency of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules. The concept of valence was developed in the second half of the 19th century and helped successfully explain the molecular structure of inorganic and organic compounds. The general formula is that the nth shell can in principle hold up to 2 n 2 electrons. The most stable electron configuration is that of a noble gas, due to the fact that its valence shell is filled.
For helium, that means two valence electrons a duet in the 1s sublevel, and for the rest it means eight valence electrons an octet in the outermost s and p sublevels. The valency of an atom is equal to the number of electrons in the outer shell if that number is four or less. Otherwise, the valency is equal to eight minus the number of electrons in the outer shell.
Once you know the number of electrons, you can easily calculate the valency. Rule 1 - Lowest energy orbitals fill first. Thus, the filling pattern is 1s, 2s, 2p, 3s, 3p, 4s, 3d, etc. Since the orbitals within a subshell are degenerate of equal energy , the entire subshell of a particular orbital type is filled before moving to the next subshell of higher energy.
Ionization Enthalpy of elements is the amount of energy that an isolated gaseous atom requires to lose an electron in its ground state. You need to provide a specific amount of energy to remove an electron from an atom. Hence, the ionization enthalpies of chemical elements are always positive. The aufbau principle , from the German Aufbauprinzip building-up principle , also called the aufbau rule, states that in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels.
The lowercase letter is the sub-shell. The sub-shells are named s, p, d and f. The number of available sub-shells increases as the energy level increases. For example, the first energy level only contains an s sub-shell while the second energy level contains both an s sub-shell and a p sub-shell. The number in superscript is the number of electrons in a sub-shell. Each sub-shell can hold only a certain number of electrons.
The s sub-shell can hold no more than 2 electrons, the p sub-shell can hold 6, the d sub-shell can hold 10 and the f sub-shell can hold as many as Since the electron configuration table lists each energy level by row, you can tell how many energy levels there are by seeing how many rows there are. As was mentioned earlier, an atom of gold contains six energy levels, as shown below:. The total number of electrons in an energy level is the sum of the electrons in each sub-shell of that energy level.
Just add the numbers in superscript together to find the number of electrons in an energy level.
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