What makes electron configurations stable

The following table shows the possible number of electrons that can occupy each orbital in a given subshell. Using our example, iodine, again, we see on the periodic table that its atomic number is 53 meaning it contains 53 electrons in its neutral state. Its complete electron configuration is 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 5.

If you count up all of these electrons, you will see that it adds up to 53 electrons. Notice that each subshell can only contain the max amount of electrons as indicated in the table above. The word 'Aufbau' is German for 'building up'. The Aufbau Principle , also called the building-up principle, states that electron's occupy orbitals in order of increasing energy.

The order of occupation is as follows:. Another way to view this order of increasing energy is by using Madelung's Rule :. Figure 1. Madelung's Rule is a simple generalization which dictates in what order electrons should be filled in the orbitals, however there are exceptions such as copper and chromium. This order of occupation roughly represents the increasing energy level of the orbitals. Hence, electrons occupy the orbitals in such a way that the energy is kept at a minimum.

That is, the 7s, 5f, 6d, 7p subshells will not be filled with electrons unless the lower energy orbitals, 1s to 6p, are already fully occupied. Also, it is important to note that although the energy of the 3d orbital has been mathematically shown to be lower than that of the 4s orbital, electrons occupy the 4s orbital first before the 3d orbital.

This observation can be ascribed to the fact that 3d electrons are more likely to be found closer to the nucleus; hence, they repel each other more strongly.

Nonetheless, remembering the order of orbital energies, and hence assigning electrons to orbitals, can become rather easy when related to the periodic table. To understand this principle, let's consider the bromine atom. Since bromine has 7 valence electrons, the 4s orbital will be completely filled with 2 electrons, and the remaining five electrons will occupy the 4p orbital.

Hence the full or expanded electronic configuration for bromine in accord with the Aufbau Principle is 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 5.

If we add the exponents, we get a total of 35 electrons, confirming that our notation is correct. Hund's Rule states that when electrons occupy degenerate orbitals i. Furthermore, the most stable configuration results when the spins are parallel i. Nitrogen, for example, has 3 electrons occupying the 2p orbital. According to Hund's Rule, they must first occupy each of the three degenerate p orbitals, namely the 2p x orbital, 2p y orbital, and the 2p z orbital, and with parallel spins Figure 2.

The configuration below is incorrect because the third electron occupies does not occupy the empty 2p z orbital. Instead, it occupies the half-filled 2p x orbital. This, therefore, is a violation of Hund's Rule Figure 2. Figure 2. A visual representation of the Aufbau Principle and Hund's Rule. Note that the filling of electrons in each orbital p x , p y and p z is arbitrary as long as the electrons are singly filled before having two electrons occupy the same orbital.

Wolfgang Pauli postulated that each electron can be described with a unique set of four quantum numbers. Therefore, if two electrons occupy the same orbital, such as the 3s orbital, their spins must be paired.

The way we designate electronic configurations for cations and anions is essentially similar to that for neutral atoms in their ground state. The electronic configuration of cations is assigned by removing electrons first in the outermost p orbital, followed by the s orbital and finally the d orbitals if any more electrons need to be removed.

In this case, all the 4p subshells are empty; hence, we start by removing from the s orbital, which is the 4s orbital. Hence, we can say that both are isoelectronic. The electronic configuration of anions is assigned by adding electrons according to Aufbau Principle. We add electrons to fill the outermost orbital that is occupied, and then add more electrons to the next higher orbital.

The rest of the representative elements Groups 1, 2, , will follow the same pattern as the Period 3 elements. The atoms of the Group 1 and 2 elements will lose their valence electrons in order to achieve the electron configuration of the noble gas in the previous period, and the atoms of the Groups elements will lose, gain, or share electrons in order to achieve the electron configuration of the noble gas at the end of that period.

But they still lose electrons to form positively charged ions with stable electron configurations, but not noble gas configurations. How can elements achieve a stable electron configurations?

Aug 20, Related questions Are polyatomic ions molecular compounds or ionic compounds? What type of atoms tend to form covalent bonds? The central structure of an atom is the nucleus, which contains protons and neutrons. This nucleus is surrounded by electrons. Although these electrons all have the same charge and the same mass, each electron in an atom has a different amount of energy. Electrons with the lowest energy are found closest to the nucleus, where the attractive force of the positively charged nucleus is the greatest.

Electrons that have higher energy are found further away. When the energy of an atom is increased for example, when a substance is heated , the energy of the electrons inside the atom is also increased—that is to say, the electrons get excited.

For the excited electron to go back to its original energy, or ground state, it needs to release energy. One way an electron can release energy is by emitting light.

Each element emits light at a specific frequency or color upon heating that corresponds to the energy of the electronic excitation. It is helpful to think of this like going up a flight of steps. You need to lift your foot to the height of the step to move on. The same goes for electrons and the amount of energy they can have. This separating of electrons into energy units is called quantization of energy because there are only certain quantities of energy that an electron can have in an atom. The energy of the light released when an electron drops down from a higher energy level to a lower energy level is the same as the difference in energy between the two levels.

We will start with a very simple way of showing the arrangement of electrons around an atom. Here, electrons are arranged in energy levels, or shells, around the nucleus of an atom. Electrons that are in the first energy level energy level 1 are closest to the nucleus and will have the lowest energy. Electrons further away from the nucleus will have higher energy. For example, the first shell can accommodate 2 x 1 2 or 2 electrons. The second shell can accommodate 2 x 2 2 , or 8, electrons.

The arrangement of electrons in a lithium atom : Lithium Li has an atomic number of 3, meaning that in a neutral atom, the number of electrons will be 3. The energy levels are shown as concentric circles around the central nucleus, and the electrons are placed from the inside out.

The first two electrons are found in the first energy level, and the third electron is found in the second energy level. As an example, fluorine F , has an atomic number of 9, meaning that a neutral fluorine atom has 9 electrons.

The first 2 electrons are found in the first energy level, and the other 7 are found in the second energy level. Though electrons can be represented simply as circling the nucleus in rings, in reality, electrons move along paths that are much more complicated. These paths are called atomic orbitals, or subshells. There are several different orbital shapes—s, p, d, and f—but we will be focusing mainly on s and p orbitals for now.

The first energy level contains only one s orbital, the second energy level contains one s orbital and three p orbitals, and the third energy level contains one s orbital, three p orbitals, and five d orbitals. Within each energy level, the s orbital is at a lower energy than the p orbitals. Orbital diagram : The positions of the first ten orbits of an atom on an energy diagram.

Note that each block is able to hold two electrons. An orbital diagram helps to determine the electron configuration of an element. There are a few guidelines for working out this configuration:. Electron configurations can be used to rationalize chemical properties in both inorganic and organic chemistry. It is also used to interpret atomic spectra, the method used to measure the energy of light emitted from elements and compounds.

Although the nucleus of an atom is very dense, the electrons around it can take on a variety of positions which can be summarized as an electron configuration. As electrons are added, they assume the most stable shells with respect to the nucleus and the electrons already present. The order in which orbitals are filled is given by the Madelung rule. According to the principle, electrons fill orbitals starting at the lowest available energy states before filling higher states e. The Madelung energy ordering rule : Order in which orbitals are arranged by increasing energy according to the Madelung Rule.

An Aufbau diagram uses arrows to represent electrons. When there are two electrons in an orbital, the electrons are called an electron pair. Electron pairs are shown with arrows pointing in opposite directions. According to the Pauli Exclusion Principle, two electrons in an orbital will not spin the same way.

That is, an Aufbau diagram uses arrows pointing in opposite directions. An arrow pointing up denotes an electron spinning one way and an arrow pointing downwards denotes an electron spinning the other way. If the orbital only has one electron, this electron is called an unpaired electron. Aufbau diagram for lithium : The electron configuration of lithium, shown on an Aufbau diagram.

Aufbau diagram for fluorine : An Aufbau diagram showing the electron configuration of fluorine. The notation describes the energy levels, orbitals, and the number of electrons in each. For example, the electron configuration of lithium is 1s 2 2s 1.

The number and letter describe the energy level and orbital, and the number above the orbital shows how many electrons are in that orbital. Using standard notation, the electron configuration of fluorine is 1s 2 2s 2 2p 5. The Aufbau principle is based on the idea that the order of orbital energies is fixed—both for a given element and between different elements. This assumption is approximately true—enough for the principle to be useful—but not physically reasonable.

However, the energy of an electron in an atomic orbital depends on the energies of all the other electrons of the atom. In a hydrogen-like atom, which only has one electron, the s-orbital and the p-orbitals of the same shell in the Aufbau diagram have exactly the same energy. However, in a real hydrogen atom, the energy levels are slightly split by the magnetic field of the nucleus. Because each atom has a different number of protons in its nucleus, the magnetic field differs, which alters the pull on each electron.

In general, the Aufbau principle works very well for the ground states of the atoms for the first 18 elements, then decreasingly well for the following elements. Interactive: Energy Levels of a Hydrogen Atom : The likely location of an electron around the nucleus of an atom is called an orbital.

The shape of an orbital depends on the energy state of the electron. A neutral hydrogen atom has one electron. Click in the boxes to set the energy of that electron and see the orbital shape describing where you are likely to find that electron around the nucleus.

Electrons will fill the lowest energy orbitals first and then move up to higher energy orbitals only after the lower energy orbitals are full. This is referred to as the Aufbau Principle, after the scientist who proposed the concept. Although the implications are clear for orbitals of different principal quantum number n , which are clearly of different energy, the filling order is less clear for degenerate sublevels.