# When we want to show how electrons are arranged in an

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When we want to show how electrons are arranged in an atom, we need to remember the following principles: 48

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CHAPTER 3. THE ATOM - GRADE 10 3.6 1s 2s 3s 4s 2p 3p E N E R G Y First main energy level Second main energy level Third main energy level Figure 3.7: The positions of the first ten orbits of an atom on an energy diagram. Note that each block is able to hold two electrons. 49
3.6 CHAPTER 3. THE ATOM - GRADE 10 Each orbital can only hold two electrons . Electrons that occur together in an orbital are called an electron pair . These electrons spin in opposite directions around their own axes. An electron will always try to enter an orbital with the lowest possible energy. An electron will occupy an orbital on its own, rather than share an orbital with another electron. An electron would also rather occupy a lower energy orbital with another electron, before occupying a higher energy orbital. In other words, within one energy level, electrons will fill an ’s’ orbital before starting to fill ’p’ orbitals. The way that electrons are arranged in an atom is called its electron configuration . Definition: Electron configuration Electron configuration is the arrangement of electrons in an atom, molecule, or other physical structure. An element’s electron configuration can be represented using Aufbau diagrams or energy level diagrams. An Aufbau diagram uses arrows to represent electrons. You can use the following steps to help you to draw an Aufbau diagram: 1. Determine the number of electrons that the atom has. 2. Fill the ’s’ orbital in the first energy level (the 1s orbital) with the first two electrons. 3. Fill the ’s’ orbital in the second energy level (the 2s orbital) with the second two electrons. 4. Put one electron in each of the three ’p’ orbitals in the second energy level (the 2p orbitals), and then if there are still electrons remaining, go back and place a second electron in each of the 2p orbitals to complete the electron pairs. 5. Carry on in this way through each of the successive energy levels until all the electrons have been drawn. Important: When there are two electrons in an orbital, the electrons are called an electron pair . If the orbital only has one electron, this electron is said to be an unpaired electron . Electron pairs are shown with arrows in opposite directions. This is because when two electrons occupy the same orbital, they spin in opposite directions on their axes. An Aufbau diagram for the element Lithium is shown in figure 3.8. 1s 2s Figure 3.8: The electron configuration of Lithium, shown on an Aufbau diagram A special type of notation is used to show an atom’s electron configuration. The notation de- scribes 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.

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• Fall '10
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