ELECTRON CONFIGURATION and BOHR-RUTHERFORD DIAGRAMS

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1 ELECTRON CONFIGURATION and BOHR-RUTHERFORD DIAGRAMS I. Electron Arrangement. The electron configuration is a way of showing how the electrons are arranged in an atom. Recall that electrons are placed in energy levels around the nucleus The energy levels can hold a maximum number of electrons. An easy way to calculate the total number of electrons that can be held in a given energy level is to use the formula 2n 2. The first energy level (n = 1) can hold 2(1) 2 = 2 electrons; the second energy level can hold 2(2) 2 = 8 electrons; the third energy level (n = 3) can hold 2(3) 2 = 18 electrons, and so on. Knowing how many electrons are contained in an energy level does not really tell us how the electrons are arranged. Hence we need some way of displaying how the electrons are arranged. Types of Notation. 1. Bohr-Rutherford Diagrams 2. Aufbau Notation II. How to draw Bohr-Rutherford diagrams. The purpose of these diagrams is to show the nucleus with its correct number of protons and neutrons, but more importantly, to give a more detailed view of how the electrons are arranged. 1. Draw the nucleus as a solid circle. 2. Put the number of protons (atomic number) in the nucleus with the number of neutrons (atomic mass atomic number) under it. 3. Place the number of electrons (same as protons) in orbits around the nucleus by drawing circles around the nucleus. # p # n # e # e When placing electrons in energy levels you have to fill the innermost shell before beginning to fill the next energy level. The maximum number of electrons in the first energy level is 2. Hydrogen would have 1 electron in the first level and helium would have 2 electrons in the first level.

An easy way to calculate the total number of electrons that can be held in a given energy level is to use the formula 2n 2.

2 Example: Hydrogen 1 p 0 n 1 e 0 e Example: Helium 2 p 2 n 2 e 0 e The maximum number of electrons in the second energy level is 8. Example: Lithium 3p 4n 1e We continue adding electrons into the 2 nd level until we reach the maximum of 8 Example: Neon 10p 10n The maximum number of electrons in the third energy level is 18. Example: Sodium 11p 12n 1e 0e We continue adding electrons into the 3 rd level until we reach 8. Example: Argon 18p 22n 0e

Example: Lithium 3p 4n 1e We continue adding electrons into the 2 nd level until we reach the maximum of 8

3 ** In the 4 th row of the Periodic Table there is a change in the placement of the electrons. We do NOT add electrons up to the maximum number of 18. Example: Calcium has 20 protons and 20 neutrons, and because it's neutral, it has 20 electrons. We first fill the first level with 2 electrons and the second level with 8 more. However, we only place 8 electrons in the 3 rd level. The last two electrons are placed in the fourth level. 20p 20n III. General Rules for the Elements # When placing electrons in energy levels you have to fill, if possible the innermost level before beginning to fill the next level. 2. The maximum number of electrons in the 1 st level is The maximum number of electrons in the 2 nd level is The maximum number of electrons in the 3 rd level is 18. However before placing more than 8 electrons in this level, you have to place two electrons in the next level (4 th ), and then come back to fill the 3 rd level. Example: Chromium 24p 28n 1 5. The maximum number of electrons in the 4 th level is 32. However before placing more than 8 electrons in this level, you have to place two electrons in the next level (5 th ), and then come back to fill the fourth level. Example: Zirconium 40p 51n 1 10e

However, we only place 8 electrons in the 3 rd level. The last two electrons are placed in the fourth level. 20p 20n III. General Rules for the Elements # 21-54. 1.

4 Example: Xenon 54p 77n 1 1 IV. Aufbau Principle. Aufbau is derived from the German word meaning building up. Within each energy level there are sub-levels: s, p, d, f and so on. Furthermore, within each sub-level consists of orbitals. Each orbital can hold a maximum of 2 electrons. V. Total number of electrons per energy level An easy way to calculate the total number of electrons that can be held by a given energy level is to use the formula 2n 2. For example, the fourth energy level (n = 4) can hold 2(4) 2 = 32 electrons. VI. Sub-levels and orbitals An orbital is a space that can be occupied by up to two electrons. As stated above, each type of sub-level holds a different number of orbitals, and therefore, a different number of electrons. s sub-levels have one orbital and can hold up to two electrons; p sub-levels have three orbitals, each of which can hold 2 electrons, for a total of 6; d sub-levels have 5 orbitals, for a possible total of 10 electrons; and f sublevels, with 7 orbitals, can hold up to 14 electrons. VII. Total number of orbitals per energy level An easy way to calculate the number of orbitals found in an energy level is to use the formula n 2. For example, the fourth energy level (n = 4) has a total of 4 2, or 16 orbitals. Combining VI and VII: The 4 th energy level (n = 4) can hold 2(4) 2 = 32 electrons. This makes sense because the 4 th energy level would have four sub-levels. The s sub-level holds 2 electrons, the p sub-level holds 6 electrons, the d sub-level holds 10 electrons and the f sub-level holds 14 electrons = 32, so the formula 2n 2 works. On the other hand, the 4 th energy level with 4 sub-levels would have an s sub-level with 1 orbital, a p sub-level with 3 orbitals, a d sublevel with 5 orbitals and an f sub-level with 7 orbitals = 16, so the formula n 2 works.

Total number of electrons per energy level An easy way to calculate the total number of electrons that can be held by a given energy level is to use the formula 2n 2.

5 The information above is summarized in the tables below: Sub-level Orbitals and electron capacity # of orbitals Maximum number of electrons S 1 2 P 3 6 D 5 10 F 7 14 Principle energy level (n) Type of sublevel Orbitals and electron capacity Number of orbitals per type Number of orbitals per level (n 2 ) Maximum number of electrons (2n 2 ) 1 s s 1 p 3 s 1 P 3 d 5 s 1 p 3 d 5 f VIII. Aufbau Notation. The electron configuration of an atom is a form of notation which shows how the electrons are distributed among the various atomic orbitals and energy levels. The format consists of a series of numbers, letters and superscripts. Example: Helium 1 s 2 The first number is the energy level, in this case, 1. The lowercase letter, in this case s, is the sub-shell.

8 9 18 16 32 VIII. Aufbau Notation. The electron configuration of an atom is a form of notation which shows how the electrons are distributed among the various atomic orbitals and energy levels.

6 The number in superscript is the number of electrons in a subshell, in this case, 2. VIIII. Order of filling sub-levels with electrons The energy sublevels are filled in a specific order that is shown by the arrow diagram to the right. Start at the beginning of each arrow, and then follow it all of the way to the end, filling in the sub-levels that it passes through. In other words, the order for filling in the sub-levels becomes: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p. Examples: Helium: 1 s 2 Lithium: 1 s 2 2 s 1 Nitrogen: 1 s 2 2 s 2 2 p 3 Silicon: 1 s 2 2 s 2 2 p 6 3 s 2 3 p 2

Start at the beginning of each arrow, and then follow it all of the way to the end, filling in the sub-levels that it passes through.

Name: Worksheet: Electron Configurations. I Heart Chemistry!

Name: Worksheet: Electron Configurations. I Heart Chemistry! 1. Which electron configuration represents an atom in an excited state? 1s 2 2s 2 2p 6 3p 1 1s 2 2s 2 2p 6 3s 2 3p 2 1s 2 2s 2 2p 6 3s 2 3p 1 1s 2 2s 2 2p 6 3s 2 Worksheet: Electron Configurations Name: