UNIT 2 - ATOMS & THE PERIODIC TABLE 2.1.d Bohr’s Model of Electron Arrangement Section 2.1 Models of the Atom BOHR’S MODEL OF ELECTRON ARRANGEMENT You have seen that electrons occupy the space outside the nucleus of an atom. When atoms approach one another, it is their electrons that interact. The first widely accepted explanation for the arrangement of these electrons was put forward by a Danish physicist, Niels Bohr (1885-1962) (shells) In 1913, Bohr proposed a model of the atom that helped explain the movement of the electrons around the nucleus of an atom. He suggested that the electrons exist around the nucleus of an atom in specific energy levels. Each energy level is a certain distance from the nucleus and can contain a certain maximum number of electrons. Energy levels are often referred to as shells. Bohr said that electrons can exist only in these energy levels or shells, but they could not exist in between the shells! Because electrons are negative, they are attracted to the positive nucleus. Therefore electrons will fill the shells closest to the nucleus before entering shells that are further away. The maximum number of electrons in each shell are shown in Table 1. Shell 1 2 3 4 Maxiumum number of electrons 2 8 8 (18*) *The Bohr model kind of breaks down at the 4th shell and so you’ll not be asked to place more than 1 or 2 electrons in it. For example, the lithium atom , which has the atomic number 3, has 3 electrons. The first two electrons will be in the first shell, at which point the first shell will be full. The third electron will be in the second shell. atomic number = # protons = # of electrons in a neutral atom UNIT 2 - ATOMS & THE PERIODIC TABLE Section 2.1 Models of the Atom 2.1.d Bohr’s Model of Electron Arrangement The element nitrogen, which has the atomic number of 7, has 7 electrons. Two of the electrons are in the first orbital and the other 5 electrons are in the second orbital. atomic number = # protons = # of electrons in a neutral atom Notice the position of the electrons in the second shell: at the top, bottom, left and right of the diagram. • • When placing electrons, it can be helpful to think of the atom as a hotel, and the shells as the floors of the hotel. Each floor has a certain number of rooms. First “floor” has only ______ room: Second “floor” has ______ rooms: Third “floor” has _______ rooms: • This hotel has no elevator, so electrons will fill the lower floors first • Each room can accommodate 2 electrons, maximum. • Nobody wants to share a room if they don’t have to, so electrons will slot in singly (one at a time) at first and only pair when there are no more empty rooms on the lowest available floor • I like to start at the top and go around the shell like a clock when filling in the e− UNIT 2 - ATOMS & THE PERIODIC TABLE NOTE: 2.1.d Bohr’s Model of Electron Arrangement Section 2.1 Models of the Atom We often write the number of protons and neutrons in the nucleus, but for this worksheet you don’t have to. Ex. Oxygen is atomic number 8, so an oxygen atom has 8 electrons. The numbers below show the order in which they fill in: 3 7 1 2 6 4 8 5 If we show all the numbers, the order of filling looks like this: 19 11 15 3 7 1 2 14 18 66 4 12 10 8 16 55 9 13 5 917 NOTE: Bohr models don’t really work after element #20, so we stop there. 20 UNIT 2 - ATOMS & THE PERIODIC TABLE 2.1.d Bohr’s Model of Electron Arrangement Section 2.1 Models of the Atom Exercises: 1. Give 2 names for the “circles” on which elements “live” in the Bohr Model. 2. How many electrons are there maximum in the (a) 1st shell __________ (b) 2nd shell __________ (c) 3rd shell __________ 3. If you imagine an atom to be like a hotel, how many “rooms” are there to hold electrons on the (a) 1st shell ________ (b) 2nd shell ________ (c) 3rd shell ________ 4. How many electrons can each “room” hold? ____________ 5. Electrons go into the rooms o_______ at a time, and only p___________ up when they have to. 6. Electrons always go into the shell when that one is full. 7. How can you find the number of electrons in a neutral atom? 8. Write numbers in the boxes on the Bohr model shown here, in the order of filling. Start by writing the number 1, then the number 2 and continue up to #20. highest / lowest circle the correct answer Order of filling: Start in the lowest shell. Electrons go into orbitals (boxes) singly and don’t pair up until all orbitals in an energy level (shell) contain 1 electron. Only move to the next shell when all the orbitals in a lower shell are filled. Start at the top and go around like a clock. shell first, and only go into the next UNIT 2 - ATOMS & THE PERIODIC TABLE 9. Section 2.1 Models of the Atom 2.1.d Bohr’s Model of Electron Arrangement Use the periodic table to find the # of electrons for each atom and write that on the line. (The number of electrons is equal to the atomic number for that element.) Draw in spots or small circles to represent the correct number and placement of electrons in each shell (energy level). UNIT 2 - ATOMS & THE PERIODIC TABLE Section 2.1 Models of the Atom 2.1.d Bohr’s Model of Electron Arrangement UNIT 2 - ATOMS & THE PERIODIC TABLE 2.1.d Bohr’s Model of Electron Arrangement Section 2.1 Models of the Atom 2.1.d KEY ANSWERS: 1. Give 2 names for the “circles” on which elements “live” in the Bohr Model. ① shells ② energy levels 2. How many electrons are there maximum in the (a) 1st shell __________ 2 nd (b) 2 shell __________ 8 rd 8 (c) 3 shell __________ 3. If you imagine an atom to be like a hotel, how many “rooms” are there to hold electrons on the 1 (a) 1st shell ________ 4 (b) 2nd shell ________ 4. 2 How many electrons can each “room” hold? ____________ 5. Electrons go into the rooms one 6. Electrons always go into the shell when that one is full. 7. How can you find the number of electrons in a neutral atom? at a time, and only pair up when they have to. highest / lowest shell first, and only go into the next circle the correct answer The # of e– in a neutral atom is equal to the 8. 4 (c) 3rd shell ________ # of p+ = atomic # Write numbers in the boxes on the Bohr model shown here, in the order of filling. Start by writing the number 1, then the number 2 and continue up to #20. (Remember, the Bohr model breaks down after element #20, Ca.) 19 11 15 3 7 1 2 14 18 66 10 55 9 13 5 917 4 12 8 16 20 UNIT 2 - ATOMS & THE PERIODIC TABLE Section 2.1 Models of the Atom 2.1.d Bohr’s Model of Electron Arrangement 2.1.d KEY 9. Use the periodic table to find the # of electrons for each atom and write that on the line. (The number of electrons is equal to the atomic number for that element.) Draw in spots or small circles to represent the correct number and placement of electrons in each shell (energy level). UNIT 2 - ATOMS & THE PERIODIC TABLE Section 2.1 Models of the Atom 2.1.d Bohr’s Model of Electron Arrangement 2.1.d KEY