The Electron Cloud THE BOHR MODEL LEARNING OBJECTIVE

The Electron Cloud THE BOHR MODEL

LEARNING OBJECTIVE � Express the arrangement of electrons in atoms through electron configurations.

VOCABULARY � Planetary model: an atomic model patterned after the motions of the planets around the sun. The electrons are arranged in concentric circular paths, or orbits, around the nucleus.

THE BOHR MODEL � � In 1913, Niels Bohr came up with a new atomic model. He proposed that electrons are arranged in concentric circular paths, or orbits, around the nucleus. Bohr’s model is often referred to as the planetary model. According to the Bohr model of the atom, electrons exist on fixed energy levels and cannot exist between energy levels. To more from one energy level to another, an electron must gain or lose just the right amount of energy.

THE BOHR MODEL FLAWS � Bohr’s model of the atom was consistent with the emission spectrum produced by the hydrogen atom, but the model does not work for more complicated atoms. � Although the Bohr model is not the current model of the atom, his model is still commonly used to help explain chemical behavior and periodic trends.

DRAWING BOHR MODELS FOR ATOMS The electrons are drawn on energy levels. 1. a) b) c) d) 2. The first energy level can hold 2 electrons. The second energy level can hold 8 electrons. The third energy level can hold 18 electrons. The fourth energy level can hold 32 electrons. Although the third and fourth energy level can hold more than 8 electrons, an outer energy level must never have more than eight electrons on it. This will affect the elements with atomic numbers greater than 18.

EXAMPLE 1 Let’s look at an example of the Bohr model for the oxygen-16 atom. � How many protons and electrons does an oxygen atom have? � How many neutrons does an oxygen atom have? Two electrons will go on the first energy level and six electrons will go on the second energy level. An easier way of writing this is 2. 6

EXAMPLE 2 Let’s look at an example of the Bohr model for aluminum-27 atom. How many protons and electrons does an aluminum atom have? How many neutrons? Two electrons will go on the first energy level. Eight electrons will go on the second energy level and three electrons will go on the third energy level. An easier way of writing this is 2. 8. 3

EXAMPLE 3 Now let’s look at an example of a Bohr model for calcium-40. Calcium has more than 18 e-. How many protons and electrons does a calcium atom have? How many neutrons? Two electrons will go on the first energy level. Eight electrons will go on the second energy level. Although the third energy level can hold up to 18 electrons, only 8 electrons will be placed in it. Remember: only eight electrons can be on an outer energy level. The remaining electrons go on the fourth energy level. An easier way of writing this is 2. 8. 8. 2.

BOHR MODEL NOTE � If you needed to add more electrons for a higher number element, you would put them on the third level until you reached 18. � For example, the element zinc has 30 electrons. There would be 2 electrons on the first energy level, 8 electrons on the second energy level, 18 electrons on the third energy level, and then 2 electrons the fourth energy level. The shorthand way of writing this would be 2. 8. 18. 2.