 # + Protons and Electrons 8.5B: The student is expected to identify that protons determine an element’s identity and valence electrons determine its chemical.

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+ Protons and Electrons 8.5B: The student is expected to identify that protons determine an element’s identity and valence electrons determine its chemical properties.

+ How can the periodic table be used to find the number of protons? The atomic number represents the number of protons in an atom. 3 Li Lithium 6.94 Atomic Number = Number of Protons

+ How can the periodic table be used to determine the number of neutrons? The Atomic Mass listed in each box on the Periodic Table represents the total number of protons and neutrons in an atom’s nucleus. Round the atomic mass to a whole number using the same rounding rules you have been using for years in math. Then subtract the number of protons from the atomic mass to determine the number of neutrons. 3 Li Lithium 6.94 Atomic Mass (rounded) - # of protons = # of neutrons 7 - 4 = 3

+ How can the number of electrons be determined? In an electrically neutral atom the electrons are equal to the protons. Positive Protons + Negative Electrons = Neutral Atom 3 Li Lithium 6.94 +3 + -3 = 0 In an electrically neutral atom of lithium there are 3 electrons.

+ Summing it Up: How can the periodic table be used to find the number of protons, neutrons, and electrons in an atom? The atomic number represents the number of protons in an atom. In an electrically neutral atom, the protons equal the electrons. The atomic mass represents the total number of protons and neutrons in an atom. Atomic mass – atomic number = number of neutrons 3 Li Lithium 6.94 Atomic Number = Number of Protons Atomic Mass = Protons + Neutrons How many protons, neutrons, and electrons does lithium have? Protons = 3 Neutrons = 4 Electrons = 3

+ What subatomic particle is used to determine an element’s identity? The atomic number on the Periodic Table represents the number of protons in the nucleus of an atom, so the protons can be used to determine the identity of an atom. How many protons does this atom have? 2 What element on the Periodic Table has an atomic number 2? Helium 2 He Helium 4.0026

+ How are electrons arranged in the electron cloud? Electrons are constantly moving, but we can use models to predict where they might be located. Electrons are arranged in to energy levels. In 8 th grade we will only use the first three energy levels. Each energy level can only hold a specific number of electrons. Nucleus 1 st Energy Level Holds 2 Electrons 2 nd Energy Level Holds 8 Electrons 3 rd Energy Level Holds 8 Electrons

+ How are electrons arranged in the electron cloud? Always start with the first energy level when placing electrons. Nucleus Put electrons here first. Use the 2 nd energy level when the 1 st is full. Use the 3 rd energy level when the 2 nd is full. When the 1 st level is full (2 electrons) move the 2 nd level and continue adding electrons. When the 2 nd level is full (8 electrons) move to the 3 rd level and continue adding electrons.

+ What are the properties of valence electrons? Valence electrons are the electrons located in an atoms outermost energy level. 8 + 8 o Valence Electrons How many valence electrons does this atom have? 6

+ What are the properties of valence electrons? Valence electrons determine an atom’s chemical properties, or its reactivity. All atoms want to have a full outer shell and will react with other atoms to steal or share electrons to make this happen. Atoms with 1, 2, 6, or 7 valence electrons are extremely reactive. (Except for Helium, its outer shell can only hold 2 electrons) Atoms with 8 valence electrons are very stable or not reactive. (Helium’s outermost energy level is full with 2 so it is stable) 8 + 8 o 10 + 10 o Is this atom reactive or stable? Reactive Stable

+ What if the protons and electrons are not equal? When the positive protons are not balanced by an equal number of negative electrons it results in an atom with an electrical charge. An atom with an electrical charge is called an ion. To determine the overall charge of an atom simply add the protons and electrons. 8 + 8 o How many protons? How many electrons? +8 -10 +8 + -10 = -2 This atom has an overall charge of -2 because it has 2 more negative electrons than positive protons.

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