Atoms – Valence Electrons

Orbital Levels - Review closest to nucleus – max 2 electrons 2nd shell – max 8 electrons 3rd shell – max 8 electrons fill shells closest to nucleus first TEACHER NOTE: the term “shell” is used here to demonstrate there are various terms for the orbitals. We recommend that you use orbital, electron shell, electron cloud and circle, interchangeable throughout this unit so students are familiar with all the terms. Since electrons can be anywhere in the electron cloud, we use a model of circles (Bohr Model) to demonstrate the orbitals. This might be a good point to talk about the limitations of this model…electrons don’t travel in circular orbits; they are found somewhere in the cloud at any given moment…as an analogy, think about winding a ball of yarn: as you wind the yarn around the ‘ball’ and say “STOP”…you are somewhere in the “cloud” at that moment.

Important! Knowing how electrons are arranged in an atom is Electrons control how atoms react with each other! H This slide requires a CLICK of the mouse to complete….after the click, the 2 Hydrogen atoms will “combine” with the Oxygen atom to create a WATER molecule: H2O O H

HAPPY ATOMS Atoms want their outer most shell to be full and will lose or gain extra electrons to achieve that… Electrons on the outer most orbital have a special name: Valence Electrons Make sure to point out that atoms with only one shell will be “happy” with 2 electrons…the second and third shells require 8 to be “happy”. NOTE: “Happy” means ‘stable’.

Vocabulary Check: Valence Electron – located on the outer most orbital (shell) determine reactivity same for all members of a group/family Last digit of group # = # of valence e- Pick several elements and have students tell you how many valence electrons the element has. [DO NOT USE the transition elements…only use those in groups 1,2, 13-18] i.e., Oxygen is in group 16 so it has 6 valence electrons; Calcium is in Group 2 so it has 2 valence electrons.

What does that have to do with anything?? Putting it all together

Sodium - - - - - - - - - - - NUCLEUS VALENCE 1 1 H Hydrogen 3 Li Lithium 7 - - VALENCE - - 11 Na Sodium 23 - - - NUCLEUS - Ask students how many electrons an atom of Sodium should have – 11 How many of the electrons will fit in the first orbital? – 2 How many electrons will fit in the second orbital? 8 How many electrons will be in the third orbital? 1 What is the valence number for Sodium? 1 Have students find group 1 on their periodic table. They should have this labeled as the most reactive group – discuss how elements that only have 1 valence electron are highly reactive because it is very easy to give up that 1 electron. Use the Group#’s to compare with the Valence #’s! Ask students what would need to occur for this atom to be “happy”. They should come up with gain 7 electrons or lose 1 electron. Explain that it takes the same amount of energy for an atom to gain or lose electrons. Would it be easier for Sodium to gain 7 electrons or lose 1 electron? – lose 1 19 K Potassium 39 - 37 Rb Rubidium 85 - -

Chlorine - - - - - - - - - - - - - - - - - NUCLEUS 17 9 F Fluorine 19 How many Valence Electrons? 17 Cl Chlorine 35 - - - - - - - - 35 Br Bromine 80 - - NUCLEUS - Ask students how many electrons an atom of Chlorine should have – 17 How many of the electrons will fit in the first orbital? – 2 How many electrons will fit in the second orbital? 8 How many electrons will be in the third orbital? 7 What is the valence number for Sodium? 7 Ask students what would need to occur for this atom to be “happy”. They should come up with lose 7 electrons or gain 1 electron. Would it be easier for Chlorine to lose 7 electrons or gain 1 electron? – lose 1 Ask students to locate group 17 on the periodic table. All of these elements are only missing one electron – it’s difficult to give up 7 but easier to gain 1 – these elements are less reactive… 53 I Iodine 127 - - - - 85 At Astatine 210 - -

Sodium Chloride “SALT” Chlorine - - - - - - - - - - - - - - NUCLEUS NUCLEUS - - - - - On the first click Sodium’s valence electron will move to Chlorine and it’s outer orbital will disappear. On the second click the names will combine. *** Have students explain how this makes both atoms in the molecule “happy”. They are happy because both elements get a “Full” valence (outer) shell. BACKGROUND INFO: Properties of Sodium: solid, silvery-white metal, highly reactive with water, & poisonous Properties of Chlorine: yellow-green gas, distinctive strong odor, rapidly reacts with a metal Properties of Sodium Chloride (Salt): white solid, non-metal, used to flavor food (safe to eat) - - - - - - - - -

Journal Questions Define Valence Electron. What is the significance of valence electrons? Use your periodic table to figure out how many valence electrons Magnesium has. Explain Magnesium’s reactivity using it’s valence number. Electrons of an element located in the outer electron shell (outer orbital). The number of valence electrons of an atom determine its reactivity. Mg is in group 2 meaning it has 2 valence electrons. Group 2 is the second most reactive group on the periodic table.