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Designing Compounds Start a new thread/topic Learning Target: How do I design a stable compound? Update TOC

What we’ve learned… Electrons move back and forth between distinct energy levels and never exist between energy levels. The difference in energy between energy levels is unique for every element on the periodic table (spectral lines).

Designing Compounds Recall that Ionization energy is the energy required to remove an electron from a neutral atom of an element. We are going to graph the ionization energies of elements on the periodic table to identify trends. The trends will help us to design stable compounds

Designing Compounds p. 182 Look at Fig p Design a layout for your plot of ionization energy (y-axis) vs. atomic number (x-axis). Make sure your plot covers most of your graph paper. Leave a few lines at the bottom for comments. Use colored pencils to help identify trends.

Designing Compounds You are only plotting the first 20 elements. What is an appropriate scale for the x-axis?? Look at Figure 4.12 p What is the largest ionization energy? What is an appropriate scale for the y-axis?? Make sure the intervals on each axis are equal!!! Label each axis and make sure your plot has a Title! Connect each point with a straight line. Label ALL high points and low points with their element symbol.

Answer #4a-d by labeling the graph.

Designing Compounds TrendElements InvolvedExplanation Ionization energy is lowest on the LHS of the periodic table Alkali MetalsAlkali metals have a weak attraction for electrons. They are very reactive!reactive! Ionization energy is highest on RHS of periodic table Noble GasesNoble gases have a very strong attraction for electrons. I.E. increases (generally) across a period Across a periodNumber of protons increases, which increases attraction for electrons I.E. decreases (generally) down a group/family Down a group/familyElectrons are further from the nucleus thus the attraction is less

Designing Compounds Which family of elements has the lowest ability to attract electrons? Which family of elements has the lowest ionization energy? Which family of elements has the highest ionization energy? Alkali Metals Noble Gases

Designing Compounds Where might the I.E. of Xe be located on our graph from step 3? Where might the I.E. of Cs be located on our graph from step 3? Since Xe is a noble gas, and it is below Kr on the periodic table, its I.E. would be less than that of Kr. Since Cs is an alkali metal, and it is below Rb on the periodic table, its I.E. would be less than that of Rb.

Designing Compounds 6b. Why are there only a few known compounds involving noble gases, yet thousands of compounds involving other elements? Noble gases have high Ionization Energies. For bonding to occur, atoms must share or exchange electrons. Since it is difficult to remove electrons from noble gases, the likelihood of them forming compounds is low.

Designing Compounds In chapter 3, we learned how to draw Bohr models of atoms. There is a simpler way to represent the part of the atom that is involved in bonding – the valence electrons. It is called an Electron Dot Diagram

Electron Dot Diagram Start with the atomic symbol in the center. Draw a dot for each valence electron. Place one dot on each side, then pair up dots until you have placed all the valence electrons. Hydrogen: 1P H Electron dot diagram Bohr Model

Electron Dot Diagram Another example: Boron: B 5P+ 6N B Electron dot diagram Bohr Model Boron has 5 protons 6 neutrons 5 electrons

Designing Compounds Open your notebook to a clean page. Draw a table and fill it out for elements #1-20 (make it BIG): For each element draw a Bohr model, and an Electron Dot Diagram in the box. H Li He

Designing Compounds The way electrons are configured around the nuclei of noble gases is very stable, which is why noble gases have such high ionization energies. When electron configurations in other elements are the same as noble gas electron configurations, those elements are also stable, but not to the same extent. How many electrons must a neutral sodium atom lose in order to have an electron configuration like neon? Answer for Magnesium. Na must lose 1 electronMg must lose 2 electrons

Designing Compounds 6d. How many electrons must a neutral chlorine atom gain in order to have an electron configuration like argon? Answer for oxygen. Cl needs to gain 1 electron O needs to gain 2 electrons

Designing Compounds 6e. Why might some atoms tend to form positive ions and other atoms tend to form negative ions? Atoms can become a stable ion either by gaining electrons (negative ion) or losing electrons (positive ion). Their position on the periodic table tells whether they will gain or lose electrons – depending on which is easier.

Designing Compounds 6f. How are the most common ions formed for elements 1-20 related to their position on the periodic table? If an element is in Group 1, 2, or 3, it is easier for it to lose electrons to become a stable positive ion. If an element is in Group 5, 6, or 7, it is easier to gain electrons to become a stable negative ion. An element in group 4 can either gain or lose electrons to become a stable ion. This are simplified rules, you will learn more detailed rules in chemistry!

Designing Compounds Noble Gases have either 2 or 8 valence electrons (a full valence shell). Stable ions have either 2 or 8 valence electrons (a full valence shell) just like Noble Gases.

Designing Compounds Stable compounds are electrically neutral. We can use this fact to predict which compounds will be stable… NaCl is a stable compound. (how do you know this from everyday experience??)

Designing Compounds Stable compounds are electrically neutral. Do you think MgCl would make a stable compound?

Designing Compounds 9. What is a rule for designing stable compounds? 1.Determine the family of each element 2.Predict the charge for a stable ion of each element 3.Combine those charges to make a neutral compound. (need the same number of positive and negative charges) 4.Sometimes you will need more than 1 ion of a particular element.

Designing Compounds 9. Which of the following formulas is correct? NaCl 2 BaF KO Li 2 F Mg 2 S Al 2 O 3

Designing Compounds How does the organization of the periodic table help you quickly predict the correct formulas of compounds? Each family of elements on the periodic table is associated with losing or gaining a certain number of electrons to obtain a noble gas configuration. If you number each column with the most common ion formed, then you can use those numbers to determine the correct formula for many binary compounds.