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02 periodic_Lewis dot

02 periodic_Lewis dot - Electronic structures tell use a...

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Electronic structures tell use a lot about the chemistry of atoms and the shapes of molecules. We will see this when we get to molecules and reactions. Electronic structures also tell us why some elements have similar physical and chemical properties. C. THE PERIODIC NATURE OF MATTER The periodic table is a listing of all of the known elements in order of increasing mass. The elements are arranged so elements with similar chemical properties are in columns. Columns of elements are called families. The families were determined by early chemist doing chemical reaction with the elements and literally putting elements with similar chemical properties in columns together. For example, we’ve seen that lithium (Li) and oxygen (O) react to make a new chemical. We haven’t had enough chemistry yet to know that the compound they make contains 2 lithiums and 1 oxygen, but we will learn how to predict this formula soon. This compound is called lithium oxide and is written Li 2 O. The subscript 2 just tells us that there are 2 lithiums. Early chemists also noted that sulfur (S) reacts with lithium in the same way. That is, S and Li react to make Li 2 S. So sulfur and oxygen were put in the same column on the periodic table. Sodium (Na) reacts with O and S the same way as Li. That is it will form Na 2 O and Na 2 S. So Na is in the same column as Li. Magnesium (Mg) and calcium (Ca) also react with O and S, but they form compounds with 1 Mg for each O (MgO) or S (MgS) and 1 Ca for each O (CaO) or S (CaS). So Mg and Ca are in a column together, but it is a different column than the column the Li and Na are in. Families of elements have very similar electronic structures. That is why they have similar chemical properties! The periodic table was started before we knew enough to know about electronic structure, but it was noted that there was a pattern (in the atomic weights) of elements with similar properties. Our understanding of electronic structure developed with the development of the periodic table. The name, “Periodic Table,” refers to the repeating period of atomic number and chemical properties. The period isn’t so simple. Let’s look at the period for elements to repeat hydrogen’s chemical properties. The first period is just 2. (H’s atomic # = 1, Li’s atomic # = 3) The second and third periods are 8. (Li’s atomic # = 3, Na’s atomic # =11 and K’s atomic # = 19) The fourth and fifth periods are 18. The sixth (and last period) is 32. But you don’t need to remember this, just remember how to use the But you don’t need to remember this, just remember how to use the periodic table. periodic table.
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Knowing electronic structures we can explain the periodic nature of the elements. The chemical properties of elements with similar arrangements of electrons in the outer most shell are similar. Thus the chemistry of H, Li, Na, K, Rb, Cs, and Fr is similar (almost identical) because they all have 1 electron in an s orbital for their outermost electron. (The outermost shell is called the valence shell and electrons in it are called valence electrons).
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