CHEM 1151K September 14, 2004 Interesting things about the organization of the Periodic Table (3 ways to know) First Way Notice the stair-step line that starts under Boron. Metals are to the left of the stair-step line Metalloids border the stair-step line Non-metals are to the right of the stair step line Trends Metallic character generally decreases across any given row (period) Metallic character generally increases down any given column Second Way Representative (Main Group) Elements are in the first two and last six columns of the Periodic Table (columns 1, 2 and 13-18) Transition elements are in columns 3 through 12 and rows 4-7. Rare Earth Elements are in the two separated rows placed at the bottom of the Periodic Chart. These two rows have their own names as well: the lanthanides and the actinides. Third Way I want you to remember the specific names of five columns (families or groups). Alkali metals - column 1 Alkali earth metals - column 2 Chalcogens - column16 (oxygen family) Halogens - column 17 (fluorine family) Noble gases (inert gases) - column 18 (helium family) Hydrogen is in a family all its own. Other families will be identified by the element at the top (ex. the Nitrogen family for column 15) Three types of trends to know 1. Atomic Radii 2. Ionization energy 3. Electron Affinity 1. Atomic Radii (here we'll only compare the radii of neutral atoms) Down a given column radius tends to increase (have more electrons going into bigger valence shells, so size gets bigger). This should make intuitive sense. Across a given row (left to right) the radius tends to decrease. This is counter-intuitive unless you look at the situation in more detail. Let's take row 2 as an example. As you go from the elements lithium to carbon to neon, each has an increasing number of electrons. This might lead you to think neon should be the biggest. WRONG. Here the electrons added as you go from one element to the next are all going into the same valence shell (shell 2 here) and that shell actually shrinks as you go from lithium to carbon to neon because of the increasing number of PROTONS. The added protons pull the valence shell closer to the nucleus. So of these three elements LITHIUM is the largest and Neon is the smallest. 2. First Ionization Energy - energy required to remove an electron from the atom Some definitions Ion = charged particle (this happens when #protons #electrons) Cation = ion with a positive charge Anion = ion with a negative charge Monatomic ion = a single atom that has a charge Polyatomic ion = a group of atoms that has a charge Across a row the first ionization energy tends to increase. The noble gases have the highest ionization energies because they are very stable and it's hard to get any electrons away from them. Alkali metals like lithium are happy to have 1 electron removed because when that happens, they end up with the same number of electrons as one of the noble gases. This gives them a very stable electron configuration. (Li+). So for the extremes of row 2, lithium has the lowest first ionization energy and neon the highest and the trend is to generally increase as you go across the row. (There are some minor exceptions as you go across the row, but we are only going to look at the overall general trend.) Also, since the radii get smaller as you go left to right across a given row, the outermost electrons are being pulled more tightly by the nucleus. This makes it harder to get them off. Down a given column the ionization energy tends to decrease. The atoms get larger as you go down a column, so the outermost electrons are held less tightly by the nucleus and easier to get off. 3. Electron Affinities - measure of ease of adding one electron to a neutral atom. The easier it is to add and electron, the higher the electron affinity. This is nearly the opposite of the ionization Across a row the electron affinity increases (up through the halogens, which are the easiest elements to add an electron to). Fluorine likes to pick up 1 electron to become the fluoride ion, F-. When it does, it ends up with the same number of electrons as neon (10), which is a very stable number to have. Down a column the electron affinity decreases (the bigger the atom, the less tendency it has to pick up extra electrons). END OF CHAPTER 2 ________________________________________________________________________ CHAPTER 3 Lewis Electron Dot Symbols for the Elements - These show only the 1 or 2 letter symbol for the element and up to 8 dots for the valence electrons. Dots are places top, bottom, left and right of the letter symbol in pairs at most. The electrons that are not valence electrons are called core or inner electrons. They are not displayed on the Lewis Structures. See the table of Lewis structures of the elements Table 3.1 page 70. Example: Carbon's electron configuration is 1s22s22p2. It has 4 valence electrons in valence shell 2. The two electrons in the 1s orbital are core electrons and are not shown on the Lewis structure. That's why the structure shown on table 3.1 has only 4 dots around the C for carbon. Notice that aside from changing the one or two-letter symbol for the element, that elements in the same column have the same Lewis structure (same number of dots). That's because elements in the same column on the Periodic Chart have the same number of valence electrons. Ions Many elements tend to either lose or gain a few electrons to gain stability. For the main group elements this results in their having the same number of electrons as one of the noble gases. Examples: Cation formation by loss of 1 electron. Na Na1+ sodium atom (neutral) sodium ion 11 protons 11 protons 11 electrons 10 electrons (same # as neon) Anion formation by gain of 1 electron. Cl Cl1- chlorine atom (neutral) chloride ion (note suffix change) 17 protons 17 protons 17 electrons 18 electrons (same # as argon) Nomenclature: Monatomic anions have the suffix -ide . Monatomic cations are just named after the element. Cations and anions attact and tend to form ionic compounds. Cations and anions combine in a ratio to a neutral (uncharged) compound. Sodium cations and chloride anions combine in a 1-to-1 ratio to make sodium chloride (table salt). Na1+ + Cl1- give NaCl Both in the naming and in the chemical formula, the cation is first, then the anion. In the names, leave off the words "ion". In the formulas of ions you must show the charge, but the formulas of final ionic compound no charges are shown. SEE THE EXPLANATION FOR THE "CRISS-CROSS METHOD" IN YOUR LABORATORY EXERCISE ON NAMING IONIC COMPOUNDS. In this method the absolute value of the charge on the cation becomes the subscript in the formula for the anion and vice versa. Subscripts are then reduced to their smallest whole number ratio. Examples: Na1+ (sodium ion) combines with S2- (sulfide ion) in a 2:1 ratio to give Na2S (sodium sulfide) This means the compound forms so that there are two Na1+ ions for every one S2- ion so the plus and minus charges balance out to give a neutral substance.