hemistry 105 Atoms, Ions and an Introduction to Lewis Dot Structures Valence Electrons Valence electrons are located in the valence or outer energy shell these are sometimes referred to as the outer electrons valence electrons are largely responsible for chemical bonding and chemical properties valence electrons can help us predict the formation of ions and ionic compounds valence electrons can help us predict the formation of covalent bonds and the shapes of molecules The number of valence electrons can be determined by counting the number of electrons in the valence or outer electron shell There are a number of ways to do this, but the simplest involves using the periodic table The number of valence electrons for a main group element is simply equal to the group number For example, group 1A is the first column in the periodic table and is composed of hydrogen and the alkali metals All elements in this group have 1 valence electron Elements in group 2A (Be through Ra) have 2 Elements in group 3A (B through Tl) have 3, etc The noble gases, group 8A, all have 8 (with the exception of helium, which has only two electrons total) Electron Dot (Lewis) Structures A Lewis or Electron Dot Structure is a convenient representation of the valence electrons in an atom An electron dot structure for an atom is simply the symbol for the element, surrounded by a number of dots equal to the number of valence electrons Avoid a common mistake: the dots represent valence electrons only, so make sure you use only the number of dots corresponding to the number of valence electrons Examples: Na a Al Draw lewis dot structures for an atom of each of following elements: P Kr 1 K 2 Si 3 Ar 4 As Ions Main group atoms strive to achieve the same configuration as a noble gas It turns out that this type of electron configuration is very stable Atoms may achieve such a configuration by losing or gaining electrons, or by sharing electrons with other atoms Losing or gaining electrons results in the formation of ions (charged species)
hemistry 105 Laboratory Meeting 2, page 2 For example, the alkali metals (group 1A) have 1 valence electron To achieve a noble gas configuration, they lose one electron For example consider potassium (K) Potassium has a total electron count of 19 It is in group 1A so it has 1 valence electron To achieve the same configuration as a noble gas, it loses that electron and thus becomes positively charged (K + ) loses an which has the K electron K + same electron Ar to become configuration as Atoms in groups 1A, 2A and 3A tend to lose their outer electrons and form cations For example, a magnesium atom, group 2A, loses its 2 valence electrons and forms Mg 2+ loses TW which has the Mg electrons Mg 2+ same electron Ne to become configuration as Atoms in groups 5A, 6A and 7A tend to gain electrons and form anions onsider chlorine (l) - it already has 7 valence electrons So, rather than losing all seven, it gains one electron to form l l has the same electron configuration as argon gains one which has the l electron l same electron Ar to become configuration as Similarly, a group 6A atom like oxygen, has six electrons It gains two electrons to reach a eight (an octet), and forms a 2 ion Phosphorus, a group 5A atom, has five electrons It gains three electrons to reach an octet, and forms a P 3 ion Identify the ion likely to be formed by the following elements: 1 a 2 N 3 Rb 4 Ba 5 l 6 Al 7 s 8 Se ovalent Bonds An atom can also achieve a noble gas configuration by sharing electrons with other atoms When two nonmetals combine, they typically share electrons in covalent bonds and form what are know as covalent compounds We can draw Lewis Electron Dot Diagrams for covalent molecules to determine what type of bonds the molecules form For example, consider two hydrogen atoms Each one has one valence electron owever, both hydrogens would like to have two electrons, to obtain the same configuration as [e] To do this, each atom shares an electron with the other, forming a covalent bond In 2, each atom has 2 electrons
hemistry 105 Laboratory Meeting 2, page 3 + 2 each has 1 e and wants 1 more s share e s forming a covalent bond shared pairs can be represented as lines between atoms a hydrogen molecule (diatomic = 2 atoms) ere s another example, this time for chlorine A l atom has 7 valence electrons, so it needs 1 more to achieve an octet (8 electrons) each l has 7 e and wants 1 more l + l l l l l a chlorine molecule each l shares 1 e s forming a covalent bond consisting of 2 electrons the shared electron pair is represented as a line Each chlorine now has 8 electrons, because you can count the shared ones: l 2 this l now has an octet of 8 electrons l l Sometimes you need to share more than one pair of electrons to achieve an octet: this l atom also has an octet + 2 each has 6 e and wants 2 more each shares 2e s forming a double covalent bond an oxygen molecule the four shared electrons are represented as two lines between the ato We can check to see that each oxygen has the appropriate number of electrons: each oxygen atom now has an octet total # valence electrons = 12 (note the 12 dots)
hemistry 105 Laboratory Meeting 2, page 4 For the following problems, calculate the total number of valence electrons in each of the following molecules Then, draw lewis dot structures for each: Br 2 l Br N 2 N This one is tricky, there is no way to achieve an octet So, get each atom as close to an octet as possible without going over Si 4 Another tricky one Take Si to be the central atom and arrange the atoms around it Try to get an octet around silicon and two electrons around each of the hydrogens
hemistry 105 Laboratory Meeting 2, page 5 Steps for Drawing Lewis Dot Structures for Larger Molecules 1 First, determine the central atom: (a) ydrogens () and halogens (F,l,Br,I) are almost always outer atoms They only want to form one bond to get to a noble gas configuration (b) If the choice is still ambiguous, the atom further to the right on the periodic table is generally an outer atom, the one further to the left is often the central atom For example: 2 The s are outer atoms, leaving and as candidates is further to the left than, so is the central atom 2 Arrange the outer atoms around the central atom: carbon is the central atom 3 ount up the valence electrons: (4) + (6) + 2 (2x1) = 12 electrons 4 Draw a bond between each outer atom and the central atom ount the electrons you have used in these bonds and subtract them from your total valence electrons 6 electrons are use in making these bonds so 12 6 = 6 electrons remain 5 Use the remaining electrons to fill octets around the outer atoms Put any remaining ones on the central atom the s can t accept any more electrons so the remaining six are place around the oxygen 6 If all the atoms don t have an octet, move a non-bonding electron pair from 1 atom into a sharing position move one of the pairs on oxygen in to a sharing position with carbon the s have 2 electrons has an octet, but does not or both and have octets and has 2 electrons
hemistry 105 Laboratory Meeting 2, page 6 Try drawing lewis dot structures for these molecules: l 4 Pl 3 Nl 3 S 2 2 l 2
hemistry 105 Laboratory Meeting 2, page 7 3 S 2 Si 3 l 2 6 (arrangement of atoms is shown below) 2 4 (arrangement of atoms is shown below)