The Basics Bonding and Molecular Structure Chapter 1

1.2 Atomic Structure Compounds made up of elements combined in different proportions Elements made up of atoms Atoms positively charged nucleus containing protons and neutrons with a surrounding cloud of negatively charged electrons

1.2 Atomic Structure Each element is distinguished by its atomic number, Z Atomic number = number of protons in nucleus Isotopes: Atoms of the same element (same # of protons) may have different masses because they have different numbers of neutrons 12C 13C 14C (6 protons 6 neutrons) (6 protons 7 neutrons) (6 protons 8 neutrons)

1.2 Atomic Structure Electrons that surround the nucleus exist in shells of increasing energy and at increasing distances from the nucleus. Valence Electrons Electrons in the outermost shell an atom uses in making chemical bonds with other atoms to form compounds The number of electrons in the valence shell is equal to the group number of the atom

1.3 Chemical Bonds Ionic (or electrovalent) bonds are formed by the transfer of one or more electrons from one atom to another to create ions Covalent bonds result when atoms share electrons In forming compounds, they gain, lose, or share electrons to give a stable electron configuration characterized by a full shell of valence electrons For the second row elements this is 8 electrons – octet rule

Electronegativity (EN) The measure of the ability of an atom to attract electrons Electronegativity is based on an arbitrary scale, with F the most electronegative (4.0) and Cs the least (0.7)

1.3A Ionic Bonds Li 1s22s1 F 1s22s2p5 Li 1s2 F 1s22s2p6 Atoms may gain or lose electrons and form charged particles called ions An ionic bond is an attractive force between oppositely charged ions Ionic bonds form between atoms with a large EN difference Li 1s22s1 F 1s22s2p5 Li 1s2 F 1s22s2p6

1.3B Covalent Bonds Molecules may also contain ions and covalent bonds:

1.4 Lewis Structures Lewis structures show the connections between atoms in a molecule or ion using only the valence electrons of the atoms involved For main group elements, the number of valence electrons a neutral atom brings to a Lewis structure is the same as its group number If the structure is an anion (negative ion) we add one electron for each negative charge and if the structure is a cation (positive ion), we subtract one electron for each positive charge In drawing Lewis structures we try to give each atom the electron configuration of a noble gas

1.4 Lewis Structures Steps for Drawing Lewis Structures Find the total # of valence electrons for all atoms Adjust the # if any negative or positive charges are indicated Arrange atoms logically – large elements in middle, hydrogens/halogens on the periphery Connect the molecule with single bonds (2 e- each) Assign remaining electrons to the most electronegative elements first as lone pairs Complete octets (if possible) with multiple bonds Assign formal charges, if necessary

Know Your Elements H fills its shell with two electrons or zero (H+) Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Know Your Elements H fills its shell with two electrons or zero (H+) Neutral C, N, O, F always follow octet rule (and usually Cl, Br, I)

Know Your Elements Group IA Metals only form +1 cations H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Know Your Elements Group IA Metals only form +1 cations Group IIA Metals only form +2 cations

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Know Your Elements Group IIIA may follow the octet rule or prefer a sextet of e- The higher halogens usually follow the octet rule, but may expand their octet if they are the central element in the molecule (F always follows the octet rule)

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Know Your Elements The main group non-metals may follow the octet rule but very often expand their octet The transition metals can form ions, follow the octet rule or expand their octets

1.5 Formal Charge Formal charge is the charge assigned to individual atoms in a Lewis structure. Formal charge is calculated as follows: The number of electrons “owned” by an atom is determined by its number of bonds and lone pairs. An atom “owns” all of its unshared electrons and half of its shared electrons.

Know Your Lewis Structures Once you practice enough Lewis structures you begin to see that the elements we cover in this course follow a similar pattern

Know Your Lewis Structures Once you practice enough Lewis structures you begin to see that the elements we cover in this course follow a similar pattern

Know Your Lewis Structures Once you practice enough Lewis structures you begin to see that the elements we cover in this course follow a similar pattern

5A. A Summary of Formal Charges © 2014 by John Wiley & Sons, Inc. All rights reserved.