Chapter 1

The BasicsBonding and Molecular Structure

1.2 Atomic StructureCompounds

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 StructureEach 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 StructureElectrons 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 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

F 1s22s2p5

Li 1s22s1

F 1s22s2p6

Li 1s2

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

NH H

H

H+

(ammonia)(3 bonds on N)

NH H

H

H

(ammonium cation)(4 bonds on N with

a positive charge on N)

1.4 Lewis StructuresLewis 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 StructuresSteps for Drawing Lewis Structures1. Find the total # of valence electrons for all atoms2. Adjust the # if any negative or positive charges are

indicated3. Arrange atoms logically – large elements in middle,

hydrogens/halogens on the periphery4. Connect the molecule with single bonds (2 e- each)5. Assign remaining electrons to the most electronegative

elements first as lone pairs6. Complete octets (if possible) with multiple bonds7. Assign formal charges, if necessary

Know Your ElementsH fills its shell with two electrons or zero (H+)

Neutral C, N, O, F always follow octet rule (and usually Cl, Br, I)

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 ElementsGroup 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 ElementsGroup 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 ElementsThe 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

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

1.5 Formal ChargeFormal 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 StructuresOnce you practice enough Lewis structures you begin to

see that the elements we cover in this course follow a similar pattern

Know Your Lewis StructuresOnce you practice enough Lewis structures you begin to

see that the elements we cover in this course follow a similar pattern

Know Your Lewis StructuresOnce 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

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