2Covalent BondingA molecule is a neutral group of atoms held together by covalent bonds.A chemical compound whose simplest units are molecules is called a molecular compound.Water is a molecular compound.molecule
3Molecular CompoundsThe models for water, oxygen, and sucrose, represent a few examples of the many molecular compounds in and around us. Atoms within molecules may form one or more covalent bonds.
4A chemical formula indicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts.H2OWater:2 hydrogen atoms1 oxygen atomC12H22O11Sucrose:12 carbon atoms22 hydrogen atoms11 oxygen atoms
5A molecular formula shows the types and numbers of atoms combined in a single molecule of a molecular compound.H2OBoth chemical formula andmolecular formula.
6A diatomic molecule is a molecule containing only two atoms.
7Formation of a Covalent Bond As atoms near each other the charged particles begin to interact.The nuclei and electrons are attracted to one another, but at the same time the two nuclei repel each other and the electrons repel each other.The attraction forces are stronger than the repulsion forces.
8Formation of a Covalent Bond Rememberthe attractionforces arestronger thanthe repulsionforces.
9Why does this bondform?As independent particles, most atoms are at relatively high potential energy.Nature, however, favors arrangements in which potential energy is minimized.This means that most atoms are less stable existing by themselves than when they are combined.By bonding with each other, atoms decrease in potential energy, thereby creating more stable arrangements of matter.
10Potential energy changes during the formation of a hydrogen-hydrogen bond. The separated hydrogen atoms do not affect each other.Potential energy decreases as the atoms are drawn together by attractive forces.Potential energy is at a minimum when attractive forces are balanced by repulsive forces.(d) Potential energy increases when repulsion between like charges outweighs attraction between opposite charges.
11Characteristics of the Covalent Bond The distance between two bonded atoms at their minimum potential energy, that is, the average distance between two bonded atoms, is the bond length.
12Bond energy is the energy required to break a chemical bond and form neutral isolated atoms. All atoms release energy as they change from isolated individual atoms to parts of a molecule.The same amount of energy must be added to separate the bonded atoms.
13The Octet RuleThe noble gases are stable because their atoms’ outer s and p orbitals are completely filled by 8 electrons.ArOther main group elements can fill their outermost s and p orbitals with electrons by sharing electrons through covalent bonding.
14Octet RuleOctet Rule – chemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level.
15Exceptions to the Octet Rule Hydrogen forms bonds in which it is surrounded by only two electrons.Boron has just three valence electrons, so it tends to form bonds in which it is surrounded by six electrons.Main group elements in Periods 3 and up can form bonds with expanded valence, involving more than eight electrons.
16Electron-Dot Notation Electron-dot notation is an electron-configuration notation in which only the valence electrons of an atom or a particular element are shown, indicated by dots placed around the element’s symbol.HHydrogen has 1 valence electron so one dot is placed around the symbol.
17C 4 F 7 How many valence electrons does carbon have? Notice one dot is placed on each side of the symbol.How many valence electrons does fluorine have?F7Notice one dot is placed on each side of the symbol before placing a second.
19Lewis StructuresElectron-dot notation can also be used to represent molecules.A hydrogen molecule, H2, is represented by combining the notations of two individual hydrogen atoms.Remember that hydrogen is “happy” with two electrons instead of eight.HHRemember eachhydrogen has onevalence electron thatit brings to the bond.Shared electron pair(called a bonding pair).
20F F Will two fluorine atoms form a bond? Each fluorine atom has 7 valence electrons so each fluorine atom wants to gain one electron to achieve an octet.FFNeither atom will give up an electron – remember fluorine ishighly electronegative.What’s the solution - what can they do to achieve an octet?
21The octet is achieved by each atom sharing the electron pair in the middle. A lone pair is a pair ofelectrons that is not involved in bonding and that belong exclusively to one atom.FFoctetoctetThis is the shared pair called thebonding pair.It is a single bonding pair and is calleda single bond.
22Often the shared pair of electrons in a covalent bond is replaced by a long dash. -Shared pairReplace with long dash.
23These representations are all Lewis Structures, formulas in which atomic symbols represent nuclei and inner-shell electrons, dot-pairs or dashes between two atomic symbols represent electron pairs in covalent bonds, and dots adjacent to only one atomic symbol represent unshared electrons.Unshared electrons.FFAtomic symbol – represents nuclei and inner-shell electrons.Electron pair incovalent bond.
24A structural formula indicates the kind, number, arrangement, and bonds but not the unshared pairs of the atoms in a molecule.FFStructural formulaFF-
25Multiple Covalent Bonds A double bond is a covalent bond produced by the sharing of two pairs of electrons between two atoms.A triple bond is a covalent bond produced by the sharing of three pairs of electrons between two atoms.
26O How will two oxygen atoms bond? Each atom has two unpaired electrons.Each atom wants to gain two electrons toachieve an octet.
27OO7 electrons7 electronsIf we share one pair of electrons will each oxygen atom have an octet?
28O O What if we share two pairs of electrons? 6 valence electronsplus 2 shared electrons = full octet6 valence electronsplus 2 shared electrons = full octetWhat if we share two pairs of electrons?Both electron pairs are shared.