Covalent Bonding And Molecular Compounds

Covalent Bonding A 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

Molecular Compounds The 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.

A chemical formula indicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts. H2O Water: 2 hydrogen atoms 1 oxygen atom C12H22O11 Sucrose: 12 carbon atoms 22 hydrogen atoms 11 oxygen atoms

A molecular formula shows the types and numbers of atoms combined in a single molecule of a molecular compound. H2O Both chemical formula and molecular formula.

A diatomic molecule is a molecule containing only two atoms.

Formation 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.

Formation of a Covalent Bond Remember the attraction forces are stronger than the repulsion forces.

Why does this bond form? 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.

Potential 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.

Characteristics 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.

Bond 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.

The Octet Rule The noble gases are stable because their atoms’ outer s and p orbitals are completely filled by 8 electrons. Ar Other main group elements can fill their outermost s and p orbitals with electrons by sharing electrons through covalent bonding.

Octet Rule Octet 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.

Exceptions 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.

Electron-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. H Hydrogen has 1 valence electron so one dot is placed around the symbol.

C 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? F 7 Notice one dot is placed on each side of the symbol before placing a second.

Electron-Dot Notation for Groups 1, 2, 13-18

Lewis Structures Electron-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. H H Remember each hydrogen has one valence electron that it brings to the bond. Shared electron pair (called a bonding pair).

F 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. F F Neither atom will give up an electron – remember fluorine is highly electronegative. What’s the solution - what can they do to achieve an octet?

The octet is achieved by each atom sharing the electron pair in the middle. A lone pair is a pair of electrons that is not involved in bonding and that belong exclusively to one atom. F F octet octet This is the shared pair called the bonding pair. It is a single bonding pair and is called a single bond.

Often the shared pair of electrons in a covalent bond is replaced by a long dash. - Shared pair Replace with long dash.

These 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. F F Atomic symbol – represents nuclei and inner-shell electrons. Electron pair in covalent bond.

A structural formula indicates the kind, number, arrangement, and bonds but not the unshared pairs of the atoms in a molecule. F F Structural formula F F -

Multiple 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.

O How will two oxygen atoms bond? Each atom has two unpaired electrons. Each atom wants to gain two electrons to achieve an octet.

O O 7 electrons 7 electrons If we share one pair of electrons will each oxygen atom have an octet?

O O What if we share two pairs of electrons? 6 valence electrons plus 2 shared electrons = full octet 6 valence electrons plus 2 shared electrons = full octet What if we share two pairs of electrons? Both electron pairs are shared.

O O two bonding pairs making a double bond