Chemical Bonds and Compounds www.physicalgeography.net

Chemical Bonds A bond is a force that holds groups of two or more atoms together and makes them function as a unit. Bond energy is the amount of energy required to break a particular bond. *determines the strength of a bond

The strongest bonds that are present in biochemicals are covalent bonds, such as the bonds that hold the atoms together within the individual bases . A covalent bond is formed by the sharing of a pair of electrons between adjacent atoms.

A typical carbon-carbon (C-C) covalent bond has a bond length of 1 A typical carbon-carbon (C-C) covalent bond has a bond length of 1.54 Å and bond energy of 85 kcal mol-1 (356 kJ mol-1). Because this energy is relatively high, considerable energy must be expended to break covalent bonds.

More than one electron pair can be shared between two atoms to form a multiple covalent bond. For example, three of the bases include carbon-oxygen (C=O) double bonds. These bonds are even stronger than C-C single bonds, with energies near 175 kcal mol-1 (732 kJ mol-1).

Table 2-3, p.40

Elements combine to form compounds Compounds have different properties from the elements that make them Atoms of different elements are held together by chemical bonds Bonds help to determine the properties of a compound

Properties of Compounds Depend on atoms in the compound Depend on how the atoms are arranged in the compound Example: C and H combine to form natural gas, auto gas, waxes in candle, plastics…etc. www.alibaba.com

Properties of Compounds are different than the elements that make them H2O (water) H and O are colorless gases at room temperature Water is a liquid at room temperature NaCl (salt) Na is a metallic solid Cl is a greenish-yellow gas that is poisonous Table salt (NaCl) is used to flavor and preserve foods www.sciencelearn.org.nz waterworks4u.com

Atoms combine in predictable numbers A given compound always contains atoms of elements in a specific ratio Ammonia NH3 always has a 1:3 ratio of Nitrogen to Hydrogen www.uh.edu

Types of Chemical Bonds: Ionic Bonding Ionic bonding occurs when there is an attraction between oppositely charges ions. Electrons are transferred to form oppositely charged ions. Ionic compounds result when an atom that loses electrons relatively easily (a metal) reacts with an atom that has a high affinity for electrons (a non-metal). Example: Sodium chloride, NaCl Na+ & Cl- ions react to form solid sodium chloride.

Ionic Bonds One or more electrons from 1 atom are removed and attached to another atom, resulting in +ve (cation) and –ve (anion) ions which attract each other

Ionic Bond Between atoms of metals and nonmetals with very different electronegativity Bond formed by transfer of electrons Produce charged ions all states. Conductors and have high melting point. This results in the metal becoming a cation, and the non-metal becoming an anion. Examples; NaCl, CaCl2, K2O

1). Ionic bond – electron from Na is transferred to Cl, this causes a charge imbalance in each atom. The Na becomes (Na+) and the Cl becomes (Cl-), charged particles or ions.

Ionic compounds Ionic compounds are very stable and their crystals are very strong. The shape of the crystals formed depends on the ratio of positive to negative ions and the sizes of the ions

Covalent Bonds

Covalent Bond Between nonmetallic elements of similar electronegativity. Formed by sharing electron pairs Stable non-ionizing particles, they are not conductors at any state Examples; O2, CO2, C2H6(Ethane) , H2O, SiC

Covalent Bond The number of covalent bonds that an atom can form depends on the number of electrons that it has available for sharing. Atoms of (O,S…)can form two covalent bonds. Atoms of (N,P…) can form three bonds Atoms of (C, Si…)can form four bonds

For some molecules, more than one pattern of covalent bonding can be written. For example, benzene can be written in two equivalent ways called resonance structures. Benzene's true structure is a composite of its two resonance structures.

A molecule that can be written as several resonance structures of approximately equal energies has greater stability than does a molecule without multiple resonance structures. Thus, because of its resonance structures, benzene is unusually stable.

Chemical reactions entail the breaking and forming of covalent bonds Chemical reactions entail the breaking and forming of covalent bonds. The flow of electrons in the course of a reaction can be depicted by curved arrows, a method of representation called “arrow pushing”. Each arrow represents an electron pair.

This makes covalently bonded materials, such as diamond and many metals, quite conductive. In contrast to covalent bonds, ionic bonds, such as the type of bonds that hold together sodium chloride (salt), electrons keep to their respective atoms, and the overall molecular structure is weaker as a result.

Lewis Structures of Simple Molecules A Lewis structure is a combination of Lewis symbols that represents the formation of covalent bonds between atoms. Lewis structure shows the bonded atoms with the electron configuration of a noble gas; that is, the atoms obey the octet rule. The bonding in carbon dioxide (CO2): all atoms are surrounded by 8 electrons, fulfilling the octet rule.

Lewis Structures The shared pairs of electrons in a molecule are called bonding pairs. In common practice, the bonding pair is represented by a dash (—). The other electron pairs, which are not shared, are called nonbonding pairs, or lone pairs. Each chlorine atom sees an octet of electrons.

Multiple Covalent Bonds The covalent bond in which one pair of electrons is shared is called a single bond. Multiple bonds can also form: In a double bond two pairs of electrons are shared. In a triple bond three pairs of electrons are shared. Note that each atom obeys the octet rule, even with multiple bonds.

Molecular Geometry Molecular geometry is simply the shape of a molecule. Molecular geometry is described by the geometric figure formed when the atomic nuclei are joined by (imaginary) straight lines. Molecular geometry is found using the Lewis structure, but the Lewis structure itself does NOT necessarily represent the molecule’s shape. A carbon dioxide molecule is linear. A water molecule is angular or bent.

Two Attachments Linear

Chemical Bonds Give all Materials their Structure Covalent Compounds (sharing valence e-) Exist as individual molecules Chemical bonds give each molecule a specific three-dimensional shape Molecular shape can affect properties of the compounds Melt and boil at lower temperatures (takes less energy to break up because atoms are organized as individual molecules)

Covalent Ionic Compounds Compounds Gases, liquids, or solids Low melting and boiling points Poor electrical conductors Many soluble in nonpolar liquids but not in water Crystalline solids High melting and boiling points Conduct electricity when melted Many soluble in water but not in nonpolar liquid

Some Typical Ions with Positive Charges (Cations) Group 1 Group 2 Group 13 H+ Mg2+ Al3+ Li+ Ca2+ Na+ Ba2+ K+

Bonds in all the polyatomic ions and diatomics are all covalent bonds

when electrons are shared equally NONPOLAR COVALENT BONDS when electrons are shared equally H2 or Cl2

2. Covalent bonds- Two atoms share one or more pairs of outer-shell electrons. Oxygen Atom Oxygen Atom Oxygen Molecule (O2)

Reversible Interactions of Biomolecules Are Mediated by Three Kinds of Noncovalent Bonds Readily reversible, noncovalent molecular interactions are key steps in the dance of life. Such weak, noncovalent forces play essential roles in the faithful replication of DNA, the folding of proteins into intricate three-dimensional forms, the specific recognition of substrates by enzymes, and the detection of molecular signals. hormones with their receptors, antibodies with their antigens.

The three fundamental noncovalent bonds are electrostatic interactions, hydrogen bonds, and van der Waals interactions. They differ in geometry, strength, and specificity. Furthermore, these bonds are greatly affected in different ways by the presence of water. Let us consider the characteristics of each:

1.Electrostatic interactions An electrostatic interaction depends on the electric charges on atoms. Thus, the electrostatic interaction between two atoms bearing single opposite charges separated by 3 Å in water (which has a dielectric constant of 80) has an energy of 1.4 kcal mol-1 (5.9 kJ mol-1).

2.Hydrogen bonds Relatively weak interactions, which nonetheless are crucial for biological macromolecules such as DNA and proteins. These interactions are also responsible for many of the properties of water that make it such a special solvent. The hydrogen atom in a hydrogen bond is partly shared between two relatively electronegative atoms such as nitrogen or oxygen.

The hydrogen-bond donor is the group that includes both the atom to which the hydrogen is more tightly linked than the hydrogen atom itself, whereas the hydrogen-bond acceptor is the atom less tightly linked to the hydrogen atom. Hydrogen bonds are fundamentally electrostatic interactions.

The relatively electronegative atom to which the hydrogen atom is covalently bonded pulls electron density away from the hydrogen atom so that it develops a partial positive charge (δ+). Thus, it can interact with an atom having a partial negative charge (δ-) through an electrostatic interaction.

. The strongest hydrogen bonds have a tendency to be approximately straight, such that the hydrogen-bond donor, the hydrogen atom, and the hydrogen-bond acceptor lie along a straight line.

Hydrogen Bonds that Include Nitrogen and Oxygen Atoms The positions of the partial charges (δ+ and δ-) are shown.

3. van der Waals interactions The basis of a van der Waals interaction is that the distribution of electronic charge around an atom changes with time. At any instant, the charge distribution is not perfectly symmetric. This transient asymmetry in the electronic charge around an atom acts through electrostatic interactions to induce a complementary asymmetry in the electron distribution around its neighboring atoms.

The resulting attraction between two atoms increases as they come closer to each other, until they are separated by the van der Waals contact distance . At a shorter distance, very strong repulsive forces become dominant because the outer electron clouds overlap.

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