II. CHEMICAL BONDS Interactions that occur between atoms are called chemical bonds. In their chemical interactions the atoms of different elements tend to achieve a stable rare gas configuration 1s2 or ns2np6. 1. Strong chemical bonds: a) ionic bond (between metals and nonmetals); b) covalent bond (between nonmetals); c) metallic bond (between metallic atoms). 2. Weak chemical bonds: a) van der Waals forces (dipole-dipole attraction) b) hydrogen bonding

II. CHEMICAL BONDS 1. Strong chemical bonds a) Ionic bond = a type of chemical bond based on the electrostatic attraction forces between ions having opposite charges. Ionic bond forms between electropositive and electronegative elements, e.g. between metal and non-metal ions.

II. CHEMICAL BONDS The metal, with a few electrons on the last shell, donates one or more electrons to get a stable electron configuration and forms positively charged ions (cations). These electrons are accepted by the non-metal to form a negatively charged ion (anion) also with a stable electron configuration. The electrostatic attraction forces between the anions and cations cause them to come together and form a bond.

II. CHEMICAL BONDS Example: the formation of ionic bond between Na and Cl Ionic bond formation in NaCl Na: 1s22s22p63s1 Na loses one e- from its outer shell Na+: 1s22s22p6 Cl: 1s22s22p63s23p5 Cl gains one e- Cl-: 1s22s22p63s23p6

II. CHEMICAL BONDS When sodium and chlorine react, the outer electron of the sodium atom is transferred to the chlorine atom to produce sodium ion Na+ and chloride ion Cl- , which are held together by the electrostatic force of their opposite charges. NaCl is an ionic compound.

II. CHEMICAL BONDS NaCl formation may be illustrated showing the outer electrons only (Lewis symbol): In a similar way, a calcium atom may lose two electrons which are accepted by two chlorine atoms forming a calcium ion Ca2+ and two chloride ions Cl-, that is calcium chloride CaCl2 :

II. CHEMICAL BONDS In sodium chloride, the ionic bonds are not only between a pair of sodium ion Na+ an chloride ion Cl-, but also between all the ions. These electrostatic interactions have as a result the formation of NaCl crystal. We write the formula of sodium chloride as NaCl, but this is the empirical formula. The sodium chloride crystal contains huge and equal numbers of Na+ and Cl- ions pocket together in a way that maximizes the electrostatic forces of the oppositely charged ions.

Sodium chloride crystal

Lithium bromide crystal

b) Covalent bonds = is a type of chemical bond formed by sharing pairs of electrons between atoms. When two electronegative atoms react together, ionic bonds are not formed because both atoms have a tendency to gain electrons. In such cases, a stable electronic configuration may be obtained only by sharing electrons. First, consider how chlorine atoms Cl react to form chlorine molecules Cl2 :

II. CHEMICAL BONDS Each chlorine atom shares one of its electrons with the other atom. The electron is shared equally between both atoms, and each atom in the molecule has in its outer shell 8 electrons – a stable electronic configuration corresponding to that of Ar. The sharing of a single pair of electrons results in a single covalent bond, often represented by a dash sign, so chlorine molecule may be written as follow: Cl — Cl

II. CHEMICAL BONDS If two pairs of electrons are shared we have a double covalent bond Ex: the oxygen molecule O2, each oxygen atom shares two electrons O ═ O If three pairs of electrons are shared we have a triple covalent bond Ex: the nitrogen molecule N2, each nitrogen atom shares three electrons. N ≡ N

II. CHEMICAL BONDS In a similar way a molecule of carbon tetrachloride CCl4 is made up of carbon and four chloride atoms. The carbon atom shares all its four electrons and the chlorine atoms share one electron each. The carbon atom forms 4 covalent bonds with 4 chlorine atoms. In this way, both the carbon and all four chlorine atoms attain a stable electronic structure.

Coordinate bond The molecule of ammonia NH3 is made up of one nitrogen and three hydrogen atoms: The nitrogen atom forms three bonds and the hydrogen atoms one bond each. In this case, one pair of electrons is not involved in bond formation and this is called a lone pair of electrons.

It is possible to have a shared electron pair in which the pair of electrons comes just from one atom and not from both. Such bond is called coordinate covalent bond. Even though the ammonia molecule has a stable configuration, it can react with hydrogen H+ by donating the lone pair of electrons, forming the ammonium ion NH4+:

Partial ionic character of covalent bonds In the chlorine molecule Cl – Cl the pair of electrons of the covalent bond is equally shared between both chlorine atom. Because there is not a charge separation between the chlorine atoms, Cl2 molecule is nonpolar.

The polar molecule of hydrochloric acid On the contrary, in HCl molecule, there is a shift of electrons toward the chlorine atom which is more electronegative than hydrogen. Such molecule, in which a charge separation exists is called a polar molecule or dipole molecule. + e- H Cl d + - The polar molecule of hydrochloric acid

The separation between the positive and negative charges is given by the dipole moment μ. The dipole moment is the product between the magnitude of the charges (δ) and the distance separating them (d): μ = δ · d The symbol δ suggests small magnitude of charge, less than the charge of an electron (1.602 · 10-19 C). The unit for the dipole moment is Debye (D): 1D = 3.34 · 10-30 C m

Dipole moment values for some molecules: Carbon dioxide CO2 μ = 0 D Carbon monoxide CO μ = 0.112 D Water H2O μ = 1.85 D Hydrochloric acid HCl μ = 1.03 D dist. between H and Cl atoms is d = 136 pm (136 10-12 m) We can calculate the charge δ for HCl molecule: The charge δ for HCl molecule represents about 16% of the electron charge (1.602 10-19 C). We can say that the covalent H – Cl bond has about 16% ionic character.

c) Metallic bond The metallic bond represents the electromagnetic attraction forces between delocalized electrons and the metal nuclei. The metallic bond is a strong chemical bond, as indicated by the high melting and boiling points of metals. A metal can be regarded as a lattice of positive metal “ions” in a “sea” of delocalised electrons.

Metal atoms contain few electrons in their outer shells Metal atoms contain few electrons in their outer shells. Metals cannot form ionic or covalent bonds. Sodium has the electronic structure 1s22s22p63s1. When sodium atoms come together, the electron from the 3s atomic orbital of one sodium atom shares space with the corresponding electron of a neighbouring atom to form a molecular orbital. All the 3s orbitals of all the atoms overlap to give a vast number of molecular orbitals which extend over the whole piece of metal. There is a huge numbers of molecular orbitals because any orbital can only hold two electrons.

Crystal structure of sodium The electrons can move freely within these molecular orbitals and so each electron becomes detached from its parent atom. The electrons are called delocalized electrons. The “free“ electrons of the metal are responsible for the characteristic metallic properties: ability to conduct electricity and heat, malleability (ability to be flattened into sheets), ductility (ability to be drawn into wires) and lustrous appearance.