Chemistry Unit 1.3 - Structure and Bonding

Notes on structure and bonding

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Chemistry Unit 1.3: Structure and Bonding
IONIC BONDING
Ionic bonds are a type of chemical bond based on electrostatic forces between two oppositely-charged ions.
In ionic bond formation, a metal donates an electron, due to a low electronegativity to form a positive ion or
cation. Often ionic bonds form between metals and non-metals. The non-metal atom has an electron
configuration just short of a noble gas structure. They have high electronegativity, and so readily gain
electrons to form negative ions or anions. The two or more ions are then attracted to each other by
electrostatic forces. Such bonds are stronger than hydrogen bonds, but similar in strength to covalent bonds.
Ionic bonding occurs only if the overall energy change for the reaction is favourable when the bonded atoms
have a lower energy than the free ones. The larger the resulting energy change the stronger the bond.
Pure ionic bonding is not known to exist. All ionic bonds have a degree of covalent bonding or metallic
bonding. The larger the difference in electronegativity between two atoms the more ionic the bond. Ionic
compounds conduct electricity when molten or in solution. They generally have a high melting point and tend
to be soluble in water.
COVALENT BONDING
Covalent bonding is a description form of chemical bonding that is characterized by the sharing of one or
more electrons between two atoms. In general bonds are defined by a mutual attraction that holds the
resultant molecule together. Often bonding occurs in such a way that the outer electron shells of the
participating atoms becomes filled. Such bonds are always stronger than the intermolecular hydrogen bond
and similar in strength to or stronger than the ionic bond.
In contrast to the ionic and metallic bond, the covalent bond is directional, i.e. the bond angles have a great
impact on the strength of the interaction. This impact arises because covalent bonds are formed by the
overlap of atomic orbitals, with greater overlap producing a greater strength of interaction. Atomic orbitals
all have highly directional character, resulting in a highly directionally-dependent interactions in bonding.
Covalent bonding most frequently occurs between atoms with similar electronegativities. For this reason,
non-metals tend to engage in covalent bonding more readily since metals have access to metallic bonding.
DATIVE BONDING
A dative covalent bond (also known as coordinate covalent bond) is a special type of covalent bond in which
the shared electrons "come from" one of the atoms only. Once the bond has been formed, its strength is no
different from that of a covalent bond. The process of forming a dative bond is called coordination. The
electron donor acquires a positive formal charge, while the electron acceptor acquires a negative formal
charge.
A compound that contains a lone pair of electrons is capable of forming a dative bond. Dative bonds can be
found in many different substances, such as in simple molecules like carbon monoxide or the
ammonium ion . Dative bonds are also formed by electron deficient compounds, such as beryllium
chloride . The Beryllium atom in this compound tends to bind two additional chlorine atoms, in which
every beryllium atom is bonded to four chlorine atoms, two with normal covalent bonding, and the other two
with dative bonds.
Dative bonding can also be found in coordination complexes involving metal ions, especially if they are
transition metal ions. In such complexes, substances in a solution donate their free pairs of electrons to the
metal ion, which accepts the electrons. Dative bonds form and the resulting compound is called a
coordination complex, while the electron donors are called ligands. A common ligand is water , which
will form coordination complexes with metal ions, like , which will form in aqueous
solution.
A dative bond is sometimes represented by an arrow pointing from the donor of the electron pair to the
acceptor of the electron pair.
METALLIC BONDING

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Metallic bonding is the bonding within metals. It involves the delocalised sharing of free electrons among a
lattice of metal atoms. Thus, metallic bonds may be compared to molten salts.
Metal atoms typically contain a high number of electrons in their valence shell compared to their period or
energy level. These become delocalised and form a sea of electrons surrounding a giant lattice of positive
ions. The surrounding electrons and the positive ions in the metal have a strong attractive force between
them.…read more

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In a complex molecule even if the bonds in the molecule are all polar bonds, the molecule overall may not be
polar because of the slight charges cancelling out. This is true in molecules such as , but not in
molecules such as where the charges do not cancel out.
Intermolecular forces
HYDROGEN BONDING
Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a small highly electronegative atom
such as nitrogen, oxygen, or fluorine. The result is a dipolar molecule.…read more

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