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13/11/11 Chemical bonds 2 © Zanichelli editore 2016 2 2

Chemical bonds and the stability of atoms 13/11/11 Chemical bonds and the stability of atoms When two atoms form a chemical bond, the attractive forces (dotted lines) and the repulsive forces (red arrows) are balanced. When atoms are bonded, their stability is higher. – + + – 3 © Zanichelli editore 2016 3 3

13/11/11 Binding energy Higher stability is associated with lower potential energy. When two atoms form a chemical bond, a certain amount of energy is released. The released energy corresponds to the amount of energy that is necessary to break the bond (binding energy). The strengh of the bond is measured by binding energy. repulsion attraction potential energy distance minimum energy 4 © Zanichelli editore 2016 4 4

Three types of primary bonds 13/11/11 Three types of primary bonds There are three types of primary bonds: covalent bond – two atoms share one or more pairs of electrons; ionic bond – one atom gives one or more electrons to another atom; metallic bond – the attraction between positively charged metal ions and their most external electrons. 5 © Zanichelli editore 2016 5 5

Electronegativity and chemical bonds 13/11/11 Electronegativity and chemical bonds Electronegativity (En) measures the force with which an atom attracts bonding electrons. We can determine if a bond is ionic or covalent by calculating the differences in the electronegativity (Δe) of the involved elements. Δe > 1,9  ionic bonds Δe < 0,4  pure covalent bonds Δe < 1,9  covalent bonds 0,4 < Δe < 1,9  polar covalent bonds 6 © Zanichelli editore 2016 6 6

Single and multiple covalent bonds 13/11/11 Single and multiple covalent bonds The strength of attraction between two atoms sharing valance electrons is called a covalent bond. A covalent bond can be: single – two atoms share one pair of electrons; double – two atoms share two pairs of electrons; triple – two atoms share three pairs of electrons. F F O O N N 7 © Zanichelli editore 2016 7 7

13/11/11 Electron promotion Many atoms have electron promotion which allows an electron to transition from a full orbital level into a higher sublayer. It is used by the atoms of some elements, e.g. carbon, in order to increase the number of unpaired electrons. 2p 2p 2s 2s Energy 1s 1s 8 © Zanichelli editore 2016 8 8

Dative covalent bonds O N O H O N O H O 13/11/11 Dative covalent bonds In dative covalent bonds a pair of shared electrons originates from one of the two atoms. O N O H O N O H O A dative bond can also be formed between a molecule and a positive ion or between two molecules – these are called coordination bonds. 9 © Zanichelli editore 2016 9 9

Valence bond theory and the shapes of molecules 13/11/11 According to valence bond theory (VB theory), atoms share electrons by overlapping valence orbitals while their cores remain unchanged. If the overlap is a full head-on bond, it is called a σ bond. If the overlap occurs on the side it is called a π bond. σ bond 10 π bond © Zanichelli editore 2016 10 10

The geometry of bonds and hybridization 13/11/11 The geometry of bonds and hybridization The geometry of covalent bonds, particularly bond angles, determines the shape of a molecule. The model of hybridization explains bond angles formed in molecules with three or more atoms. 11 © Zanichelli editore 2016 11 11

Hybridization in bioelements 13/11/11 Hybridization in bioelements Hybridization is the transformation of two or more atomic orbitals into hybrid orbitals, with the same shape and energy level. It is common in carbon, nitrogen, oxygen and phosphorus. 12 © Zanichelli editore 2016 12 12

Delocalized electrons 13/11/11 Delocalized electrons Some molecules, represented in the form of resonance hybrids, have delocalized electrons that make the molecules very stable. 13 © Zanichelli editore 2016 13 13

The polarity of molecules 13/11/11 The polarity of molecules Molecules that have pure covalent bonds are non-polar. Diatomic molecules that have polar covalent bonds are always polar. δ+ δ – H C Poliatomic molecules are polar only if they contain polar covalent bonds displaced in a way that creates an unbalanced distribution of bonding electrons. 14 © Zanichelli editore 2016 14 14

13/11/11 Molecular substances Materials formed by the same molecules, which behave as independent units, are called molecular substances. In molecular substances, covalent bonds determine chemical properties, while physical properties are determined by intermolecular bonds. Apolar substances have low melting and boiling points; intermolecular bonds are very weak. Polar substances have higher boiling points. 15 © Zanichelli editore 2016 15 15

13/11/11 Covalent solids In covalent solids, atoms form a unique pattern, creating a gigantic molecule. In these solids, physical properties are determined by the strength and geometry of covalent bonds. Quarz, diamond and graphite are examples of natural covalent solids. 16 © Zanichelli editore 2016 16 16

Bonds in ionic compounds 13/11/11 Bonds in ionic compounds In ionic compounds, positive and negative ions are strongly linked and form a stable and ordered reticulum. The attraction between ions with opposite charges is called an ionic bond. This bond is non-directional and has the same strength in any direction. + 2- + Na O + Na Na + O Na 17 © Zanichelli editore 2016 17 17

The properties of ionic solids 13/11/11 The properties of ionic solids Ionic solids are neutral overall and individual molecules are not identifiable within them. They are very stable solids with an ordered, crystal lattice structure. They present high levels of hardness and fragility. They are poor conductors of electricity and have a high melting temperature. 18 © Zanichelli editore 2016 18 18

13/11/11 Metallic bonds Metals are chemical elements with low ionization energy and a small number of electrons in the external orbital. In solid metals, there are positive ions immersed in a sea of delocalized electrons. The strength of attraction between cations and delocalized electrons is called a metallic bond. + + + + + + 19 © Zanichelli editore 2016 19 19

Properties of solid metals 13/11/11 Properties of solid metals The presence of delocalized electrons explains the physical properties of solid metals. They are: sometimes deformed; good conductors of electricity and heat; shiny; melted at a high temperature (but lower than ionic solids). 20 © Zanichelli editore 2016 20 20