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Chemical bonds 2
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3. Chemical bonds and the stability of atoms
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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
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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
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5. Three types of primary bonds
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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
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6. Electronegativity and chemical bonds
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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
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7. Single and multiple covalent bonds
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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
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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
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9. Dative covalent bonds O N O H O N O H O
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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
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10. Valence bond theory and the shapes of molecules
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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
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11. The geometry of bonds and hybridization
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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
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12. Hybridization in bioelements
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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
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13. Delocalized electrons
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Delocalized electrons
Some molecules, represented in the form of resonance hybrids, have delocalized electrons that make the
molecules very stable. 13
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14. The polarity of molecules
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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
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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
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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
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17. Bonds in ionic compounds
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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
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18. The properties of ionic solids
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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
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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
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20. Properties of solid metals
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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
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