BONDING

Atoms are generally found in nature in combination held together by chemical bonds. –A chemical bond is a mutual electrical attraction between the nuclei and valence electrons of different atoms that binds the atoms together. There are two types of chemical bonds: ionic, and covalent. Atoms are generally found in nature in combination held together by chemical bonds. –A chemical bond is a mutual electrical attraction between the nuclei and valence electrons of different atoms that binds the atoms together. There are two types of chemical bonds: ionic, and covalent. Introduction to Bonding

What determines the type of bond that forms? –The valence electrons of the two atoms involved are redistributed to the most stable arrangement. –The interaction and rearrangement of the valence electrons determines which type of bond that forms. Before bonding the atoms are at their highest possible potential energy What determines the type of bond that forms? –The valence electrons of the two atoms involved are redistributed to the most stable arrangement. –The interaction and rearrangement of the valence electrons determines which type of bond that forms. Before bonding the atoms are at their highest possible potential energy Introduction to Bonding

There are understandings of bond electron interaction –One understanding of the formation of a chemical bond deals with balancing the opposing forces of repulsion and attraction Repulsion occurs between the negative e - clouds of each atom Attraction occurs between the positive nuclei and the negative electron clouds There are understandings of bond electron interaction –One understanding of the formation of a chemical bond deals with balancing the opposing forces of repulsion and attraction Repulsion occurs between the negative e - clouds of each atom Attraction occurs between the positive nuclei and the negative electron clouds Introduction to Bonding

When two atoms approach each other closely enough for their electron clouds to begin to overlap –The electrons of one atom begin to repel the electrons of the other atom –And repulsion occurs between the nuclei of the two atoms When two atoms approach each other closely enough for their electron clouds to begin to overlap –The electrons of one atom begin to repel the electrons of the other atom –And repulsion occurs between the nuclei of the two atoms Introduction to Bonding

As the optimum distance is achieved that balances these forces, there is a release of potential energy –The atoms vibrate within the window of maximum attraction/minimum repulsion The more energy released the stronger the connecting bond between the atoms As the optimum distance is achieved that balances these forces, there is a release of potential energy –The atoms vibrate within the window of maximum attraction/minimum repulsion The more energy released the stronger the connecting bond between the atoms Introduction to Bonding

Another understanding of the form- ation of a chemical bond between two atoms centers on achieving the most stable arrangement of the atoms’ valence electrons –By rearranging the electrons so that each atom achieves a noble gas-like arrangement of its electrons creates a pair of stable atoms (only occurs when bonded) Another understanding of the form- ation of a chemical bond between two atoms centers on achieving the most stable arrangement of the atoms’ valence electrons –By rearranging the electrons so that each atom achieves a noble gas-like arrangement of its electrons creates a pair of stable atoms (only occurs when bonded) Introduction to Bonding

Sometimes to establish this arrange- ment one or more valence electrons are transferred between two atoms –Basis for ionic bonding Sometimes valence electrons are shared between two atoms –Basis for covalent bonding Sometimes to establish this arrange- ment one or more valence electrons are transferred between two atoms –Basis for ionic bonding Sometimes valence electrons are shared between two atoms –Basis for covalent bonding Introduction to Bonding

A good predictor for which type of bonding will develop between a set of atoms is the difference in their electronegativities. –Remember, electronegativity is a measure of the attraction an atom has for e - s after developing a bond The more extreme the difference between the two atoms, the less equal the exchange of electrons A good predictor for which type of bonding will develop between a set of atoms is the difference in their electronegativities. –Remember, electronegativity is a measure of the attraction an atom has for e - s after developing a bond The more extreme the difference between the two atoms, the less equal the exchange of electrons Introduction to Bonding

Let’s consider the compound Cesium Fluoride, CsF. –The electronegativity value (EV) for Cs is.70; the EV for F is The difference between the two is 3.30, which falls within the scale of ionic character. When the electronegativity difference between two atoms is greater than 2.1 the bond is mostly ionic. Let’s consider the compound Cesium Fluoride, CsF. –The electronegativity value (EV) for Cs is.70; the EV for F is The difference between the two is 3.30, which falls within the scale of ionic character. When the electronegativity difference between two atoms is greater than 2.1 the bond is mostly ionic. Introduction to Bonding

The take home lesson on electro- negativity and bonding is this: –The closer together the atoms are on the P.T., the more evenly their e - interact, and are therefore more likely to form a covalent bond –The farther apart they are on the P.T., the less evenly their e - interact, and are therefore more likely to form an ionic bond. The take home lesson on electro- negativity and bonding is this: –The closer together the atoms are on the P.T., the more evenly their e - interact, and are therefore more likely to form a covalent bond –The farther apart they are on the P.T., the less evenly their e - interact, and are therefore more likely to form an ionic bond. Introduction to Bonding metal w/nonmetal = ionic nonmetal w/nonmetal = covalent

In a co-valent bond: –The electronegativity difference between the atoms involved is not extreme So the interaction between the involved electrons is more like a sharing relationship –It may not be an equal sharing relationship, but at least the electrons are being “shared”. In a co-valent bond: –The electronegativity difference between the atoms involved is not extreme So the interaction between the involved electrons is more like a sharing relationship –It may not be an equal sharing relationship, but at least the electrons are being “shared”. Introduction to Covalent Bonding

Lets look at the molecule Cl 2 Covalent Bonds Cl Shared Electrons Shared Electrons Cl

Shared electrons are counted with both atoms Cl Notice 8 e - in each valence shell!!!

Cl H H H H Covalent Bonds How about the molecule HCl? (Polar Covalent) shared, but not evenly

To be stable the two atoms involved in the covalent bond share their electrons in order to achieve the arrangement of a noble gas. So what’s the bottom line?

In an ion - ic bond: –The electronegativity difference is extreme, So the atom with the stronger pull doesn’t really share the electron –Instead the electron is essentially transferred from the atom with the least attraction to the atom with the most attraction In an ion - ic bond: –The electronegativity difference is extreme, So the atom with the stronger pull doesn’t really share the electron –Instead the electron is essentially transferred from the atom with the least attraction to the atom with the most attraction Introduction to Ionic Bonding

An electron is transferred from the sodium atom to the chlorine atom

Notice 8 e - in each valence shell!!! Both atoms are happy, they both achieve the electron arrangement of a noble gas. Both atoms are happy, they both achieve the electron arrangement of a noble gas.

Very Strong Electrostatic attraction established… IONIC BONDS

To be stable the two atoms involved in the ionic bond will either lose or gain their valence electrons in order to achieve a stable arrangement of electrons. So what’s the bottom line?

As we’ve learned so far ionic com- pounds are formed by the transfer of electrons from a metal to a nonmetal The ionic compound is held together by the strong electrostatic attraction between oppositely charged ions. There is a tremendous amount of energy stored in the bonds formed in an ionic compound. As we’ve learned so far ionic com- pounds are formed by the transfer of electrons from a metal to a nonmetal The ionic compound is held together by the strong electrostatic attraction between oppositely charged ions. There is a tremendous amount of energy stored in the bonds formed in an ionic compound. Bond Energies and Bonding

It takes a lot of energy (A.K.A. bond energy) to pull the two ions apart once they have established their stable arrangement through bonding Energy can be released or absorbed when ions form —Removing electrons from atoms requires an input of energy —Remember from last chapter this energy is called ionization energy It takes a lot of energy (A.K.A. bond energy) to pull the two ions apart once they have established their stable arrangement through bonding Energy can be released or absorbed when ions form —Removing electrons from atoms requires an input of energy —Remember from last chapter this energy is called ionization energy Bond Energies and Bonding

Energy and Ionic Bonding On the other hand adding electrons to atoms releases energy into the environment —Remember this has to do with the atoms affinity for electrons —Sometimes this energy is used to help remove the electron from another atom The ionization energy to remove 1 e - from each atom in a mole of Na atoms is kJ On the other hand adding electrons to atoms releases energy into the environment —Remember this has to do with the atoms affinity for electrons —Sometimes this energy is used to help remove the electron from another atom The ionization energy to remove 1 e - from each atom in a mole of Na atoms is kJ

A mol of Cl atoms releases kJ when an e - is added to the atom —Notice that it takes more energy to remove Na’s e - than the amount released from the Cl atoms. Forming an ionic bond is a multi-step process —The final step releases a substantial amount of energy (a.k.a. the driving force) A mol of Cl atoms releases kJ when an e - is added to the atom —Notice that it takes more energy to remove Na’s e - than the amount released from the Cl atoms. Forming an ionic bond is a multi-step process —The final step releases a substantial amount of energy (a.k.a. the driving force) Energy and Ionic Bonding

At the beginning there is solid sodium and chlorine gas. Na(s) & Cl 2 (g) Crystal Formation ENERGY IN A mol of sodium is converted from a solid to a gas: Na(s) + energy  Na(g)

One electron is then removed from each sodium atom of form a sodium cation Na(g) + energy  Na + (g) + e - One electron is then removed from each sodium atom of form a sodium cation Na(g) + energy  Na + (g) + e - ENERGY IN Energy is required to break the bond holding 0.5mol of Cl 2 molecules together to form a mole of chlorine atoms Cl 2 (g) + energy  2Cl(g) Energy is required to break the bond holding 0.5mol of Cl 2 molecules together to form a mole of chlorine atoms Cl 2 (g) + energy  2Cl(g) ENERGY IN

The next step involves adding an electron to each chlorine atom to form a chloride anion: Cl(g) + e -  Cl - (g) + energy ENERGY OUT The final step provides the driving force for the reaction. Na + (g) + Cl - (g)  NaCl(s) + energy The final step provides the driving force for the reaction. Na + (g) + Cl - (g)  NaCl(s) + energy ENERGY OUT

Energy released in the final step is called the lattice energy —Energy released when the crystal lattice of an ionic solid is formed For NaCl, the lattice energy is kJ/mol, which is greater than the input of energy in the previous steps —The lattice energy provides enough energy to allow for the formation of the sodium ion Energy released in the final step is called the lattice energy —Energy released when the crystal lattice of an ionic solid is formed For NaCl, the lattice energy is kJ/mol, which is greater than the input of energy in the previous steps —The lattice energy provides enough energy to allow for the formation of the sodium ion Crystal Formation

We can use the lattice energy as a method for measuring the strength of the bond in ionic compounds. —The amount of energy necessary to break a bond is called bond energy. This energy is equal to the lattice energy, but —Bond energy moves into the system —Lattice energy moves out of the system We can use the lattice energy as a method for measuring the strength of the bond in ionic compounds. —The amount of energy necessary to break a bond is called bond energy. This energy is equal to the lattice energy, but —Bond energy moves into the system —Lattice energy moves out of the system Crystal Formation

MgO CaF NaI NaBr NaCl LiI LiBr LiCl kJ/mol (in) kJ/mol (out) Bond energy Lattice energy Compound

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In the construction of a crystal lattice, depending on the ions involved there can be small “pores” develop between ions in the ionic crystal. —Some ionic compnds have enough space between the ions that water molecules can get trapped in between the ions Ionic compounds that absorb water into their pores form a special type of ionic compound called a hydrate. In the construction of a crystal lattice, depending on the ions involved there can be small “pores” develop between ions in the ionic crystal. —Some ionic compnds have enough space between the ions that water molecules can get trapped in between the ions Ionic compounds that absorb water into their pores form a special type of ionic compound called a hydrate. Hydrate Formation

Hydrates typically have different properties than their dry versions - A.K.A. anhydrides —Anhydrous CuSO 4 is nearly colorless —CuSO 4 5 H 2 O is a bright blue color When Copper (II) Sulfate is fully hydrated there are 5 water molecules present for every Copper ion. —The hydrated name would be Copper (II) Sulfate Pentahydrate Hydrates typically have different properties than their dry versions - A.K.A. anhydrides —Anhydrous CuSO 4 is nearly colorless —CuSO 4 5 H 2 O is a bright blue color When Copper (II) Sulfate is fully hydrated there are 5 water molecules present for every Copper ion. —The hydrated name would be Copper (II) Sulfate Pentahydrate Hydrate Formation

Have you ever bought a new purse or camera and found a small packet of crystals labeled – do not eat? —These crystals are there to absorb water that might lead to mildew or mold The formula of a hydrate is X A Y B Z H 2 O (Z is a coefficient indicating how many waters are present per formula unit) Have you ever bought a new purse or camera and found a small packet of crystals labeled – do not eat? —These crystals are there to absorb water that might lead to mildew or mold The formula of a hydrate is X A Y B Z H 2 O (Z is a coefficient indicating how many waters are present per formula unit) Hydrate Formation