Chemical Bonds

Atom – the smallest unit of matter “indivisible” Helium atom

electron shells Atomic number = number of Electrons Electrons vary in the amount of energy they possess, and they occur at certain energy levels or electron shells. Electron shells determine how an atom behaves when it encounters other atoms

Electrons are placed in shells according to rules: The 1st shell can hold up to two electrons, and each shell thereafter can hold up to 8 electrons.

Octet Rule = atoms tend to gain, lose or share electrons so as to have 8 electrons C would like to N would like to O would like to Gain 4 electrons Gain 3 electrons Gain 2 electrons

Why are electrons important? Elements have different electron configurations different electron configurations mean different levels of bonding

Electron Dot Structures Symbols of atoms with dots to represent the valence-shell electrons 1 2 13 14 15 16 17 18 H He:            Li Be  B   C   N   O  : F  :Ne :                    Na Mg  Al  Si  P S :Cl  :Ar :        

Chemical bonds: an attempt to fill electron shells Ionic bonds – Covalent bonds – Metallic bonds

Learning Check A. X would be the electron dot formula for  A. X would be the electron dot formula for 1) Na 2) K 3) Al   B.  X  would be the electron dot formula 1) B 2) N 3) P

Two major categories of compounds are ionic and molecular (covalent) compounds. Ionic compounds are formed when a metal combines with a nonmetal. Ionic compounds have ionic bonds. Molecular compounds are formed between two nonmetals. Molecular compounds have covalent bonds.

Regents Question: Which compound contains ionic bonds? NO NO2 CaO

Regents Question: þ Nitrogen – nonmetal Oxygen – nonmetal Which compound contains ionic bonds? NO NO2 CaO (4) CO2 Nitrogen – nonmetal Oxygen – nonmetal Calcium – metal Carbon – nonmetal þ

Regents Question: Which formula represents an ionic compound? (1) NaCl (2) N2O (3) HCl (4 )H2O

Chemical bonds are formed when valence electrons are transferred from one atom to another (ionic) shared between atoms (covalent) mobile within a metal (metallic)

IONIC BOND bond formed between two ions by the transfer of electrons

Formation of Ions from Metals Ionic compounds result when metals react with nonmetals Metals lose electrons to match the number of valence electrons of their nearest noble gas Positive ions form when the number of electrons are less than the number of protons Group 1 metals  ion 1+ Group 2 metals  ion 2+ Group 13 metals  ion 3+

Formation of Sodium Ion Sodium atom Sodium ion Na  – e  Na + 2-8-1 2-8 ( = Ne) 11 p+ 11 p+ 11 e- 10 e- 0 1+

Formation of Magnesium Ion Magnesium atom Magnesium ion  Mg  – 2e  Mg2+ 2-8-2 2-8 (=Ne) 12 p+ 12 p+ 12 e- 10 e- 0 2+

Some Typical Ions with Positive Charges (Cations) Group 1 Group 2 Group 13 H+ Mg2+ Al3+ Li+ Ca2+ Na+ Sr2+ K+ Ba2+

Learning Check A. Number of valence electrons in aluminum 1) 1 e- 2) 2 e- 3) 3 e- B. Change in electrons for octet 1) lose 3e- 2) gain 3 e- 3) gain 5 e- C. Ionic charge of aluminum 1) 3- 2) 5- 3) 3+

Solution A. Number of valence electrons in aluminum 3) 3 e- B. Change in electrons for octet 1) lose 3e- C. Ionic charge of aluminum 3) 3+

Learning Check Give the ionic charge for each of the following: A. 12 p+ and 10 e- 1) 0 2) 2+ 3) 2- B. 50p+ and 46 e- 1) 2+ 2) 4+ 3) 4- C. 15 p+ and 18e- 2) 3+ 2) 3- 3) 5-

Ions from Nonmetal Ions In ionic compounds, nonmetals in groups 15, 16, and 17 gain electrons from metals Nonmetal add electrons to achieve the octet arrangement Nonmetal ionic charge: 3-, 2-, or 1-

Fluoride Ion     1 - : F  + e : F :     2-7 2-8 (= Ne) unpaired electron octet     1 - : F  + e : F :     2-7 2-8 (= Ne) 9 p+ 9 p+ 9 e- 10 e- 0 1 - ionic charge

Ionic Bond Between atoms of metals and nonmetals with very different electronegativity Bond formed by transfer of electrons Produce charged ions all states. Conductors and have high melting point. Examples; NaCl, CaCl2, K2O

Ionic Bonds: One Big Greedy Thief Dog!

1). Ionic bond – electron from Na is transferred to Cl, this causes a charge imbalance in each atom. The Na becomes (Na+) and the Cl becomes (Cl-), charged particles or ions.

Ionic Bonds: Metals and Non-metals Metals… lose e-. Non-metals… gain e-. They can exchange electrons to form a bond.

Metals tend to have low ionization energies, so they lose e- easily. Non-metals tend to have high electronegativities, so they gain e’ readily.

To add to notes… The two Elements involved in an ionic bond have a difference in electronegativity (E.N.D.) that is greater than or equal to 1.7 Ex: NaCl E.N.D.= 2.3 (How do you find this? Look up electronegativity of each element on Table S and subtract.) Na = 0.9 Cl = 3.2 3.2 – 0.9 = 2.3

Summary of Concepts: Ionic Bonds Occur between metals (+) and non-metals (-) Involve a transfer of electrons from the metal to the nonmetal. The two elements have an electronegative difference of 1.7 or greater We call ionic compounds “salts”

Aim: To identify what covalent bonds are and to compare and contrast them to ionic bonds. Do Now: What is an ionic bond? In order for a bond to be considered ionic, what does the electronegativity difference between the two elements have to be? List two properties of ionic bonds.

COVALENT BOND bond formed by the sharing of electrons

Covalent Bond Between nonmetallic elements of similar electronegativity. Formed by sharing electron pairs Stable non-ionizing particles, they are not conductors at any state Examples; O2, CO2, C2H6, H2O, SiC

Covalent Bonds

Bonds in all the polyatomic ions and diatomics are all covalent bonds

Covalent Bonds vs. Ionic Bonds

Two Hydrogen Atoms The valence shells overlap and the electrons are shared making a more stable molecule.

Covalent Bonds Involves SHARING electrons because both elements have high electronegativities. Sharing of electrons can be equal (non-polar) or unequal (polar) Usually is between two NON-METALS (ex. H, C, O, N…) Covalent bonding like Ionic bonding results in a more stable compound, because the atoms involved meet the “octet rule”.

Covalent Bonding (cont.) A “shared” pair of electrons makes a SINGLE BOND (2 total e-). 2 “shared” pairs makes a DOUBLE BOND (4 total e-). 3 “shared” pairs makes a TRIPLE BOND (6 total e-). We call them MOLECULES. Covalent bonds are very strong bonds.

when electrons are shared equally NONPOLAR COVALENT BONDS when electrons are shared equally H2 or Cl2

2. Covalent bonds- Two atoms share one or more pairs of outer-shell electrons. Oxygen Atom Oxygen Atom Oxygen Molecule (O2)

when electrons are shared but shared unequally POLAR COVALENT BONDS when electrons are shared but shared unequally H2O

Polar Covalent Bonds: Unevenly matched, but willing to share.

- water is a polar molecule because oxygen is more electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.

Comparing the properties: compounds with ionic bonds and compounds with covalent bonds. Properties of molecular compounds Low melting and boiling points (weak attraction between molecules) Nonelectrolytes: Do not conduct electricity as solids or when dissolved or molten – no charged particles (ions) to move Solids are soft Forms molecules Properties of ionic compounds Solids with high melting and boiling points (strong attraction between ions) Electrolytes: Do not conduct electricity as solids but do when dissolved or molten – ions are charged particles that are free to move No individual molecules

Ionic solids conduct electricity when dissolved or molten Ionic solids conduct electricity when dissolved or molten. Molecular solids do not. Solution conducts electricity Solution doesn’t conduct electricity Ionic Solid dissolved in water Molecular Solid dissolved in water

Regents Question: Which of the following solids has the highest melting point? (1) H2O(s) (2) Na2O(s) (3) SO2 (s) (4) CO2 (s)

Regents Question: Which of the following solids has the highest melting point? (1) H2O(s) (2) Na2O(s) (3) SO2 (s) (4) CO2 (s) þ

Regents Question: Testing of an unknown solid shows that it has the properties listed below. (1) low melting point (2) nearly insoluble in water (3) nonconductor of electricity (4) relatively soft solid State the type of bonding that would be expected in the particles of this substance. Explain in terms of attractions between particles why the unknown solid has a low melting point. Explain why the particles of this substance are nonconductors of electricity.

Regents Question: Covalent Testing of an unknown solid shows that it has the properties listed below. (1) low melting point (2) nearly insoluble in water (3) nonconductor of electricity (4) relatively soft solid State the type of bonding that would be expected in the particles of this substance. Explain in terms of attractions between particles why the unknown solid has a low melting point. Explain why the particles of this substance are nonconductors of electricity. Covalent The attraction between particles is weak because there are no charged particle. Molecular substances are non-electrolytes – they do not form ions.

When nonmetals combine with nonmetals, they share electrons When nonmetals combine with nonmetals, they share electrons. The attraction of two atoms for a shared pair makes a covalent bond. Electrons are always shared in pairs. Sharing electrons fills the valence shell with 8 electrons (2 for hydrogen.) Hydrogen chloride water ammonia methane

Regents Question: Which type of chemical bond is formed between two atoms of bromine? Metallic (2) Hydrogen (3) ionic (4) covalent

Regents Question Which type of chemical bond is formed between two atoms of bromine? Metallic (2) Hydrogen (3) ionic (4) covalent þ

In a multiple covalent bond, more than one pair of electrons are shared between two atoms. Diatomic oxygen has a double bond O=O (2 shared pairs) because oxygen needs 2 electrons to fill its valence shell Diatomic nitrogen has a triple bond NN (3 shared pairs) because nitrogen needs 3 electrons to fill its valence shell Carbon dioxide has two double bonds

Regents Question: þ Which molecule contains a triple covalent bond? (4) Cl 2 þ

Molecular polarity can be determined by the shape of the molecule and the distribution of charge. Possible shapes Linear (X2 HX CO2) Bent (H2O) Pyramidal (NH3) Tetrahedral (CH4 CCl4) A polar molecule is called a dipole. It has a positive side and a negative side – uneven charge distribution.

Symmetrical molecules are not dipoles. Symmetrical (nonpolar) molecules include CO2 , CH4 , and diatomic elements. .. Symmetrical molecules are not dipoles.

Asymmetrical (polar) molecules include HCl, NH3 , and H2 O. (5.2l) The negative side of the molecule is the side that has the atom with the higher electronegativity.

Physical properties of substances can be explained in terms of chemical bonds and intermolecular forces. These properties include conductivity, malleability, solubility, hardness, melting point, and boiling point.

The stronger the intermolecular forces, the higher the boiling points and melting points. Strongest Ionic Solids Molecules with Hydrogen bonds Polar molecules Nonpolar molecules Weakest For nonpolar molecules, the greater the mass, the greater the force of attraction.

Regents Question: The table below shows the normal boiling point of four compounds. Compound Normal Boiling Point (°C) HF (l) 19.4 CH3Cl (l) –24.2 CH3F (l) –78.6 HCl (l) –83.7 Which compound has the strongest intermolecular forces? (1) HF(l) (2) CH3Cl(l) (3)CH3F(l) (4)HCl(l)

Regents Question: The table below shows the normal boiling point of four compounds. Compound Normal Boiling Point (°C) HF (l) 19.4 CH3Cl (l) –24.2 CH3F (l) –78.6 HCl (l) –83.7 Which compound has the strongest intermolecular forces? (1) HF(l) (2) CH3Cl(l) (3)CH3F(l) (4)HCl(l) þ

Electronegativity indicates how strongly an atom of an element attracts electrons in a chemical bond.

Electronegativity values are assigned according to arbitrary scales Fluorine is assigned the value 4.0 – the highest of any element Nonmetals have high electronegativity – they want to attract electrons so they can fill their valence shell Metals have low electronegativity – they want to lose electrons to get rid of their valence shell

Regents Question: (1) electronegativity (2) ionization energy The strength of an atom’s attraction for the electrons in a chemical bond is the atom’s (1) electronegativity (2) ionization energy (3) heat of reaction (4) heat of formation

Regents Question: The strength of an atom’s attraction for the electrons in a chemical bond is the atom’s (1) electronegativity (2) ionization energy (3) heat of reaction (4) heat of formation þ

The electronegativity difference between two bonded atoms is used to assess the degree of polarity in the bond. Polar covalent bonds form between two different nonmetals Polar bonds have a negative side and a positive side The electrons are attracted more to the atom with the higher electronegativity. The atom with the higher electronegativity is the negative side of the bond.

RECALL! When the electronegativity difference between two elements is greater than or equal to 1.7, it is an IONIC bond. When the electronegativity difference between two elements is less than 1.7, it is a COVALENT bond. Complete the worksheet!

Regents Question: Which of these formulas contains the most polar bond? (1) H–Br (2) H–Cl (3) H–F (4) H–I

Regents Question: Which of these formulas contains the most polar bond? (1) H–Br (2) H–Cl (3) H–F (4) H–I þ

Three types of bonds Ionic bonds– transfer of electrons- occur between a metal and a nonmetal Polar bonds– unequal sharing- occur between two different nonmetals Nonpolar bonds– equal sharing- occur between two of the same nonmetals

Electron-dot diagrams (Lewis structures) can represent the valence electron arrangement in elements, compounds, and ions. atom ion molecular compound ionic compound

Magnesium atom Magnesium ion When metals lose electrons to form ions, they lose all their valence electrons. The Lewis Dot Structure of a metal ion has no dots. The charge indicates how many electrons were lost. Magnesium atom Magnesium ion

When nonmetals gain electrons, they fill up their valence shell with a complete octet (except hydrogen.) The ion is placed in brackets with the charge outside the brackets.

The formula for magnesium fluoride is MgF2 A (+) metal ion is attracted to a (–) nonmetal ion (opposites attract) forming an ionic compound. We can use Lewis dot structures to represent ionic compounds. The formula for magnesium fluoride is MgF2

Regents Question: Draw the electron-dot (Lewis) structure of an atom of calcium. Draw the electron-dot (Lewis) structure of an atom of chlorine. Draw the electron-dot (Lewis) structure of calcium chloride.

Regents Question: Draw the electron-dot (Lewis) structure of an atom of calcium. Draw the electron-dot (Lewis) structure of an atom of chlorine. Draw the electron-dot (Lewis) structure of calcium chloride.

Regents Question: Draw an electron-dot diagram for each of the following substances: A. calcium oxide (an ionic compound) B. hydrogen bromide C. carbon dioxide

Regents Question: Draw an electron-dot diagram for each of the following substances: A calcium oxide (an ionic compound) B hydrogen bromide C carbon dioxide

Polyatomic ions are groups of atoms covalently bonded together that have a negative or positive charge.

Polyatomic ions are held together by covalent bonds but form ionic bonds with other ions. + Ionic bond H - Covalent bonds H N H Cl H

METALLIC BOND bond found in metals; holds metal atoms together very strongly

The bonds holding metals together in their crystal lattice are called metallic bonds. All metals have metallic bonds “Positive ions immersed in a sea of mobile electrons” Bonds are between Kernels, leaving the valence electrons free to move from atom to atom Mobile electrons give metals the ability to conduct electricity

Metallic Bond Formed between atoms of metallic elements Electron cloud around atoms Good conductors at all states, lustrous, very high melting points Examples; Na, Fe, Al, Au, Co

Metallic Bonds: Mellow dogs with plenty of bones to go around.

Regents Question: Metallic bonding occurs between atoms of (1) sulfur (2) copper (3) Fluorine (4) carbon

Regents Question: þ Metallic bonding occurs between atoms of (1) sulfur (2) copper (3) Fluorine (4) carbon þ

Regents Question: The high electrical conductivity of metals is primarily due to (1) high ionization energies (2) filled energy levels (3) mobile electrons (4) high electronegativities

Regents Question: 01/03 #15 The high electrical conductivity of metals is primarily due to (1) high ionization energies (2) filled energy levels (3) mobile electrons (4) high electronegativities þ

Regents Question: Which substance contains metallic bonds? (1) Hg(l) (2) H2O(l) (3) NaCl(s) (4)C6H12O6(s)

Regents Question: þ Which substance contains metallic bonds? (1) Hg(l) (2) H2O(l) (3) NaCl(s) (4)C6H12O6(s) þ

Metallic Bond, A Sea of Electrons

Metals Form Alloys Metals do not combine with metals. They form Alloys which is a solution of a metal in a metal. Examples are steel, brass, bronze and pewter.

Regents Question: A chemist performs the same tests on two homogeneous white crystalline solids, A and B. The results are shown in the table below. The results of these tests suggest that (1) both solids contain only ionic bonds (2) both solids contain only covalent bonds (3) solid A contains only covalent bonds and solid B contains only ionic bonds (4) solid A contains only ionic bonds and solid B contains only covalent bonds

Regents Question: A chemist performs the same tests on two homogeneous white crystalline solids, A and B. The results are shown in the table below. The results of these tests suggest that (1) both solids contain only ionic bonds (2) both solids contain only covalent bonds (3) solid A contains only covalent bonds and solid B contains only ionic bonds (4) solid A contains only ionic bonds and solid B contains only covalent bonds þ

Regents Question: Each molecule listed below is formed by sharing electrons between atoms when the atoms within the molecule are bonded together. Molecule A: Cl2 Molecule B: CCl4 Molecule C: NH3 Draw the electron-dot (Lewis) structure for the NH3 molecule. Explain why CCl4 is classified as a nonpolar molecule. Explain why NH3 has stronger intermolecular forces of attraction than Cl2 . Explain how the bonding in KCl is different from the bonding in molecules A, B, and C.

Regents Question: Draw the electron-dot (Lewis) structure for the NH3 molecule. Explain why CCl4 is classified as a nonpolar molecule. Explain why NH3 has stronger intermolecular forces of attraction than Cl2 . Explain how the bonding in KCl is different from the bonding in molecules A, B, and C. It is symmetrical. There is an even charge distribution around the molecule. Ammonia has hydrogen bonds, chlorine is nonpolar. KCl is ionic and molecules A, B and C are molecular with covalent bonds.