Classifying Chemical Compounds

Review of Gr.10 Science

Section A: Complete the chart using a periodic table to help you.

Answer these questions: An atom that gains one or more electrons will have a ____________________ charge. An atom that loses one or more electrons will have a ____________________ charge. An atom that gains or loses one or more electrons is called an ____________. A positive ion is called a ______________ and a negative ion is called an _______________. POSITIVE NEGATIVE ION CATION ANION “An-Eye-On” “Cat-Eye-On”

What is an ionic bond? Atoms will transfer one or more ________________ to another to form the bond. Each atom is left with a ________________ outer shell. An ionic bond forms between a ___________ ion with a positive charge and a ________________ ion with a negative charge. Example B1: Sodium + ChlorineExample B2: Magnesium + Iodine ELECTRONS COMPLETE METAL NONMETAL

Example B3: Potassium + IodineExample B4: Sodium + Oxygen Example B5: Calcium + ChlorineExample B6: Aluminum + Chlorine

What is a covalent bond? Atoms ___________ one or more electrons with each other to form the bond. Each atom is left with a ________________ outer shell. A covalent bond forms between two _________________. Example C1: Hydrogen + HydrogenExample C2: 2 Hydrogen + Oxygen SHARE COMPLETE NONMETAL

Example C3: Chlorine + ChlorineExample C4: Oxygen + Oxygen Example C5: Carbon + 2 OxygenExample C6: Carbon + 4 Hydrogen

Elements There are about 90 naturally occurring elements. Most are not found as pure elements. The majority of elements are found combined with other elements to form compounds. Gold, silver and platinum are rare examples of metals found in elemental form (precious metals).

Classification Systems Since the 90 elements can form thousands of different compounds, classification systems have been developed. The classification of compounds is based on their properties to help our understanding of compounds. Example: melting point, boiling point, hardness, etc.

Properties of Ionic and Covalent Compounds

What is a crystalline solid? What does it look like? A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions.

Thought Lab: Ionic or Covalent? With the person sitting next to you, take a look at the following chart See if you can figure out what compound belongs with what sample number. The compounds used were: Ethanol Carbon tetrachloride Glucose Table salt (sodium chloride) Water Potassium permanganate

Experimental Results covalent ionic covalent

What is Bonding?

Chemical Bonds forces that attract atoms to each other to form compounds involves the interactions of valence electrons between atoms usually the bond forms a compound that is more stable than the atoms individually.

The Octet Rule atoms bond in order to achieve the same number of electrons as the nearest noble gas in the periodic table. bonded atoms are said to be isoelectronic to the nearest noble gas. (All noble gases have 8 valence electrons except He.)

Ionic Bonds a chemical bond formed when one atom loses valence electron(s) an another atom gains electron(s). occurs between metal and nonmetal atoms

Cl – Na + Cl – Ionic Bonding Ionic bonding involves 3 steps 1) loss of an electron(s) by one element 2) gain of electron(s) by a second element 3) attraction between positive and negative Na Cl e–e– 1) 2) 3) Na +

Ionic Bonding

Ionic Bonding Examples Using Lewis Structures In some bonds, multiple electrons can be exchanged:

Examples Continued Some ionic compounds involve bonds between more than two elements.

Structure of Ionic Compounds an ion of one charge is totally surrounded by ions of a opposite charge in the solid state. the charged ions that make the ionic substance also explain why these compounds are soluble in polar solvents such as water. also explains conductivity of ionic solutions

Structure of Ionic Compounds- Crystal Lattice the ions in these compounds are not free to move around in the solid state this explains the high melting point and non- conductivity of these compounds the structured lattice also explains why the crystals are brittle

Covalent Bonds form when two atoms share valence electrons occurs between two non metal atoms

Covalent Bonds consist of a shared pair of electrons between two nonmetals most often each atom of a bond contributes one electron to the bond atoms share electrons so that they may achieve a stable octet in their valence shell

Pure Covalent Bond two atoms of the same element share their electrons equally these are called diatomic elements. ex. H 2, N 2, O 2, F 2, Cl 2 when atoms of two different elements share electrons and the Δ EN is less than 0.5.

Multiple Covalent Bond some atoms will in order to complete octets will share more than one electron and form double or triple bonds.

Covalent Bonding Examples CCl 4 C Cl HCl H Cl NH 3 H N H H In covalent bonding one atom does not take electrons from another atom, but rather the two atoms share electrons.

Properties of Covalent Compounds wide range of properties ex. some dissolve in water and some do not the atoms in the bond are held together very tightly and do not break apart during change of state or during the dissolving process, and thus these compounds are called molecular compounds.

Intra- and Inter- molecular Forces Intramolecular Forces:forces that bond an atom together in a molecule ex. covalent bonds Intermolecular Forces:forces that keep the molecules close to each other in the solid and liquid state these forces are weaker than intramolecular forces and are responsible for the low mp and bp of covalent compounds.

Electronegativity (EN) a measure of an atoms ability to attract electrons in a chemical bond. a property of an atom involved in a bond

How can we use electronegativity to predict bond type? When two atoms form a bond the difference in electronegativity (ΔEN) can help to determine the bond type. | | |

Examples HCl = 0.96 Polar Covalent CrO = 1.78 Ionic Br = 0 Covalent

Polar Covalent Bonds and Polar Molecules The variety of differences in electronegativities of covalent bonds accounts for the variety of properties observed in covalent compounds. ex. -some dissolve in water, others not some are solids while others are liquids or gases at room temperature

Polar Covalent Bonds- The “In Between Bonds” electrons shared between two atoms unequally ΔEN is between 0.5 and 1.7 the atom with the higher electronegativity is partially negative ( δ - ) the atom with the lower electronegativity is partially positive ( δ + )

What does this look like?

Molecular Shape To understand why molecules have different shapes, consider how electron arrangement affects shape. Electron pairs that are not involved in bonding are called lone pairs. Electron pairs that are involved in bonding are called bonding pairs. Electron pairs are arranged around molecules so that they are a maximum distance from each other. The shape that allows four electron pairs to be a maximum distance from each other around an atom is a tetrahedron.

Dipoles The polarity of a bond in a molecule can be shown using a vector called a dipole. The magnitude of the dipole is determined by the Δ EN The direction is determined by EN of each atom in the bond. ex. Water

Dipoles A polar molecule is identified by the vector addition of all dipoles. If the resultant vector, call the dipole moment, is nonzero, then the molecule is polar. This mean one end of the molecule carries a partial positive charge and the other a partial negative charge.

Example: Carbon Dioxide Through vector addition we see that the resultant vector (dipole moment) equals zero, therefore carbon dioxide is a nonpolar molecule.

Example: Hydrogen Chloride

Example: Ammonia

Example: Carbon Tetrafluoride

In General symmetrical molecules with no lone pairs of electrons on the central atom are nonpolar molecules with lone pairs of electrons and polar bonds are polar.

Properties Polar moleculesNonpolar molecules -Molecules stick to themselves (Strong intermolecular forces: Hydrogen bonds or Dipole-Dipole) -high melting points -dissolve ionic or polar covalent compounds -low or weak attractions for molecules (London dispersion forces) -low melting points -dissolve non polar molecules

Determining Molecular Polarity Depends on: dipole moments molecular shape ++ –– ++ –– ++ –– ++ –– HCl ++ ––

BF 3 F F F B Determining Molecular Polarity Nonpolar Molecules Dipole moments are symmetrical and cancel out.

H2OH2O H H O Determining Molecular Polarity Polar Molecules Dipole moments are asymmetrical and don’t cancel. net dipole moment

CHCl 3 H Cl Determining Molecular Polarity Therefore, polar molecules have... asymmetrical shape (lone pairs) or asymmetrical atoms net dipole moment

Dipole Moment Nonpolar Polar.. HH O C OO Bond dipoles Overall dipole moment = 0 Bond dipoles Overall dipole moment The overall dipole moment of a molecule is the sum of its bond dipoles. In CO 2 the bond dipoles are equal in magnitude but exactly opposite each other. The overall dipole moment is zero. In H 2 O the bond dipoles are also equal in magnitude but do not exactly oppose each other. The molecule has a nonzero overall dipole moment.

Examples:

109.5 o

Tetrahedron

Central Atom Central Atom

Central Atom Central Atom

Substituents

Methane, CH 4

.. Predicting Polar Molecules H Cl Polar A molecule has a zero dipole moment because their dipoles cancel one another. HH O Polar FF B F Nonpolar H H H N Polar Nonpolar FF Cl F F F Xe FF Cl C NonpolarPolar Cl H C H H

Drawing Lewis Structures of Molecules All atoms are most stable in chemical bonds when they have an octet in its outermost shell (valence shell) *The Octet Rule. Except : transition metals H hydrogen Be beryllium B boron Decide which atom is going to be the central atom: Hints: Draw the Lewis symbol of each element look for the element that needs the most number of electrons to complete the octet. look for symmetry in oxy-acids, the H is attached to the oxygen Count all the valence electrons. Place 2 electrons in each bond. Complete the octets of all atoms attached to the central atom by adding pairs of electrons. Place any remaining electrons on the central atom in pairs. If the central atom does not have an octet, form double or triple bonds.

Predicting the Shapes of Molecules VSEPR Valence Shell Electron Pair Repulsion Theory Atoms around a central atom in a molecule will position themselves so that they are as far away as possible form each other. This is due to the repulsion between electron pairs in the valence shell of the central atom. The shape of molecules depends on the number of electron pairs around the central atom.

Some Notes For the VSEPR Theory treat double and triple bonds as one bonding pair. Any two atom molecule is a linear molecule. Shape Diagrams: Must show lone pairs of e - on the central atom only Indicates projection out of the paper Indicates projection behind the paper

Number of Bonding Pairs Number of Lone Pairs Shape of the MoleculesName of shape of Molecule Bond Angle 1 0Linear180 o 2 0Linear180 o 3 0Trigonal Planar120 o 4 0Tetrahedral109.5 o 3 1Trigonal Pyramidal109.5 o 2 2Angular/Bent109.5 o