The Study of ChemistryThe Study of ChemistryThe The AtomicAtomic and and MolecularMolecular Perspective of Chemistry Perspective of Chemistry

• Matter is the physical material of the universe.• Matter is made up of relatively few (ca. 100) elements. • Elements are the building blocks of matter.• On the nano (ultramicroscopic) level, matter consists of atoms. An atom is a “nano-basketball” -- nano = 10 -9.• Atoms usually are found in the combined state, either molecules, salts, or alloys. • Molecules may consist of the same type of atoms or different types of atoms.

ElementsElements

Classification of MatterClassification of Matter

The next five elements are:Na 2%, K 2%, Mg 2%, H 1%, Ti 0.5%.

The next six elements are:N 3%, Ca 1.5%, P 1%, K,S,Na 0.75%

Elements in the Human Body –Elements in the Human Body – including trace elementsincluding trace elements

The Periodic TableThe Periodic Table

Bring your Periodic Tableto each class!

Properties of MatterProperties of Matter

When a substance undergoes a physical change, its physical appearance changes, but its chemical nature does not.

Physical and Chemical ChangesPhysical and Chemical Changes

Example: the melting of ice (physical change) results in a solid being converted into a liquid, but it is still water. Physical changes do not result in a change of composition.

When a substance changes its composition, it undergoes a chemical change

Example: when pure hydrogen and pure oxygen react completely, they form pure water. In the flask containing water, there is no oxygen or hydrogen left over.

Units of MeasurementUnits of MeasurementPowers of ten are used for convenience with smaller or larger units in the SI system.

What is a GigaByte?

Units of Measurement - Units of Measurement - TemperatureTemperature

There are three temperature scales:Kelvin Scale (used in science)

Same temperature increment as Celsius scale.Lowest temperature possible (absolute zero) is zero Kelvin. Absolute zero: 0 K = -273.15oC.

Celsius Scale (used in science)Also used in science.Water freezes at 0oC and boils at 100oC.To convert: K = oC + 273.15.

Fahrenheit Scale (used in US engineering and commerce)Water freezes at 32oF and boils at 212oF. To convert:

32-F9

5C 32C

5

9F

Units of Measurement - Units of Measurement - TemperatureTemperature

A user-friendly way to view the Celsius Scale:0° - Cold! (coat)10° - Cool (sweat shirt)20° - Pleasant (long sleeves)25° - Room temperature (short sleeves)30° - Very warm (T-shirt)40° - Hot! (swimming pool!)

Units of Measurement - Units of Measurement - TemperatureTemperature

Units of Measurement - Units of Measurement - DensityDensity

• Used to characterize substances.

• Defined as: density = mass /volume.

• Units: g/cm3, also known as specific gravity.

• Originally based on mass -- the density was defined as the mass of 1.00 g of pure water.

• John Dalton: Elements are composed of atoms.– All atoms of an element are identical (chemically).

(Dalton stressed “identical in weight” but he didn’t know about isotopes)

– In chemical reactions, the atoms are not changed.– Compounds are formed when atoms of more than one

element combine.

The Atomic Theory of Matter

(e.g., H2O, C6H6, C12H22O11 but not H2, Cl2)

The ancient Greeks were the first to postulate that matter consists of indivisible constituents.Later scientists realized that the atom consisted of charged (+ or -) entities.

The Discovery of Atomic StructureThe Discovery of Atomic Structure

Cathode Rays and ElectronsCathode Rays and ElectronsA cathode ray tube (CRT) is a hollow vessel with an electrode at either end. A high voltage is applied across the electrodes.

A charged particle will have its path bend in either an electric or magnetic field.

The ancient Greeks were the first to postulate that matter consists of indivisible constituents.Later scientists realized that the atom consisted of charged (+ or -) entities.

The Discovery of Atomic StructureThe Discovery of Atomic Structure

Cathode Rays and ElectronsCathode Rays and ElectronsA cathode ray tube (CRT) is a hollow vessel with an electrode at either end. A high voltage is applied across the electrodes.

A charged particle will have its path bend in either an electric or magnetic field.

The Discovery of Atomic StructureThe Discovery of Atomic Structure

•a spot which is not affected by the electric field,

Three spots are noted on the detector:

•a spot in the direction of the positive (+) plate,

•a spot in the direction of the negative (-) plate.

-radiation: Large deflection toward the positive plate corresponding to radiation which is negatively charged and of low mass. These particles are light and of low mass. particles are electrons.

-radiation: No deflection; neutral (zero charge) radiation.

-radiation: Small deflection toward the negative plate corresponding to high mass, positively charged radiation.

The Nuclear AtomThe Nuclear AtomFrom the separation of radiation we conclude that the atom consists of neutral, positively, and negatively charged entities.J. J. Thomson assumed all these charged species were found in a sphere.

The Discovery of Atomic StructureThe Discovery of Atomic Structure

The Nuclear AtomThe Nuclear AtomRutherford’s -particle experiment:

The Discovery of Atomic StructureThe Discovery of Atomic Structure

The Nuclear AtomThe Nuclear AtomIn order to get the majority of -particles through a piece of foil to be undeflected, the majority of the atom must consist of a low mass, diffuse negative charge the electron.

The Discovery of Atomic StructureThe Discovery of Atomic Structure

To account for the small number of high deflections of the -particles, the center or nucleus of the atom must consist of a dense positive charge.

The Nuclear AtomThe Nuclear AtomRutherford modified Thomson’s model as follows:assume the atom is spherical but a massive positive charge must be located at the center, with a diffuse light negative charge surrounding it.

The Discovery of Atomic StructureThe Discovery of Atomic Structure

The atom consists of positive, negative, and neutral entities (protons, electrons, and neutrons).

The Modern View of Atomic StructureThe Modern View of Atomic Structure

Protons and neutrons are located in the nucleusof the atom, which is small. Most of the mass of the atom is due to the nucleus.

Electrons are located outside of the nucleus. Most of the volume of the atom is due to electrons.

(1Å = 10-8cm =10-10 m)

The Nucleus

Ångstrom unit:

Columns in the periodic table are called groups (numbered from 1A to 8A or 1 to 18).

The Periodic TableThe Periodic Table

Metals are located on the left hand side of the periodic table (most of the elements are metals).Non-metals are located in the top right hand side of the periodic table. Elements with properties similar to both metals and non-metals are called metalloids and are located at the interface between the metals and non-metals.

Rows in the periodic table are called periods.

Some elements occur naturally as diatomic molecules

(Most elements can be viewed as uniatomic; but thereare unusual elemental molecules, e.g., P4, S8, C60.)

Some of the groups in the periodic table are given special names.

The Periodic TableThe Periodic Table

These names indicate the similarities between group members:

Group 1A: Alkali metals - “al kali” = “the ashes” (of a fire)Group 2A: Alkaline earth metals (“earths” historically were oxides that were difficult to reduce to the metal).Group 6A: Chalcogens - “ore formers”Group 7A: Halogens - “salt formers”Group 8A: Noble gases - “unreactive” gasesAt the bottom are the lanthanides (“rare earths”) and the actinides.

The Periodic TableThe Periodic Table

Metals

Non-Metals

Metalloids“semiconductors”

COMPARISON OF METALS AND NONMETALS

PROPERTY METALS NONMETALSAppearance Metallic luster Flat, matte appearanceConductivity of electricity High LowConductivity of heat High LowChange of form Malleable BrittleAffinity for electrons Loses electrons Gains electronsOxide chemistry Oxide forms bases Oxide forms acids MO + H2O → MOH XO + H2O → HXO Examples: Na2O + H2O → 2NaOH SO3 + H2O → H2SO4

AlkaliMetals

AlkalineEarths

Noble or Inert Gases

Halogens

Chalcogens

Lanthanides (rare earths)

Actinides

Transition Metals

Navigating the Periodic TableNavigating the Periodic Table

Different Kinds of CompoundsDifferent Kinds of Compounds

A salt, formed by ionic bonding,is formed between a metal anda nonmetal, (e.g., NaCl, Ag2O).

A molecule, formed by covalentbonding, is formed between a nonmetal and a nonmetal, (e.g., CO2, PBr3, H2O).

Different Kinds of CompoundsDifferent Kinds of Compounds

Different Kinds of CompoundsDifferent Kinds of Compounds

An alloy, formed by metallicbonding, is formed between a metal and a metal, (e.g., brass or nickel-steel)

The Wave Nature of LightThe Wave Nature of Light

Na

H

Bohr’s Model of the Hydrogen AtomBohr’s Model of the Hydrogen AtomLineLine Spectra SpectraColors from excited gases arise because electrons move between energy states in the atom. These arecalled line spectra.

Absorption Emission

E1

E2

E3

E4

Bohr’s Model of the Hydrogen AtomBohr’s Model of the Hydrogen Atom

The MoleThe MoleThe mole connects the visible with the invisible.A fluorine molecule (F2) weighs 38.000 amu.A mole of fluorine molecules weighs 38.000 grams.The number of fluorine molecules in a mole isan incredibly large number, called Avogadro’s Number, N, which is 6.022 x 1023.We will be using the mole concept very often.

Amedeo Avogadro1776-1856

The MoleThe MoleExamples:A mole of H is 1.008 grams.A mole of H2 is 2.016 grams.A mole of CO2 is 44.011 grams.A mole of CO is 28.01 grams.A mole of octane (C8H18) is 114.22 grams.A mole of copper (Cu) is 63.54 grams.A mole of table salt (NaCl) is 58.44 grams.A mole of sodium bicarbonate (NaHCO3) is 84.01 grams.A mole of Ag2O is 231.74 grams.A mole of glucose (C6H12O6) is 180.16 grams.A mole of chlorophyll (C55H72MgN4O5) is 893.51 grams.

Picturing MoleculesPicturing Molecules

Molecules and Molecular CompoundsMolecules and Molecular Compounds

When an atom or molecule loses electrons, it becomes positively charged.

Ions and Ionic CompoundsIons and Ionic Compounds

•For example, when Na loses an electron, it becomes Na+. Positively charged ions are called cations.

When an atom or molecule gains electrons, it becomes negatively charged.

Ions and Ionic CompoundsIons and Ionic Compounds

For example when Cl gains an electron it becomes Cl.Negatively charged ions are called anions.An atom or molecule can lose more than one electron.

12p+ 12 e- lose 2 e-

12p+ 10 e-

Ions and Ionic CompoundsWhen an atom or molecule loses two electrons, itbecomes doubly positively charged.

For example, when Mg loses two electrons, it becomes Mg2+

Mg atom Mg2+ ion “magnesium atom” “magnesium ion”

16 p+ 16 e- gain 2 e-

16p+ 18 e-

Ions and Ionic CompoundsWhen an atom or molecule gains two electrons, itbecomes doubly negatively charged.

For example, when S gains 2 electrons, it becomes S2-

S atom S2- ion“sulfur atom” “sulfide ion”

Important: note that there are no easily identified NaCl molecules in the ionic lattice. Therefore, we cannot use molecular formulas to describe ionic substances.

Ions and Ionic CompoundsIons and Ionic Compounds

Ionic CompoundsIonic Compounds I.e., 3Mg atoms need to form 3Mg2+ ions (total 3x2+ charges) and 2 N atoms need to form 2N3- ions (total 2x3- charges).Therefore, the formula is Mg3N2.

Ions and Ionic CompoundsIons and Ionic Compounds

Mg N

N3-Mg2+

3 2Be careful! what’s the ionic compound formed between magnesium and oxygen?)

MgO

• Lavoisier: mass is conserved in a chemical reaction.• Chemical equations: descriptions of chemical reactions.• Two parts to an equation: reactants and products:

Chemical Equations

2H2 + O2 2H2O

Reactants Product

H2 O2H2O

• Stoichiometric coefficients: numbers in front of the chemical formulas give numbers of molecules or atoms reacting (and numbers being produced).

CH4 + O2 CO2 + H2O

Count atoms:

Reactants: Products: 1 C 1 C

4 H 2 H2 O 3 O

is not balanced. (Why?)

Law of Conservation of Mass:All reactions must be balanced

Balance reactions only by changing coefficients, not by altering chemical formula