Chapter 2Chapter 2Chapter 2Chapter 2

Atoms, Molecules, and Ions

Group Activity

• Draw you idea of what an atom “looks like”.

• Answer the following questions:

– How do you know what an atom “looks like” at the atomic level?

– How do scientists know what at atom “looks like”?

– Can scientists see the parts of an atom? Explain.

Before chemistry there was…

Alchemy’s demise

1661: Robert Boyle, the father of modern chemistry,

publishes The Sceptical Chymist,

Alchemy

Conservation of Mass

1774: Priestley isolated oxygen by heating HgO

(mercury (II) oxide). When carried out in a closed container the total mass of the contents within the

container didn’t change.

1785: Lavoisier formulated The Law of Conservation

of Mass.

In a chemical reaction, matter is neither created nor destroyed

Mass of reactants must = mass of products + any unreacted material

8g H2

34g O2

+36g H2O

6g H2

reaction

42g total 42g total

Law of Definite Proportions

Joseph Louis Proust discovered that all samples of a specific substance will have the same ratio of elements by mass.

For example, all samples of pure:

•water: 1 part H, 8 part O by mass

•carbon dioxide: 1 part C, 2.7 parts O by mass

Elements within a compound combine in specific proportions

Compounds are not a random mixture of elements

• Dalton's Theory:�All matter is composed of atoms; indivisible particles that are exceedingly (really, really) small.

�All atoms of a given element are identical, both in mass and in chemical properties. However, they are different from atoms of other elements.

�Atoms are not created or destroyed in chemical reactions. Reactions only change how atoms are arranged.

�Atoms combine in small, whole-number ratios to form compounds.

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Dalton’s Atomic Theory

Plum Pudding?

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Based on J.J. Thompson’s experiments, the “plum pudding” model of the atom was developed

Large sphere of positive charge

Negatively charged electrons scattered

throughout

Nuclear Atom• In 1911, Hand Geiger and Ernest Marsden carried

out an experiment under the direction of Ernest Rutherford.

• Gold foil experiment: Positive alpha particles were “shot” to toward gold foil.

1010

(Radium)

What was expected vs. what was observed

Expected data: no large deflections. Plum

pudding model fits.

Actual data: large

deflections. New model of the atom needed.

1 out of every 8000 α

particle had a large deflection

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--?

Discovery of the Neutron

• 1932!

• Sir James Chadwick (Irene Joliot-Curie, Frederic Joliot-Curie)

• Bombarded beryllium with α particles

• The radiation that was emitted was electrically neutral

• This radiation could eject protons from wax

• Used multiple elements

• Determined it had the same mass as a proton

Discovery of the neutron

• Simplified experimental set-up

• Why did it take so long to discover the neutron?

Can we see atoms? Can we see inside them?

STM image (7 nm x 7 nm)

of a single zigzag chain of cesium atoms (red) on a

gallium-arsenside surface (blue)

Photo courtesy National Institute of Standards and Technology (NIST)

Nickel atoms – STM

http://www.almaden.ibm.com/vis/stm/atomo.html

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Number4

Relative Sizes

• Nucleus is to an atom what a pea is to a stadium

Atomic #, Mass #, and Isotopes

How many of each particle are in the following atoms (protons, electrons, neutrons)?

11H

+ 21H

31H

126C

136C

146C

188O

178O

168O

2-

147N

157N

3- 158O

23892U

23492U

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Atomic Mass vs. Atomic Mass Number

• Atomic mass and atomic number are NOT the same

• Atomic mass is the mass found on the periodic table, which is an average of isotopes. This mass is not a whole number. It has units.

• Isotopic mass is the mass of a specific isotope. Must look up from a source. Except for one important exception (12C), it is not a whole number. It has units.

• Mass number is the number of protons plus the number of neutrons for a specific isotope. This MUST be a whole number! It has no units.

OAtomic mass

15.999 amu

16OIsotopic mass

15.994 amu

16OMass number

16

Molecules, Ions, and Chemical Bonds

• Which side of the periodic table gains electrons? Loses electrons?

– Metals tend to lose electrons, have a positive charge (cation)

– Nonmetals tend to gain electrons, have a negative charge (anion)

• Na � Na1+

• Cl � Cl1-

• Mg � Mg2+

• S � S2-

1+ 2+ 1-2-3-

Covalent Chemical Bonds

• Covalent bonds form when atoms share electrons; typically between nonmetals

• Hydrogen, oxygen and other elements exist as diatomic molecules

– H2, N2, F2, O2, I2, Cl2, Br2

Ionic Chemical Bonds

• Ionic bonds form when atoms transfer electrons; typically between metal and nonmetal

• Ions are atoms or groups of atoms that have a positive or negative charge; they have gained or lost electrons.

Cations have lost

one or more

electrons; cations have

positive charges

Anions have

gained one or more electrons;

since electrons

are negative, anions have

negative charges

Representations of ionic compounds

• Do they appear different than covalent compounds?

Ionic vs. Covalent Bonding

• Can you determine which is most likely ionic bonding and which is covalent bonding?

Metallic Bonding

Electrons are shared by all of the metal atoms. The are said to be delocalized electrons.

Sometimes referred to as a “sea of electrons”.

Terminology

Molecule: individual unit made up of two or more atoms covalently bonded

Atom: one sphere in an image

Formula Unit:individual unit made up of two or more ionsbonded

H2O

NaCl

Compounds

• Compound: composed of one or more atoms of different elements bonded covalently or ionically.

Are the following compounds? Molecules?

O2 O3 NaCl

• Pure substance: all atoms, compounds, or molecules are the same in a

sample

• Mixture: blends of two or more different substances

– Homogeneous: uniform throughout (seawater)

– Heterogeneous: not uniform (water and oil)

Physical

Separation

Chemical

Separation

Classifications of Matter

• Describe each picture below using the terms

– mixture or pure substance

– atom, element, molecule, and/or compound

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Classifications of Matter

• How would you classify the following?

– Air

– Distilled water

– Ice Cream

– Rocks

– Gold jewelry

– Diamond

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Chemical Formulas

• Chemical (or molecular) formulas indicate the composition of molecules and ions

– Which atoms are present and the ratios in which they have combined

• Structural formulas show how atoms are bonded together

• Empirical formulas give the smallest whole-number ratio of a molecular formula. (H2O2 becomes HO.) These formulas are often used in analysis.

Chemical Formulas

• Why do we have different types of models?

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Sodium Chloride

NaCl

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