Chapter 2

Atoms, Molecules and Ions

Pentacene on a copper surface (AFM image)

Science, Volume 325, Issue 5944, pp. 1110 – 1114 (28 August 2009).

(Dalton’s) Atomic Theory• Each element is made up of atoms.

• Atoms of a given element are identical while atoms of different elements differ.

• Chemical compounds are made up of specific whole number ratios of atoms.

• Chemical Reactions involve the reorganization of atoms – atoms (and masses) do not change.

John Dalton, 1766 -1844 Lithograph

Properties of Charged Particles

Thomson was awarded the 1906 Nobel Prize.

Sir Joseph John Thompson, 1856 - 1940

Electron mass/charge = -5.6857 x 10-9 g coulomb-1

An evacuated tube, containing a small amount of a gas was attached to a power supply:

• The same cathode rays (electrons) were seen no matter what gas the tube was filled with.

• But the positively charged portion left over had a different mass/charge dependent on the type of gas used.

Thompson’s Conclusion:

Electrons are a fundamental unit of all materials, and atoms look like a “sea” of positive charge with imbedded electrons:

Millikan’s Oil Drop Experiment

Determined the charge on an electron = 1.592 X 10-19 coulombs

•γ-rays are high-energy light.

•α-particles are helium

nuclei.

•β-particles are high energy

electrons

Types of Radiation(Marie Curie/Rutherford)

Marie Curie1867-1934

•Met Pierre Curie, Physics Professor at the Sorbonne in 1894

•Succeeded her husband as Head of the Physics Laboratory at the Sorbonne

•Following the death of Pierre Curie in 1906, she took his place as Professor of General Physics in the Faculty of Sciences, the first time a woman had held this position.

•Received the Nobel Prize for Physics in 1903

•Received a second Nobel Prize in Chemistry in 1911

•Discovered two elements (Polonium and Radium)

Marie Curie(Maria Sklodowska)

1867-1934

•Met Pierre Curie, Physics Professor at the Sorbonne in 1894

•Succeeded her husband as Head of the Physics Laboratory at the Sorbonne

•Following the death of Pierre Curie in 1906, she took his place as Professor of General Physics in the Faculty of Sciences, the first time a woman had held this position.

•Received the Nobel Prize for Physics in 1903

•Received a second Nobel Prize in Chemistry in 1911

•Discovered two elements (Polonium and Radium)

Rutherford’s Gold Foil Experiment

The Composition of the Atom

The nucleus is made up of protons and neutrons

Rutherfordprotons 1919

James Chadwickneutrons 1932

The “Modern” Atom

Particle Mass Charge

g amu Coulombs (e)

Electron 9.109 x 10-28 (0) –1.602 x 10-19 –1Proton 1.673 x 10-24 1 +1.602 x 10-19 +1Neutron 1.675 x 10-24 1 0 0

The Fundamental Particles

Atoms are uncharged species – thus they contain the same number of protons and electrons (charged species are known as ions)

The Concept of Atomic Number

The Concept of Atomic Number

The atomic number (Z) = # of protons

The Concept of Atomic Number

The atomic number (Z) = # of protons

The atomic mass (A) = protons + neutrons

Atomic Mass Units

The atomic mass (A) = protons + neutrons

• Mass of carbon-12 = 1.9926 X 10-23

g

• Mass of carbon-12 = 12 amu (atomic mass units - amu or u)

Isotopic Distributions

Why does the periodic table say that the mass of carbon is 12.011 amu?

Isotopic Distributions

Why does the periodic table say that the mass of carbon is 12.011 amu?

98.9% of all carbon atoms have A = 12 (6 neutrons)

1.1% of all carbon atoms have A = 13 (7 neutrons)

12.011 is the average mass of a large sample of carbon atoms. When we measure “large” amounts of atoms, we can always use the average atomic mass.

Mass Spectromeric Isotope Indentification

Mass Spectrometric Isotope Identification

200Hg

202Hg

201Hg

199Hg

198Hg

196Hg

204Hg

How many atoms are in 1 g of copper?

How many atoms are in 1 g of copper? If Cu has an atomic mass of 63.546 amu, how does this easily convert to grams?

Avogadro’s Number (the mole) relates atomic mass to sample mass:

NA = 6.022 X 1023 = 1 mole

1 mole of atoms = mass in grams equal to the atomic mass

How many atoms are in 1 g of copper? If Cu has an atomic mass of 63.546 amu, how does this easily convert to grams?

Avogadro’s Number (the mole) relates atomic mass to sample mass:

NA = 6.022 X 1023 = 1 mole

1 mole of atoms = mass in grams equal to the atomic mass

The molar mass, M, is the mass of one mole of a substance.

A charged species is called an ion.

Metals tend to lose electrons to form positively charged ions – cations.

Nonmetals tend to gain electrons to form negatively charged ions – anions.

When forming compounds, the metal and nonmetal ions combine so that the net charge on a formula unit is zero.

Ions and Ionic Compounds

The atomic charge = protons – electrons

Ions and Ionic Compounds

Ionic compounds are held together by the electrostatic attraction of opposite charges - this is also known as coulombic attraction. This attraction is an ionic bond.

Ionic compounds behave as an aggregate, not individual molecules

Predicting Ionic Charges

Main group elements tend to lose or gain electrons in order to equal the electron count of the nearest noble gas

Predicting Ionic Charges

Main group elements tend to lose or gain electrons in order to equal the electron count of the nearest noble gas

Transition Metals Are Able To Form Multiple Cations: +1, +2, +3 and/or +4