Atoms, Ions and Molecules The Building Blocks of Matter Chapter 2

Atoms, Ions and MoleculesThe Building Blocks of Matter

Chapter 2

Chapter Outline

2.1 The Rutherford Model of Atomic Structure

2.2 Nuclides and Their Symbols

2.3 Navigating the Periodic Table

2.4 The Masses of Atoms, Ions, and Molecules

2.5 Moles and Molar Mass

2.6 Making Elements

2.7 Artificial Nuclides

Experiments in Atomic Structure

• J. J. Thompson (1906 Nobel Prize in Physics) - cathode ray tube experiments; discovery of the electron; measurement of the charge-to-mass ratio.

• Robert Millikan (1923 Nobel Prize in Physics) - oil-drop experiments; measured the mass of the electron, therefore calculate the charge

• Ernest Rutherford (1908 Nobel Prize in Physics) - gold-foil experiments; the nuclear atom

• James Chadwick (1935 Nobel Prize in Physics) - discovery of the neutron

J.J. Thomson Cathode Ray Tube Experiments - Electrons

Results of “Cathode Ray” Experiments

• Travel in straight lines• invisible• independent of cathode composition• bend in a magnetic field like a

negatively-charged particle would• charge/mass = -1.76 x 108 C/g

Thompson’s “Plum Pudding” Model of the Atomelectrons distributed throughout a diffuse, positively charged sphere.

Robert Millikan’s oil drop Experiment - measured the mass of the electron

Millikan’s Results• The air molecules in the chamber were ionized by a beam of X-rays,

producing electrons and positively-charged fragments

• Fine mist of oil introduced into chamber; electrons adhere to the

droplets

• Negatively-charged droplets settle to bottom of chamber under

influence of gravity

• Charged repeller plates adjusted until droplets were suspended in

mid-air

• From the physics and knowledge of the size of the gravitational and

electrostatic forces, the charge on each droplet could be calculated

• Discovered that each droplet was a whole-number multiple of 1.60 X

10-19 C, so the mass = 9.11 X 10-28 g

Radioactivity and the Nuclear AtomSpontaneous emission of particles and/or radiation from a

decaying, unstable nucleus

-particles =

-particles =

-rays =

Ernest Rutherford - the nuclear atom

Rutherford's Observations

1. the majority of particles penetrated undeflected

2. some particles were deflected at small angles

3. occasionally -particles scattered back at large angles

b) Expected results from “plum pudding” model.

c) Actual results.

Rutherford’s Conclusions

• The atom is mainly empty space because most of the -particles passed through undeflected

• The nucleus is very dense and positively charged because some of the -particles were repulsed and deflected

• Electrons occupy the space around the nucleus

• The atom is electrically neutral

atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Rutherford’s Model of the Atom

If the nucleus was the size of an orange, then the radius of the atom would be 2.5 miles

mass p mass n = 1840 x mass e-

Chapter Outline






2.6 Making Elements


Atomic Mass Units

• Atomic Mass Units (amu)

• Comprise a relative scale to express the masses of atoms and subatomic particles.

• Scale is based on the mass of 1 atom of carbon:

» 6 protons + 6 neutrons = 12 amu.

• 1 amu = 1 Dalton (Da)

Isotopes: Experimental Evidence

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

Isotopes (nuclides) are atoms of the same element (X) with different numbers of neutrons in the nucleus

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

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Mass Number

Atomic NumberElement Symbol

• Use the format AX to write the symbol for the nuclides having 28 protons and 31 neutrons.

Practice: Isotopic Symbols

• Collect and Organize:• Analyze:• Solve:• Think about It:

• Complete the missing information in the table.Practice: Identifying Atoms and Ions

• Collect and Organize:• Analyze:• Solve:• Think about It:

Chapter Outline






2.6 Making Elements


Mendeleev’s Periodic Table

Dmitrii Mendeleev (1872):

• Ordered elements

by atomic mass.

• Arranged elements in

columns based on similar

chemical and physical

properties.

• Left open spaces in the table

for elements not yet

discovered.

The Periodic Table of the Elements

The Modern Periodic Table

• Also based on a classification of elements in terms of their physical and chemical properties.

• Horizontal rows: called periods (1 → 7).

• Columns: contain elements of the same family or group (1 →18).

• Several groups have names as well as numbers.

Navigating the Modern Periodic Table – Groups and Families

Groups of Elements (cont.)

These 7 elements occur naturally as diatomics (memorize) -

H2 N2 F2 O2 I2 Cl2 Br2

Metals

• found to the left of the “diagonal line”

• lose electrons in chemical reactions

• solids (except for Hg, Cs, and Fr)• conduct electricity• ductile (draw into a wire)• malleable (roll into sheets)• form alloys ("solid-solution" of

one metal in another)

Nonmetals

• found to the right of the “diagonal line”• like to gain electrons from metals, or

share electrons among themselves• found as solids, liquids (Br), and gases

(Inert gases, H, N, O, F, Cl)• “diatomics” - H2, N2, F2, O2 ,I2, Cl2, Br2

• oxygen also exist as ozone, O3

• insulators (except for graphite or C)

Helium-Neon lasers

Metalloids

• elements next to the “diagonal line”

• B, Si, Ge, As, Sb, and Te• physical properties of a

metal (can be “convinced” to conduct electricity) and chemical properties of a nonmetal

Elemental Si is used in the semiconductor

industry

Chapter Outline






2.6 Making Elements


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AM = (mass 1)(abn) + (mass 2)(abn) + (mass 3)(abn) +………

Average Atomic Mass

Weighted average mass of natural sample of an element, calculated by multiplying the natural abundance of each isotope by its exact mass in amu’s and then summing up these products.

One Mole of:

C S

Cu Fe

Hg

Molecular Mass – the sum of the average atomic masses of the atoms in it.

NOTE: the terms mass and weight are used interchangeably, e.g. molecular weight (MW) or atomic weight (AW)

Molecular Mass

e.g. H2SO4

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Formula Units – for ionic compounds, the smallest electrically neutral unit in an ionic compound

Formula Mass – the sum of the average atomic masses of the cations and anions that make up a neutral formula unit

Formula Units and Formula Mass

e.g. NaCl

Chapter Outline






2.6 Making Elements


38

The Mole - The mole is the Chemist’s counting unit

Avogadro’s Number (NA) = 6.022 X 1023

= 1 mole of atoms, molecules, ions, etc.

pair = 2

dozen = 12

gross = 144

ream = 500

Experiment – how many atoms must be added together so that the mass in grams = mass in amu’s?

Analogy using coins:

Mass ratio = 1 : 5 : 25

Significance of the Mole

NA of carbon atoms weighs __________

NA of iron atoms weighs__________

Mass in amu’s

Mass in grams/mole

Equivalent to

• To convert between number of particles and an equivalent number of moles.

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Moles, Mass, and Particles

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Sample Exercise 2.5

The silicon used to make computer chips has to be extremely pure. Fpr example, it must contain less than 3 x 10-10 moles of phosphorus (a common impurity in Si) per mole of silicon. What is this level of impurity expressed in atoms of phosphorus per mole of Si?

e.g. carbon

e.g. H2SO4 sulfuric acid

Using the Molar Mass as a Conversion Factor for Atoms & Molecules

mole

s12.011grams12.011gram

mole 1

mole

grams 98.0grams 98.0

mole 1

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Moles, Mass, and Particles

grams of atoms or

molecules

moles of atoms or

molecules

Numbers of atoms or

molecules

How many moles of K atoms are present in 19.5 g of potassium?

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Practice: Mole Calculations #1

How many moles are present in 58.4 g of chalk (CaCO3)?

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The uranium used in nuclear fuel exists in nature in several minerals. Calculate how many moles of uranium are found in 100.0 grams of carnotite, K2(UO2)2(VO4)2•3H2O.

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Atoms, Ions and Molecules The Building Blocks of Matter Chapter 2