Ch. 3Atoms: The Building Blocks of Matter

Table of Contents

Chapter 3 Atoms: The Building Blocks of Matter

Section 1 The Atom: From Philosophical Idea to Scientific Theory Section 2 The Structure of the Atom

Section 3 Counting Atoms

•Young people should not smoke.

•Smoking at an early age may make it more difficult to quit smoking later.

Which of the above statements is an opinion and which is a theory?

Theories Vs. Opinions

Democritus (460 BC)

Can matter be divided forever?

Atom• From the Greek word atomos

• “Not to be cut”• “Indivisible”

Chemical Reactions• Process in which the physical and chemical properties of the original

substances change as new substances with different physical and chemical properties are formed.

• What could be a shorter definition?

The Law of Conservation of Mass

*Mass can be neither created nor destroyed during a chemical reaction.

Law of Definite Proportions• Law of definite proportions: compounds contain the same

elements in exactly the same proportions by mass

• Example: Sodium Chloride – NaCl

– 39.34% sodium and 60.66% chlorine

Law of Multiple Proportions

The ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers

Dalton’s Atomic Theory• All matter is composed of extremely small particles called atoms.

• Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.

• Atoms cannot be subdivided, created, or destroyed.

• Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

• In chemical reactions, atoms are combined, separated, or rearranged.

Modern Atomic Theory• Not all aspects of Dalton’s atomic theory have

proven to be correct. We now know that:

Some important concepts remain unchanged.

• Atoms are divisible into even smaller particles.

• A given element can have atoms with different masses.

• All matter is composed of atoms.

• Atoms of any one element differ in properties from atoms of another element.

Structure of an atom• Subatomic particles

– Smaller than an atom

– 1. Proton +– 2. Neutron =– 3. Electron -

http://www.youtube.com/watch?v=-FWxd78sOZ8

In search of giants video

Discovery of the Electron• *1897 English physicist Joseph John Thomson

– *Performed cathode ray tube experiments and discovered electron

– a. Cathode ray tube produces a ray with a constant charge to mass ratio

– b. All cathode rays are composed of identical negatively charged particles (electrons)

Inferences from the properties of electrons

You know = 1. Atoms are neutral2. Electrons are negative3. Electrons have little mass

• What does this mean for the rest of the atom?

The Ernest Rutherford Experiment• *1911 – performed Gold foil experiment to discover atomic

nucleus

1. Alpha particles (helium nuclei) fired at a thin sheet of gold– a. Assumed that the positively charged particles were bounced back if they approached a

positively charged atomic nucleus head-on (Like charges repel one another)

2. Very few particles were greatly deflected back from the gold sheet -a. nucleus is very small, dense and positively charged -b. most of the atom is empty space

Question

• Electrons are attracted to the positive nucleus.

• How does the nucleus hold together if it is a bunch of positive charges?

Composition of the Atomic Nucleus

• When two protons are extremely close to each other, there is a strong attraction between them.

Forces in the Nucleus

*nuclear forces – hold the protons and neutrons together

A similar attraction exists when neutrons are very close to each other or when protons and neutrons are very close together.

2.3 Counting Atoms

• 1. Atomic Number• 2. Isotopes• 3. Mass Number• 4. Designating Isotopes

Atomic Number

• the number of protons of each atom of that element.

Isotopes• Isotopes are atoms of the same element that have different

masses.– Different number of ______________

• Nuclide is a general term for a specific isotope of an element.

Mass Number• total number of protons and neutrons that make

up the nucleus of an isotope.

Designating Isotopes• Hyphen notation: The mass number is written with a

hyphen after the name of the element.• uranium-235

• *Nuclear symbol: The superscript indicates the mass number and the subscript indicates the atomic number.

• What would the nuclear symbol for carbon-13 be?• 6 protons and 7 neutrons

235 92 U

Calculating Protons, Electrons and Neutrons

• General Rules

Atomic number = number of protons = number of electrons

Mass number = number of neutrons + number of protons

Announcements

• Vocab Quiz Today

Ch. 3 ArticleTwo New Elements Join the Periodic Table

• 5 sentence summary – 10 points– Includes topic sentence and 4 key points

• 4 sentence opinion – 10 points– Should elements that are only around for less than a second be added to

the table?– Why? Why not?

– Since I did not ask you a personal question (I didn’t use you) you are not allowed to use I in the paragraph.

• MLA format– Align left– Name– Teacher Name– Class– Date 9 October 2012

Article Discussion

• Who agreed?

• Who disagreed?

Relative Atomic Mass• Mass of an atom in grams is very small so we use a standard

measurement.

*1 atomic mass unit (amu), is exactly 1/12 the mass of a carbon-12 atom.

• The atomic mass of any atom is determined by comparing it with the mass of the carbon-12 atom.

Average Atomic Mass

*the weighted average of the atomic masses of the naturally occurring isotopes of an element.

Calculating Average Atomic Mass

Copper consists of 69.15% copper-63, which has an atomic mass of 62.929 601 amu, and 30.85% copper-65, which has an atomic mass of 64.927 794 amu.

The average atomic mass of copper can be calculated by multiplying the atomic mass of each isotope by its relative abundance (expressed in decimal form) and adding the results.

• (0.6915 62.929 601 amu) + (0.3085 64.927 794 amu) =

Relating Mass to Numbers of AtomsMole (mol)

Avogadro’s number - 6.022 1415 1023—is the number of particles in exactly one mole of a pure substance.

-amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12.

SI unit for amount of substance.

Molar MassThe mass of one mole of a pure substance is called the

molar mass of that substance.-written in units of g/mol. -molar mass = atomic mass (amu)

Solving Mole Problems

For Today

1. Work on your study guide

2. Work on Mole practice problems sheet

*Tomorrow – Mole-atoms-grams calculation quiz

ReviewSection 1

1. 3 Laws2. Dalton’s Atomic Theory3. Modern Atomic Theory

Section 21. Structure of an atom2. J.J Thompson & Ernest Rutherford

Section 31. Atomic Number2. Isotopes3. Mass Number4. Moles