Atoms: The Building Blocks of Matter

Chapter 3

MatterAnything that

occupies space and has mass

AtomsTiny particles too

small to seeMolecules

Atoms bonded together

States of Matter Solid

Fixed Volume Rigid Shape

Liquid Fixed volume Assume the shape of the container

Gas Atoms separated by large

distances Assume the volume and shape

of the container

CompositionPure substance

One type of atom or molecule

MixtureTwo or more

types of atoms or molecules

MixturesHeterogeneous Mixture

Two or more regions with different composition

Homogeneous MixtureUniform composition

ElementsCannot be broken into

simpler substancesCompounds

Composed of two or more elements

Antoine LavoisierLaw of Conservation

of Mass“Matter is neither

created nor destroyed”

The transformation of a substance or substances into one or more new substances is known as a chemical reaction.

Law of conservation of mass: mass is neither created nor destroyed during ordinary chemical reactions or physical changes

Total amount of matter remains constant in a chemical reaction

58 grams of butane burns in 208 grams of oxygen to form 176 grams of carbon dioxide and 90 grams of water.

58 grams + 208 grams = 176 grams + 90 grams

266 grams = 266 grams

EnergyLaw of

Conservation of Energy

“Energy is neither created nor destroyed”

Law of definite proportions: a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound

Law of multiple proportions: if two or more different compounds are composed of the same two elements, then 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

1.1. All matter is composed of extremely All matter is composed of extremely small particles called atoms.small particles called atoms.

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

3.3. Atoms cannot be subdivided, created, Atoms cannot be subdivided, created, or destroyed.or destroyed.

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

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

Not all aspects of Dalton’s atomic theory have proven to be correct. We now know that:

• Atoms are divisible into even smaller Atoms are divisible into even smaller particles.particles.

• A given element can have atoms with A given element can have atoms with different masses.different masses.

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

• All matter is composed of atoms.All matter is composed of atoms.

Even though the two shapes look different, the characteristics of the various parts that compose them are the same.

The same is true with the atom.Though atoms of different elements

display different properties, isolated subatomic particles have the same properties.

Think about it….

An atom is the smallest particle of an element that retains the chemical properties of that element.

The nucleus is a very small region located at the center of an atom.

The nucleus is made up of at least one positively charged particle called a proton and usually one or more neutral particles called neutrons.

Surrounding the nucleus is a region occupied by negatively charged particles called electrons.

Protons, neutrons, and electrons are often referred to as subatomic particles.

Cathode Rays and Electrons:

• Experiments in the late 1800s showed that cathode rays were composed of negatively charged particles.

• These particles were named electrons.

Charge and Mass of the Electron

• Joseph John Thomson’s cathode-ray tube experiments measured the charge-to-mass ratio of an electron. Plum-pudding model of an atom.

• Robert A. Millikan’s oil drop experiment measured the charge of an electron.

• With this information, scientists were able to determine the mass of an electron.

More detail of the atom’s structure was provided in 1911 by Ernest Rutherford and his associates Hans Geiger and Ernest Marsden.

The results of their gold foil experiment led to the discovery of a very densely packed bundle of matter with a positive electric charge.

Rutherford called this positive bundle of matter the nucleus.

Except for the nucleus of the simplest type of hydrogen atom, all atomic nuclei are made of protons and neutrons.

A proton has a positive charge equal in magnitude to the negative charge of an electron.

Atoms are electrically neutral because they contain equal numbers of protons and electrons.

A neutron is electrically neutral.

Properties of Subatomic Properties of Subatomic ParticlesParticles

The nuclei of atoms of different elements differ in their number of protons and therefore in the amount of positive charge they possess.

Thus, the number of protonsnumber of protons determines that atom’s identity.

Forces in the Nucleus:• When two protons are extremely close to

each other, there is a strong attraction between them.

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

• The short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together are referred to as nuclear forces.

The radius of an atom is the distance from the center of the nucleus to the outer portion of its electron cloud.

Because atomic radii are so small, they are expressed using a unit that is more convenient for the sizes of atoms.

This unit is the picometer, pm.

Atoms of different elements have different numbers of protons.

Atoms of the same element all have the same number of protons.

The atomic number (Z) of an element is the number of protons of each atom of that element.

•The mass number is the total number of protons and neutrons that make up the nucleus of an isotope.

Isotopes are atoms of the same element that have different masses.

The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons.

Most of the elements consist of mixtures of isotopes.

Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element.

Calculating Weighted Average Atomic MassThe average atomic mass of an element depends

on both the mass and the relative abundance of each of the element’s isotopes.

Imagine that your semester grade depends 60% on exam scores and 40% on laboratory explorations.

Your exam scores would count more heavily toward your final grade.

The atomic mass of an element is a weighted average of the masses of the naturally occurring isotopes of that element.

If you had three sumo wrestlers each weighing 500 pounds and one ballerina weighing 100 pounds, what is the average weight?

A weighted average would be (3 X 500 + 1 X 100)/4 = 400(small sumo wrestlers or large ballerinas)

The chlorine atom has two isotopes. One of the isotopes weighs 35 and other isotope weighs 37. Since they are both chlorine atoms, they each have 17 protons (the atomic number for chlorine). One of the isotopes has 18 neutrons and the other isotope has 20 neutrons. The average weight ofchlorine is 35.5. Why?

17

ClChlorine35.5

The isotope that weighs 35 comprises 75% of the atoms of chlorine and the isotope that weighs 37 comprises the other 25%. Convert the percentages to decimal fractions and proceed:

0.75 X 35 + 0.25 X 37 = 35.5 That is the weighted average of atomic

weights.

Gallium has two naturally occurring isotopes: Ga-69 with mass 68.9256 amu and a natural abundance of 60.11% and Ga-71 with mass 70.9247 amu and a natural abundance of 39.89%. Calculate the atomic mass of gallium.

Solution: Convert the percent natural abundance into decimal form.

Ga-69 0.6011Ga-71 0.3989

Atomic Mass

Atomic Mass = 0.6011 (68.9256 amu) + 0.3989 (70.9247 amu)

= 69.72 amu

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.

235 92 U

The number of neutrons is found by subtracting the atomic number from the mass number.

mass number − atomic number = number of neutrons

235 (protons + neutrons) − 92 protons = 143 neutrons

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

Sample ProblemHow many protons, electrons, and neutrons are there in an atom of chlorine-37?

Solution: atomic number = number of protons = number of electronsmass number = number of neutrons + number of protons

The standard used by scientists to compare units of atomic mass is the carbon-12 atom, which has been arbitrarily assigned a mass of exactly 12 atomic mass units, or 12 amu.

One atomic mass unit, or 1 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.

Stores sell nails by the poundEasier than selling

individuallyTimely

How many nails in a pound?Need a certain number for

a projectAnalogy

How many atoms in a given mass of an element?

Atoms are too small to count

A hardware store customer buys 2.60 pounds of nails. A dozen of the nails has a mass of 0.150 pounds. How many nails did the customer buy?

Solution map:

2.60 lb. x 1 doz x 12 nails = 208 nails

0.150 lbs 1 doz.

The customer bought 2.60 lbs of nails and received 208 nails. He counted the nails by taking a mass. If he purchased a different size nails, the mass of 1 dozen of them would change.

The mass of the same idea works for atoms of different elements. Different elements have different masses.

Mole6.022 x 1023

Avogadro’s NumberConvenient number for

dealing with atomsMole vs. Dozen

Certain number of objectsAtoms

AmedeoAvogadro1776-1856

There are other numbers similar to the mole that we use all of the time:pair = twodozen = 12score = twentyGross = twelve dozen or 144and least but not last:MOLE = 6.02 X 1023

This is called Avogadro’s number after the man who discovered this number.

There are 6.02 X 1023 atoms in one mole of atoms

There are 6.02 X 1023 molecules in one mole of molecules

There are 6.02 X 1023 elephants in one mole of elephants

There are 6.02 X 1023 doughnuts in one mole of doughnuts

There are 6.02 X 1023 “thingys” in one mole of “thingys” !

1 mole of atoms = 6.022 X 1023 atomsConvert 3.5 moles of He to He atoms

3.5 moles X 6.022 X 1023 atoms = 2.1 X 1024 atoms

1 mole of atoms

Nail AnalogyMass of nailsNumber of nails

Molar MassConverts number of atoms to massMolar mass in grams per moleCopper

63.55 amu63.55 grams per mole

Sulfur32.06 amu32.06 grams per mole (g/mol)6.022 X 1023 atoms

Carbon12.01 amu12.01 grams per mole (g/mol)6.022 X 1023 atoms

Gram/Mole Conversions

4.00 g He2.00 mol He = 8.00 g He

1 mol He

• Chemists use molar mass as a conversion factor in chemical calculations.

• For example, the molar mass of helium is 4.00 g He/mol He.

• To find how many grams of helium there are in two moles of helium, multiply by the molar mass.

Sample Problem:What is the mass in grams of 3.50 mol of the element copper, Cu?

Sample Problem:A chemist produced 11.9 g of aluminum, Al. How many moles of aluminum were produced?

Sample Problem:How many moles of silver, Ag, are in 3.01 1023 atoms of silver?

Carbon and Diamond: How many carbon atoms in 0.58 grams of diamond?(Use conversion factors to solve.)

grams moles atoms

0.58 g X 1 mol X 6.022 X 1023 atoms = 2.9 X 1022 atoms 12.01 g 1 mol

Sample ProblemWhat is the mass in grams of 1.20 108 atoms of copper, Cu?