Atoms and Elements

Mr. Matthew TotaroLegacy High SchoolHonors Chemistry

Experiencing Atoms• Atoms are incredibly small, yet they compose

everything.• Atoms are the pieces of elements.• Properties of the atoms determine the properties

of the elements.

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Experiencing Atoms•There are about 91 elements found in

nature.Over 20 have been made in laboratories.

•Each has its own, unique kind of atom.They have different structures.Therefore they have different properties.

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Early Theories of Matter

• The ancient Greeks believed that all matter was composed of only four elements: earth, air, water, and fire.

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Ancient Greek Theory of Matter

Four Elements of the Ancient Greeks

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First Atomic Theory

• Democritus (c.a. 460 BC) was the first thinker ever to reason that matter was not infinitely divisible.

• He called the smallest piece of matter ‘atomon’, which in Greek means ‘not cutable’.

Democritus

Aristotle

The Ancient Greeks

• Aristotle (c.a. 380 BC) reasoned, however, that matter was infinitely divisible.

• Aristotle’s ideas agreed more closely with those of the Christian church, therefore, they were accepted as fact for over two millenia.

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Enlightenment Thinking

• Robert Boyle (c.a. 1661) wrote a book entitled The Sceptical Chymist. In it, Boyle argues that the four classical elements of the Greeks were actually not elements at all.

• He also argued that matter was composed of indivisible particles or atoms.Robert Boyle

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The First Modern Atomic Theory• After two hundred

years of experimental results, the first modern atomic theory was developed by John Dalton (c.a. 1805).

• Dalton reasoned that indivisible, spherical particles made up all of matter. He also argued that these particles were rearranged during a chemical reaction.

John Dalton

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Dalton’s Original Theory of the Atom and of Reactions

Dalton’s Atomic Theory1. Each Element is composed of tiny,

indestructible particles called atoms. Tiny, hard, indivisible, spheres.

2. All atoms of an element are identical. They have the same mass, volume, and other

physical and chemical properties. So, atoms of different elements are different. Every carbon atom is identical to every other

carbon atom. They have the same chemical and physical

properties. However, carbon atoms are different from

sulfur atoms. They have different chemical and physical properties.

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Dalton’s Atomic Theory3. Atoms combine in simple,

whole-number ratios to form molecules of compounds.

Because atoms are unbreakable, they must combine as whole atoms.

The nature of the atom determines the ratios in which it combines.

Each molecule of a compound contains the exact same types and numbers of atoms. Chemical formulas

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Discovery of the Subatomic Particles

• J. J. Thomson (c.a. 1897) performed experiments involving cathode rays that proved that atoms could be broken down into smaller particles.

• Thompson called the particle that he discovered an ‘electron’, which means unit of electricity in Greek.J. J. Thomson

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Cathode Ray Tube (Discovery of the Electron)

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Thomson’s Results• the cathode rays are made of tiny particles

• these particles have a negative chargebecause the beam always deflected toward

the + plate

• the amount of deflection was related to two factors, the charge and mass of the particles

• every material tested contained these same particles

• Thomson called these particles ‘electrons’, because they were the components of all electricity

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Some Notes on Charges

• There are two kinds of charges, called positive and negative.

• Opposite charges attract.+ attracted to –.

• Like charges repel.+ repels +.– repels –.

• To be neutral, something must have no charge or equal amounts of opposite charges.

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Thomson’s Plum Pudding Atom

• the structure of the atom contains many negatively charged electrons

• these electrons are held in the atom by their attraction for a positively charged electric field within the atomthere had to be a source of positive

charge because the atom is neutralThomson assumed there were no

positively charged pieces because none showed up in the cathode ray experiment

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Predictions of the Plum Pudding Atom

•the mass of the atom is due to the mass of the electrons within itelectrons are the only particles in Plum

Pudding atoms

•the atom is mostly empty spacecannot have a bunch of negatively

charged particles near each other as they would repel

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Radioactivity

• in the late 1800s, Henri Becquerel and Marie Curie discovered that certain elements would constantly emit small, energetic particles and rays

• these energetic particles could penetrate matter• Ernest Rutherford discovered that there were three

different kinds of emissionsalpha, a, particles with a mass 4x H atom and + chargebeta, b, particles with a mass ~1/2000th H atom and –

chargegamma, g, rays that are energy rays, not particles

Radioactivity• Three types of radiation were

discovered by Ernest Rutherford: particles particles rays

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Discovery of the Nucleus

• Ernest Rutherford (c.a. 1908) utilized radioactivity in the gold foil experiment to prove that the plum-pudding model of the atom was wrong.

• In so doing, he discovered the atomic nucleus and the proton.

Ernest Rutherford

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The Gold-Foil Experiment

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Rutherford’s Results•Over 98% of the particles went

straight through•About 2% of the particles went

through but were deflected by large angles

•About 0.01% of the particles bounced off the gold foil“...as if you fired a 15” cannon shell at a

piece of tissue paper and it came back and hit you.”

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Rutherford’s Conclusions

•Atoms were mostly empty spacebecause almost all the particles went straight

through

•Atoms contain a dense particle that was small in volume compared to the atom but large in mass because of the few particles that bounced back

•This dense particle was positively chargedbecause of the large deflections of some of the

particles

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Nuclear Atom

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Plum PuddingAtom

if atom was likea plum pudding, all the particles

should go straight through

most particles go straight through

some particles go through, but are deflected

a few of the particles

do not go through

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Rutherford’s Interpretation – the Nuclear Model

1) The atom contains a tiny dense center called the nucleus the amount of space taken by the nucleus is only about

1/10 trillionth the volume of the atom

2) The nucleus has essentially the entire mass of the atom the electrons weigh so little they give practically no

mass to the atom

3) The nucleus is positively charged the amount of positive charge balances the negative

charge of the electrons

4) The electrons are dispersed in the empty space of the atom surrounding the nucleus (electron cloud)

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Structure of the Atom• Rutherford proposed that the

nucleus had a particle that had the same amount of charge as an electron but opposite signbased on measurements of the nuclear

charge of the elements

• these particles are called protonscharge = +1.60 x 1019 C mass = 1.67262 x 10-24 g

• since protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons

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The Nuclear Atomic Model

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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

2.2

“If the atom is the Cowboys Stadium, then the nucleus is a marble on the 50-yard line.”

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Atomic Theory from 1800-1911

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Some Problems•How could beryllium have 4 protons stuck

together in the nucleus?shouldn’t they repel each other?

• If a beryllium atom has 4 protons, then it should weigh 4 amu; but it actually weighs 9.01 amu! Where is the extra mass coming from?each proton weighs 1 amuremember, the electron’s mass is only about

0.00055 amu and Be has only 4 electrons – it can’t account for the extra 5 amu of mass

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There Must Be Something Else There!

• to answer these questions, Rutherford proposed that there was another particle in the nucleus – it is called a neutron

•neutrons have no charge and a mass of 1 amumass = 1.67493 x 10-24 g

slightly heavier than a proton

no charge

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Subatomic

Particle

Mass

g

Mass

amu

Location

in atom

Charge Symbol

Proton 1.67262

x 10-24

1.00727 nucleus +1 p, p+, H +

E lectron 0.00091

x 10-24

0.00055 empty

space

-1 e, e-

Neutron 1.67493

x 10-24

1.00866 nucleus 0 n, n0

The Sub-Atomic Particles

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Elements•each element has a unique number of

protons in its nucleusthe number of protons define the element

• the number of protons in the nucleus of an atom is called the atomic numberthe elements are arranged on the Periodic

Table in order of their atomic numbers

•Each element also has a corresponding mass number.Protons + neutrons.

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Elements• Each element has a unique name and symbol.

The symbol is either one or two lettersOne capital letter or one capital letter + one lower case letter.

H = Hydrogen = “water-former”Br = Bromine = ‘stench’

Liquid Bromine

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The Periodic Table of Elements

Atomic number

Atomic mass

Element symbol

Atomic number (Z) = number of protons in nucleus

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

= atomic number (Z) + number of neutrons XA

Z

U23892

Mass Number

Atomic NumberElement Symbol

2.3

Elemental & Isotope Notation

Isotope Notation = Uranium-238

Example: How many protons, electrons, and neutrons are in an

atom of ?

for most stable isotopes, n0 > p+Check:

Z = 24 = # p+

# e- = # p+ = 24

Solution:

in neutral atom, # p+ = # e-

mass number = # p+ + # n0

Concept Plan:

Relationships:

therefore A = 52, Z = 24# p+, # e-, # n0

Given:Find:

Cr5224

Cr5224

symbol atomicnumber

# p+ # e-

symbol atomic & massnumbers

# n0

A = Z + # n0

52 = 24 + # n0

28 = # n0

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Isotopes:• Isotopes = atoms of the same element with different

masses.

• Isotopes have different numbers of neutrons.

116C

126C

136C

146C

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Carbon-11 Carbon-12 Carbon-13 Carbon-14

Neon

9.25%221210Ne-22 or

0.27%211110Ne-21 or

90.48%201010Ne-20 or

Percent natural abundance

A, mass number

Number of neutrons

Number of protonsSymbol

Ne2010

Ne2110

Ne2210 41

Mass Spectrometer

Tro: Chemistry: A Molecular Approach, 2/e

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Mass Spectrum•A mass spectrum is

a graph that gives the relative mass and relative abundance of each particle

•Relative mass of the particle is plotted in the x-axis

•Relative abundance of the particle is plotted in the y-axis

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By definition: 1 atom 12C “weighs” 12 amu

1 amu = 1.6606 x 10-24 g

On this scale

1H = 1.008 amu

16O = 16.00 amu

Atomic mass is the mass of an atom in atomic mass units (amu)

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Atomic Mass

Average Atomic Mass

•Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations.

•Average atomic mass is calculated from the isotopes of an element weighted by their relative abundances.

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Natural lithium is:

6Li (6.015 amu) = 7.42%

7Li (7.016 amu) = 92.58%

(7.42 x 6.015) + (92.58 x 7.016) / 100

= 6.941 amu

Average atomic mass of lithium:

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Charged Atoms•The number of protons determines the

element.All sodium atoms have 11 protons in the

nucleus.

• In a chemical change (aka: a reaction), the number of protons in the nucleus of the atom doesn’t change, however the number of electrons may.

•Atoms in a compound that gain or lose electrons and have a charge, these are called ions.

Types of IonsCation – ion with a positive charge

If a neutral atom loses one or more electronsit becomes a cation.

Anion – ion with a negative chargeIf a neutral atom gains one or more electronsit becomes an anion.

Na 11 protons11 electrons

Na+ 11 protons10 electrons

Cl 17 protons17 electrons

Cl-17 protons18 electrons

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Ions of Hydrogen

Ions

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A monatomic ion contains only one atom Na+, Cl-, Ca2+, O2-, Al3+, N3-

A polyatomic ion contains more than one atom

OH-, CN-, NH4+, NO3

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Nitrate Polyatomi

c Ion

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Practice—Fill in the Table.

Ion p+ e-

Cl-

K+

S2-

Sr2+

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Practice—Fill in the Table, Continued.

Ion p+ e-

Cl1- 17 18

K1+ 19 18

S2- 16 18

Sr2+ 38 36

Example —Find the Number of Protons and Electrons in Ca2+.

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Review• What is the atomic number of boron, B? • What is the atomic mass of silicon, Si? • How many protons does a chlorine atom have? • How many electrons does a neutral neon atom

have? • Will an atom with 6 protons, 6 neutrons and 6

electrons be electrically neutral? • Will an atom with 27 protons, 32 neutrons, and

27 electrons be electrically neutral? • Will an Na atom with 10 electrons be

electrically neutral?