Atoms, Ions, & Nomenclature

History of Atomic TheoryDemocritus 460-370 B.C.: proposes idea of

matter being made up of small, indivisible particles (atoms)

Antoine Lavoisier 1743-1794: Law of Conservation of Mass

Joseph Proust 1754-1826: Law of Constant Composition (Law of Definite Proportion)

John Dalton 1766-1844: Law of Multiple Proportion & Dalton’s Atomic Theory

Dalton’s Atomic TheoryEach element is made up of tiny, indivisible

particles called atoms (indivisible disproved)The atoms of a given element are identical

(disproved); the atoms of different elements are different in some fundamental way or ways.

Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms.

Chemical reactions involve reorganization of the atoms—changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

19th & 20th CenturiesWilliam Crookes: Cathode Ray Tube; negative

particle exist; e-

J.J. Thomson: Cathode ray deflection; mass/charge ratio; e-

Robert Millikan: Oil Drop; charge; e-

Ernest Rutherford: Gold Foil; small, dense nucleus present; + nucleus

James Chadwick: proved the existence of neutrons

Niels Bohr: proposed the idea that the atom is made up of a nucleus containing p+ and n0 that was being orbited by e-s in orbits (disproved)This particle model of the e- and atom was

expanded a few years after Bohr’s ideas to include the wave nature of electrons

Charge Mass Position

Proton +1 1 amu nucleus

Neutron 0 1 amu nucleus

Electron -1 1/1836 amu Outside nucleus

Periodic TableIsotope NotationAtomic #s

Referred to as “Z”# of p+

For neutral atoms, also # of e-

Mass #s Referred to as “A” # of p+ + # of n0

IsotopesIsotopes: atoms with same # of protons and

electrons, but different # of neutronsLeads to modification of Dalton’s Atomic Theory

All atoms of the same element contain the same number of protons and electrons, but may have different numbers of neutrons

Since it is the electrons in atoms that affect chemical properties of a substance, isotopes of the same element have the same chemical properties

Mass Numbers are not integersAtomic mass of Cl is 35.5 and can be

“represented” by the following symbol: 35.5ClDoes not mean 17 p+, 17 e-, and 18.5 n0

Not possible to have a fraction of a neutronNon-integer means there is more than 1

isotope of Cl that exists in nature, 35Cl and 37ClThe Cl isotopes exist naturally in the following

abundance: 35Cl = 75% and 37Cl = 25%

Average = Σ(% of each isotope • atomic mass of each isotope

Atomic Mass 100

Chlorine Example:

Average Atomic mass = ((35)(75%)) + ((37)(25))

100

= 35.5 amu

RadioactivityThe spontaneous decay of certain atoms with

the evolution of alpha, beta, gamma, and positron particles. The radiation comes from the nucleus (it is a nuclear reaction)

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Alpha α

Beta β Gamma γ

Helium nucleus

Essentially electons

Always products

Electromagnetic

radiationHigh

energyHigh

Frequency

Charge +2 -1 0

Mass 4 1/1840 0

Movement

To neg. plate

To pos. plate

None

Penetration

LeastStoppe

d bypaper

Intermediate

Stopped by

lead or glass

GreatestThick layers

oflead or

concrete

Types of Radioactive DecayAlpha Emission

42He or 4

2αRestricted to heavy nucleiBoth protons and neutrons need to be reduced

in order to stabilize the nucleus.Example:

21884Po → 4

2He + 21482Pb

Both mass number and atomic number change

Radioactive DecayBeta Emission

0-1β

In order to decrease the number of neutrons, a neutron can be converted into a proton & an electron.

10n → 1

1p + 0-1β

An electron is emitted from the nucleus as a β particle. 14

6C → 147N + 0

-1 β Mass of the nucleus doesn’t change, only the

atomic number. β particles are associated with elements above the

band of stability. They are always found on the product side of the

rxn.

Radioactive DecayPositron Emission

0+1β

Decreases the # of protons by converting into a neutron by emitting a positron

11p → 1

0n + 0+1β

Has the same mass as an electron, but (+) charge.

Example:38

19K → 3818Ar + 0

+1β

Radioactive DecayElectron Capture

0-1e

Too many protons in the nucleusInner orbital electrons are captured by nucleusElectron + proton = neutron

0-1e + 1

1p → 10n

Electron capture will always be found on the reactant side.

Example:37

18Ar + 0-1e → 37

17Cl

Radioactive DecayGamma Emission

00γ

Highest energy; electromagnetic wavesHas no chargeNever see these rays (frequency is too high)Most powerful

Half LifeThe half-life of a radioactive nucleus is the

time taken for half the atoms to decayIt is independent of the initial quantity of

atoms. There are 3 methods of determining half-life:GraphicallyUse of the expressions (see next slide)Use of expression

Fraction of remaining activity = 1/2n

Where n = # of ½ lives

it is easy to determine how much of a sample remains after a whole number of half lives

it is more difficult to determine how much remains when a complete half life has not passed

in order to do this, you need to apply two different equations

ln Nt/No= -kt and k= 0.693/t1/2

Nt is the amount of substance left after a given time

No is the original amount of the substance

t is the amount of time that has passed

t1/2 is the half life of the substance

Transmutation of Elements

Possible by nuclear reactions to artificially produce elements

ExampleAlpha Bombardment: 14

7N + 4 2α 1

1H + 17

8OAccelerated heavier nuclei:

250 98Cf + 11 5B 257 103Lr + 4 1

0n

(bombard little one w/ big one)

Mass Deficit (Mass Defect)When atoms are formed by the combination

of p+, n0, and e-….the mass of the atom is found to be less than that of the sum of the individual particles

Contradicts law of conservation of massWhen particles combine, a small amount of

the mass is converted to energy (binding energy) and released to the surroundings

E = mc2

Predicting StabilityStable nuclei tend to have neutron-proton

ratios close to 1:1 or atomic numbers below 83

Zone of Stability is a ratio of 1 – 1.5Nuclei with higher ratios tend to want to

lower the ratio by converting a neutron to a proton and e-

Electrons are then released as β particles

Nuclear Fission vs. FusionFission Heavy nuclei capturing neutrons, splitting to

form other, smaller nuclei and releasing more neutrons

Large amounts of energy can be released, leading to a potential chain reaction

Fusion Combination of smaller nuclei into larger

ones with the release of energy Less easy to perform since they involve the

combination of two nuclei that are positively charged and therefore repel one another

Uses of RadioactivityMedicine: 133I for thyroid and brain imaging

67Ga for lung functionIsotopic datingThickness control in engineeringLeak detectionNuclear Fission (power and atomic bomb):

Uranium nuclei can be bombarded with neutrons and converted to other nuclei

MoleculesFormed when a definite number of atoms are

joined together by chemical bondsCan consist of atoms of one element or atoms

of many different elements, but always in a fixed proportion Molecules can be elements or compounds

Usually formed between non-metal elementsFormulas show the number of each type of

atom present written as subscriptsH2, N2, O2, F2, Cl2, I2,

Br2

elements diatomic

H2O compound polyatomic

NH3 compound polyatomic

IonsWhen atoms lose or gain e-s, particles

become chargedPositive ions (cations): number of p+ is

greater than the number of e-

Negative ions (anions): number of e- is greater than the number of p+

Metals form cations; non metals form anionsOppositely charged ions form ionic

compounds by attracting one another

Na+ cation monoatomic

Cl- anion monoatomic

CO32- anion polyatomic

NH4+ cation polyatomic

Nomenclature of Inorganic CompoundsBinary compounds (ionic compounds)

Formed between 2 elementsmetal and non metal

To find the formula, positive and negative charges must be balanced

To name: the unmodified name of the positive ion is written first followed by the root of the negative ion with the ending modified (-ide)

Transition metals can carry more than 1 charge, so when writing the name of the compound, parentheses must be shown to indicate the charge

Binary AcidsFor nomenclature purposes, an acid can be

defined as a compound that produces hydrogen ions when dissolved in water

Formed when hydrogen ions combine with monoatomic anions

To name: use prefix hydro-followed by the other nonmetal name modified to an –ic ending Example: HCl = Hydrochloric acid

Polyatomic ions and oxoanionsPolyatomic ions are those where more than one

element is combined to create a species with a charge

Polyatomic ions where oxygen is combined with another non-metal are called oxoanions

Certain non-metals (Cl, N, P, and S) form a series of oxoanions containing different numbers of oxygen atomsHypo(element) ite increasing # of

(Element) ite oxygen atoms(Element) ate

Per(element) ate

Some oxoanions contain hydrogen and are named accordingly, example: HPO4

2-, hydrogen phosphate

Prefix (thio-) means that a sulfur atom has replaced an atom of oxygen in an anion

To name an ionic compound that contains a polyatomic ion, the unmodified name of the positive ion is written first followed by the unmodified name of the negative ion

Example: K2CO3 , potassium carbonate

OxoacidsOxoacids are formed when hydrogen ions

combine with polyatomic oxoanionsGives a combination of hydrogen, oxygen, and

another non-metalTo name: use the name of the oxoanion and

replace the (-ite) ending with (-ous) or the (-ate) ending with (-ic). Then add the word acid. Example: H2SO4 , hydrogen sulfate becomes

Sulfuric acid

HClO ClO- Hypochlorite

Hypochlorous acid

HClO2 ClO2- Chlorite Chlorous acid

HClO3 ClO3- Chlorate Chloric acid

HClO4 ClO4- perchlorate

Perchloric acid

Binary Compounds of 2 non-metalsTwo non-metals combine, then the compound is

molecularTo name: unmodified name of 1st element is

followed by the root of the 2nd element with the ending (-ide)

In order to distinguish between compounds of the same element, use prefixes mono, di, tri, tetra, penta, ….. Example: SO2 , sulfur dioxide

HydratesIonic formula units with water associated with themWater molecules are incorporated into the solid

structure of the ionsTo name: use the normal name of the ionic

compound followed by the term hydrate with an appropriate prefix to show the number of water molecules per ionic formula. Example:

CuSO4•5H2O , copper (II) sulfate pentahydrate

Strong heating can generally drive off the water in these salts. With water removed, they become anhydrous

Mass SpectrometerMost direct way to determine the atomic and

molecular weightsWhat Happens?

A gas is introduced and bombarded by a stream of high energy electrons (vaporization)

Collisions between the electrons and the atoms or molecules of the gas produce positive ions, mostly with a +1 charge (ionization)

Ions are accelerated toward a (-) charged wire grid (acceleration)

After they pass through grid, they come to 2 slits that only allow a narrow beam of ions to pass @ a given time: no magnets

This beam pass through 2 magnetic poles which deflect the ions into a curved path (deflection) •extent of curve depends on mass

(high mass = small deflection)•ions are separated by masses

By changing the strength of the magnetic field or acceleration voltage on grid, ions of varying masses can be selected to enter detector @ end of instrument (detection)