CHAPTER 2Atoms, Molecules, and Ions

History of the Atom

Democritus (400 BC)

• Proposed that matter was composed of tiny, invisible particles.

• Gr. atomos

Aristotle

• Said that all substances are composed of four elements: Earth, Wind, Fire, and Water.

John Dalton (1803 AD)• the first coherent atomic theory

• Matter is composed of small particles called atoms.

• All atoms of an element are identical.

• During chemical reactions, atoms are neither created nor destroyed, but are simply rearranged.

J.J. Thompson (1897 AD)

• Used cathode rays to calculate the atomic mass of atoms.

Cathode Ray Tube Experiment

Ernest Rutherford (1910 AD)

• alpha particles (He nuclei) were shot at a thin piece of gold foil.

• most of the particles passed through the foil with little or no deflection, some were deflected to a great degree.

Gold Foil Experiment

History400 -370 BC - Democritus thought that there

must be atoms, “invisible particles”.

384-322 BC – Aristotle refused this theory.

1700 – Isaac Newton again favored the idea of smaller invisible particles.

1800 – John Dalton formed the atomic theory.

Dalton’s Atomic Theory Elements composed of small particles called

atoms. All atoms of the same element are identical in

physical properties but different from atoms of other elements.

Atoms of one element can not change into atoms of different elements with chemical reactions.

Compounds are composed of atoms of different elements and are consistent in number and type of elements.

Dalton’s Atomic TheoryVocabulary Atom – the smallest particle of an element that

retains the chemical identity of the element.

Compound – contains atoms of two or more elements.

Conservation of matter – atoms can neither be created nor destroyed in a chemical reaction.

Continuing History1897 – J. J. Thomson – using a cathode ray determined

the presence of negative particles, electrons, and the “plum pudding” model.

1911 –Ernest Rutherford – using alpha particles through gold foil determined electrons were not evenly spaced and determined the presence of a nucleus.

1919 – Rutherford- determined the presence of protons.

1932 – James Chadwick – determined the presence of neutrons.

Modern Atomic StructureAtoms consist of subatomic particles:

Particle Charge Mass (amu)

Location

Proton Positive 1.0073 Nucleus

Neutron Neutral 1.0087 Nucleus

Electron negative 5.486 x 10-4

Electron cloud

Atomic NumberThe number of protons in an atom of an

element.

Each element has a different atomic number or number of protons.

Each element has no charge.

Each element has the same number of electrons as protons to keep neutral.

Periodic Table of Elements 1750 only 17 elements 1800 – 31 elements 1865 – 63 elements Today – 117 elements

Antoine Lavoisier – categorized elements into metals, nonmetals, gases, and earths

Dmitri Mendeleev (Russia)- 1865 – categorized 63 elements according to atomic weight along with Lothar Meyer (Germany).

Modeled chart of elements after the solitaire card game.

Arranged the elements into rows in the order of increasing mass so that elements with similar properties were in the same column like suits in the card game.

Within columns, atomic masses increased from top to bottom leading to the periodic table.

Mendeleev

Mendeleev Since many elements were still

undiscovered, he left gaps in the chart where he thought the undiscovered elements should be.

The structure of the table lead to the prediction and discovery of gallium which had similar properties as aluminum.

1913 Rutherford’s nuclear model of the atom lead to atomic numbers, verification of Mendeleev’s table.

Periodic Table Organized by increasing atomic number. Basic Info:

17 = atomic numberCl = symbol

35.453 = atomic mass

17Cl

35.453

OrganizationTable is configured into: Periods – rows on the periodic table. Groups – columns on the table with elements

in the same group having similar physical properties.

Further organized into: Metals Metalloids Nonmetals

OrganizationMetals good conductors of heat and electricity Malleable (hammered into thin sheets) Ductile (drawn into thin wires) Lustrous (shiny)

Nonmetals Poor conductors Mostly gases If metal then brittle

Metalloid Demonstrate both metal and nonmetal properties

OrganizationGroups Group 1 – Alkali metals Group 2 – Alkaline earth metals Group 7 – Halogens Group 8 – Noble gases Group B – Transition metals

OrganizationAlkali metals – very reactive with water

and oxygen. They have low densities and melting points. They all have 1 valence electron so readily give away 1 electron in s orbital.

Ex. Sodium and potassium react violently with water such that they will react with the water in human skin igniting the hydrogen molecules and burn the skin.

OrganizationAlkaline earth metals – have 2 valence

electrons. Differences in reactivity along these elements is shown by the ways they react with water. More dense and higher melting temperatures

Ex. Calcium, strontium and barium react easily with water. Magnesium reacts with hot water. Beryllium has no reaction in water.

OrganizationHalogens – highly reactive with metals. They

all have 7 valence electrons.

Nobel gases – mostly nonreactive colorless, odorless gas that give off different colors when excited.

Ex. Helium-pinkNeon-orange/redArgon-lavenderKrypton-whiteXenon-blue

Atomic & Ionic Radii Trends Columns of periodic table – radius increases

from top to bottom.

Periods of periodic table – radius decreases from left to right.

Cations have smaller radii than parent elements.

Anions have larger radii than parent elements.

MoleculeAssembly of two or more atoms tightly bound

together with no net charge.Chemical formula - representation of the number

and type of elements in a compound.H2O, CO2, CH4

Molecular formula– actual chemical formula of a molecule indicating the actual number of molecules of each atom

Empirical formula – simplified formula of a molecule indicating the smallest ratio of atoms of elements.

Writing Chemical Formulas1. Each atom (element) is represented

by it symbol.2. The number of atoms of each

element is represented by a subscript.

3. When the number of atoms is 1 then the subscript is not written and understood to be one.

Chemical FormulasExamples of formulas

Ca3(PO4)3 Al(NO3)3 H2SO4 3CO2

Diatomic moleculesMolecules that exist is pairs

Hydrogen Nitrogen Oxygen Fluorine Chlorine Bromine Iodine

IonsA charged entity produced by taking a neutral

atom and adding or removing one or more electrons.

Cation-positively charged particle due to the removal of an electron.K+ , Mg2+

Anion-negatively charged particle due to the acceptance of an electron (tend to be nonmetals).Cl- , S2-

How many electrons are contained in each of the following ions?

Ba2+

P3-

Sn2+

Cl-

Ions

There must be both positive ions (cations) and negative ions (anions) present.

The numbers of cations and anions must have a net charge of zero or the sum of the oxidation numbers is zero.

Cation is first then the anion.

Writing Formulas

Ex. Na and Cl Na+ Cl- → NaClCharge: +1 Charge: -1 Net = 0Oxidation number = charge

Ex. Mg and Cl

Mg2+ Cl - Cl- → MgCl2Charge: +2 2 x (-1) net =0

Writing Formulas

Calcium and chlorine Sodium and sulfur Lithium and nitrogen Phosphorus and Calcium Barium and oxygen Sulfur and Aluminum Potassium and phosphorus

Writing Formulas

Writing Formulas Ionic Compound – a molecule that contains both

a metal and a nonmetallic elements.NaClMgCl2

Molecular Compound – a molecule that contains only nonmetals.CO2

CH4 Polyatomic ions – atoms joined as a molecule

with a charge.

Writing Formulas

Iron (II) and chlorine Cooper (I) and fluorine Calcium and hydroxide Chromium (II) and peroxide Hydrogen and oxygen Hydrogen and phosphate

NomenclatureNaming cations: Metals with single charge – cation is

metal name with “ion” added Metal with multiple charge – cation is

metal name with charge indicated by Roman numeral and “ion” added.

Polyatomic – nonmetal name with “ium” replacing end.

NomenclatureNaming anions: Monatomic – element name with end

replaced with “ide” . Polyatomic –

With oxygen (oxyanion)– element name with end replaced with “ate”.

With hydrogen & oxyanion – polyatomic name prefix “hydrogen” added.

Nomenclature Naming ionic compounds: Cation is named

first using cation name then anion name. Naming molecular compounds:

1. Name element further to left first.2. If same group, name higher atomic number first.3. Anion is named using anion name (ide)4. Atoms are indicated by following prefixes:

One – mono six - hexa Two – di seven - hepta Three – tri eight - octa Four – tetera nine - nona Five - penta ten - deca

NomenclatureNaming acids: If anion name ends in “ide” change to

“ic” and add “hydrogen” to name. These are molecules with no oxygen.

If anion name ends in “ate” change to “ic” and add “acid”.

If anion name ends in “ite” change to “ous” and add “acid”.

Naming Flow Chart

Is it Ionic or molecular?

Ionic Molecularnonmetals

Transition metalWith multiple charges? Use prefixes to tell

number of each atom present

YesMetal – roman numeral

Nonmetal – “ide” endingOr polyatomic ion name

NoMetal – metal name

Nonmetal – “ide” endingOr polyatomic ion name

Isotopes

Radioactive Decay

C14 Decay

Carbon Dating

Carbon Dating VideoClick on the picture above to view.