Chapter 2

Atoms, Molecules, and Ions

History of Atomic Theory

• Started with the Greeks and four elements (earth, air, water and fire)• Democritus termed “atomos”• Aristotle defined elements• Robert Boyle provided an experimental definition of elements• Lavoisier is the Father of Modern Chemistry

Laws of Chemistry• Law of Conservation of Mass

– Mass cannot be created nor destroyed

• Law of Definite Proportions– Compounds have a definite composition– Compounds react in ratios by mass

• Law of Multiple Proportions– Elements combine in small whole number ratios– Water vs. hydrogen peroxide

• 2 H with 1 O vs. 2 H with 2 O

Dalton’s Atomic Theory

1.Each element is made up of atoms2.Atoms of a given element are

identical; atoms of different elements are different

3.Chemical compounds form when atoms combine. Given compounds always have the same relative numbers

4.Chemical reactions involve the rearrangement of atoms (atoms not changed)

Helpful Observations

• Gay-Lussac observed under the same conditions of temperature and pressure, compounds react in whole number ratios by volume

• Avogadro developed Avogadro’s hypothesis that at the same temperature and pressure, equal volumes of different gases contain the same number of particles

Experiments of the Atom

• J.J. Thomson used cathode ray tubes

• Thomson’s experiment

Voltage source

+-

Thomson’s experiment

Voltage source

Passing an electric current makes a beam appear to Passing an electric current makes a beam appear to move from the negative to the positive end.move from the negative to the positive end.

+-

Thomson’s experiment

Voltage source

+

-

By adding an electric field, he found that the By adding an electric field, he found that the moving pieces were negativemoving pieces were negative

Thomson’s ModelFound the electron.Couldn’t find positive

(for a while). Said the atom was

like plum pudding.A bunch of positive

stuff, with the electrons able to be removed.

Radioactivity

• Discovered by accident• Bequerel• Three types

alpha- helium nucleus (+2 charge, large mass)

Was used in early experiments about the atom

beta- high speed electrongamma- high energy light

Rutherford’s Experiment

• Used alpha particles to shoot at gold foil• Hypothesized alpha particles should go straight through the foil• Found particles were directed in many directions, including back in the original direction• Used gold foil because it could be made atoms thin

Lead block

Uranium

Gold Foil

Florescent Screen

Rutherford’s Experiment

Rutherford’s Experiment

What he expected

Rutherford’s Experiment

Because, he thought the mass was evenly distributed in the atom.

Rutherford’s Experiment

What he got

Rutherford’s Experiment

•How Rutherford explained it– Atom is mostly empty.– Small dense, positive piece at center.– Alpha particles are deflected by it if they get close enough.– Stated his results could only be explained through a nuclear atom (dense ceter)

Modern Atomic View

• Two main components to an atom– Nucleus

• Protons are positively charged• Neutrons have no charge• Contains almost all the mass of an atom

– Electron cloud • Electrons are negatively charged• Contains most of the space of an atom

Symbols for elements Z - atomic number = number of protons determines type of atom. A - mass number = number of protons + neutrons. Number of protons = number of electrons if neutral.

XAZ Na23

11

Chemical bonds The forces that hold atoms together. Covalent bonds share electrons making molecules.

Chemical formula- the number and type of atoms in a molecule.

C2H6 - 2 carbon atoms, 6 hydrogen atoms, Structural formula shows the connections, but not necessarily the shape.

Ions Atoms or groups of atoms with a charge.

Cations- positive ions - get by losing electrons(s).Anions- negative ions - get by gaining electron(s).

Ionic bonding- held together by the opposite charges. Ionic solids are called salts.

MetalsConductors

Lose electrons

Malleable and ductile

NonmetalsBrittle

Gain electrons

Covalent bonds

Semi-metals or Metalloids

Naming Ionic Compounds

1. Cation is named first, anion named second

2. Monatomic cation takes name of the element

3. Monatomic anion has root of element name and -ide suffix

4. Polyatomic ions must be memorized (p. 62)

5. For example, sodium chloride and potassium iodide

Naming Ionic Compounds1. Cations with more than one

charge must have the charge designated

2. Use a Roman numeral to indicate the charge

3. Examples are:CuCl - copper (I) chloridePbS - lead (II) sulfide

Naming covalent compounds

1. Name the first element first2. Name the second element as if it

were an anion3. Use prefixes to denote the

number of each element (except mono on the first element)

4. Prefixes are on page 63

Naming acids1. Binary acids use the prefix

hydro and name the second as if an anion

2. With oxyacids, depends on the number of oxygenIf anion ends in -ate, use the suffix -icIf anion ends in -ite, use the suffix -ous