Atoms, Molecules and Atoms, Molecules and IonsIons

Chapter 2Chapter 2

Atomic Theory of MatterAtomic Theory of Matter

The theory that atoms are the fundamental The theory that atoms are the fundamental building blocks of matter reemerged in the early building blocks of matter reemerged in the early 19th century, championed by John Dalton.19th century, championed by John Dalton.

Dalton's PostulatesDalton's Postulates

Each element is composed of extremely small Each element is composed of extremely small particles called atoms.particles called atoms.

(pg 38)(pg 38)

Dalton's PostulatesDalton's Postulates

All atoms of a given element are identical to one All atoms of a given element are identical to one another in mass and other properties, but the another in mass and other properties, but the atoms of one element are different from the atoms of one element are different from the atoms of all other elements.atoms of all other elements.

Dalton's PostulatesDalton's Postulates

Atoms of an element are not Atoms of an element are not changed into atoms of a different changed into atoms of a different element by chemical reactions; element by chemical reactions; atoms are neither created nor atoms are neither created nor destroyed in chemical reactions.destroyed in chemical reactions.

Law of Conservation of Mass

Dalton’s PostulatesDalton’s Postulates

Compounds are formed when atoms of Compounds are formed when atoms of more than one element combine; a more than one element combine; a given compound always has the same given compound always has the same relative number and kind of atoms.relative number and kind of atoms.

J.J. Thomson, measured charge/mass of e-

(1906 Nobel Prize in Physics)

Figure 2.4

Fig. 2.3(b)

Fig. 2.3(c)

The Atom, circa 1900

• The prevailing theory was that of the “plum pudding” model, put forward by Thompson.

• It featured a positive sphere of matter with negative electrons imbedded in it.

Figure 2.5

Once the charge/mass ratio of the electron was known, determination of either the charge or the mass of an electron would yield the other.

Millikan’s Oil-drop Experiment

Measured mass of e-

(1923 Nobel Prize in Physics)

(Uranium compound)

Figure 2.8

1. Atom’s positive charge is concentrated in the nucleus2. Proton (p) has opposite (+) charge of electron (-)

particle velocity ~ 1.4 x 107 m/s(~5% speed of light)

(1908 Nobel Prize in Chemistry)

Figure 2.10

The Nuclear Atom

• Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom.

• Most of the volume of the atom is empty space.

atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Rutherford’s Modelof the Atom

Subatomic Particles (Table 2.1)

Particle Charge Relative Mass

(amu)

electron (e-) -1 ≈ 0

proton (p) +1 1

neutron (n) 0 1

Atomic number (Z) = number of protons in nucleus

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

= atomic number (Z) + number of neutrons

Isotopes - atoms of the same element (X) with different numbers of neutrons in their nuclei

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

How many protons, neutrons, and electrons are in C146 ?

How many protons, neutrons, and electrons are in C116 ?

6 protons, 8 (14 - 6) neutrons, 6 electrons

6 protons, 5 (11 - 6) neutrons, 6 electrons

Do You Understand Isotopes?

Average MassAverage Mass

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

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

The Three Isotopes of HydrogenThe Three Isotopes of Hydrogen

99.9885% 0.0115% Trace%

H1

1.00794

Periodicity

When one looks at the chemical properties of elements, one notices a repeating pattern of reactivities.

Fig 2.15

Period

Group

Alkali M

etal

Noble G

as

Halogen

Alkali E

arth Metal

Figure 2.16