Chapter 3

Chemistry

The Atom

I. Early ideasA. ___________ (450 BC)- all matter is

made of tiny indivisible particles – atomos._________- the smallest particle of an element

that retains the properties of the element.B. ____________________- (late 1700’s)

1. Law of Conservation of Matter

Early Ideas Cont:

C. __________- (1799) – Law of Constant Composition

Atoms form cmpds in specific ratios determined by mass.

Early Ideas Cont:

D. _________________ (1803) English1. Atomic Theory of Matter

a.) Each element is composed of atoms (Democritus)

b.) All atoms of an element are the same; atoms of different elements are different

c.) Atoms are not created or destroyed in rxns. (Lavoisier)

d.) Cmpds have the same relative # and kinds of atoms. (Proust)

Early Ideas Cont:

E. __________________ (late 1700’s) a) electric charges: like repel; opposites

attract

F. ____________(1839) - atoms contain particles with electric charges

G. ____________________ (1870’s) –a. Crookes’ Tube exp.

Early Ideas Cont:

H. _______(1896) – Cathode ray tubes

I. _____________ (1896) – radioactivity

J. ________________ (1909) – Oil Drop Exp.

K. ___________________ (1909) – Gold Foil Exp. (Alpha scattering exp.)

II. Early ExperimentsA. ____________________ – key experiment 1. Results – two kinds of charge: + & - like

charges repel; opp. attract

B. Crookes’ Tube Experiment 1. Partially evacuated tube filled w/a low pressure

gas. 2. 2 electrodes inside – anode (+) cathode (-)

Early Exp. Cont:

3. When voltage was applied, a green glow originated in the tube.

a) produced a shadow at the end opposite the cathode; it had to originate at the cathode CATHODE RAY; the tube was then called a CATHODE RAY TUBE.

4. Was it light or charged particles?? a) Beam was deflected by a magnet, it had to be a

stream of charged particles. www.chem.uidaho.edu/~honors/crookes.html http//library.thinkquest.org/28582/history/dalton.htm

Early Exp. Cont:

C. Thomson’s Exp.1. Build a CRT with a fluorescent screen at

end opposite the cathode.Glowed where particles struck.

2. Added a magnet and a pair of charged plates.

a) the beam was 1st deflected by the magnet, but was attracted to the + charged plate; therefore the particles had to have a – charge.

Early Exp. Cont:

3. Changing the electrodes and the gases used always yielded the same results; the particles were identical and were found in all atoms. These particles were called _______________________.

Early Exp. Cont:

D. The PROTON1. If atoms had – particles, they also had to

have + particles, because atoms were electrically neutral.

2. Discovered through exp. w/CRTs.3. 1885- Thomson used H gas in a CRT.

When voltage was applied, the ray moved to the cathode (-); therefore it had to be positive.

Early Exp. Cont:

4. The amount of deflection varied with the different gases.

5. _____________________- electrons were randomly distributed throughout a + charged “gel”.

Early Exp. Cont:

E. Becquerel’s discovery- __________1. Placed a piece of uranium on a

photographic plate and placed it in a dark drawer.

2. Later found the plate had been exposed by some unknown type of radiation originating from the U.

Early Exp. Cont:

F. Ernest Rutherford- radioactivity1. Through exp. determined the 3 types of

radiation

G. Ernest Rutherford (1909)-1. Gold Foil Exp. (alpha-scattering Exp.)

a) Aimed a beam of high speed α particles (+) at a very thin piece of gold foil.

b) Almost all particles passed through

Early Exp. Cont:

c) Some particles had slight deflectionsd) Very few particles reversed direction and

came straight back.e) Results: All + charge is concentrated at the

center of the atom – __________________. 1. Credited with the

___________________________ of the ATOM.

III. Atomic Structure

A. Nucleus1. + Charged2. Composed of p+ (+) and n (no chg)

B. Electron Cloud1. e- travel around nucleus in discrete

energy levels2. In a neutral atom, the # e- = the # p +

IV. Atomic Numbers

A. Discovered by Henry Moseley1. Atoms of @ element have a unique +

chg. on the nucleus

B. The identity of the element depends on the # p+ in nucleus ATOMIC NUMBER

V. Ions

A. Ions- an electrically charged atom

B. Formed when atoms gain or lose e-; or share e-

C. ATOMS NEVER LOSE PROTONS

D. If gain e- = - chg; If lose e- = + chg

Isotopes

________________ – atoms of the same element with the same number of protons but different numbers of neutronsEx: H-1, H-2, H-2 C-12, C-13, C-14

Atomic Mass

Atoms are too small to see individually

They are also too small to mass with normal everyday lab balances

So, how is the mass of a SINGLE atom measured????

Atomic Mass

Atomic Mass Unit – the mass of a single proton or neutron

Protons and neutrons have essentially the same mass – they make up 99% of the mass of the atom

Therefore, if we add those numbers up we get the mass of a single atom

Atomic Mass

Scientists chose to define an atomic mass unit in terms of an arbitrary standard – the C-12 atom

1 atom of C-12 is exactly 12 amu

So, why are the atomic masses listed on the periodic table not whole numbers?

Atomic Mass

_____________________ – the fractional occurrence in nature of a specific isotope of an element

_________________________ – the weighted average of all the isotopes of an element.

Atomic Mass

Example: