Atoms: The Building Blocks of Matter

Dalton’s Atomic Theory (1803)

1. All matter is made of atoms.

2. Atoms of the same element are identical. Atoms of different elements differ.

3. Atoms cannot be divided, created, nor destroyed.

4. Atoms combine in simple whole number ratios to form compounds.

5. Atoms are rearranged, separated, or combined in chemical reactions.

Dalton’s model of atom (1803)

Solid, Indivisible Sphere of Matter

J.J. Thomson & Cathode Ray (1897)

J.J. Thomson & Cathode Ray (1897)

• Rays repelled by negatively charged object.

• Charge to mass ratio same regardless of type of metal or gas used in the tube

• Concluded that identical negative particles are in all atoms. Named them electrons.

• Since atoms are neutral, they must also contain a positive charge.

J J Thomson’s Model

Millikan & Oil Drop Experiment (1909)

Millikan & Oil Drop Experiment (1909)

• Measured charge of electron

• Allowed mass of electron to be determined from charge and charge to mass ratio.

• Mass of electron much smaller than mass of atom, so the atom must contain other particles to account for most of the mass.

Rutherford and Gold Foil Experiment (1911)

Result of experiment

Results and Conclusions

• Most alpha particles went through foil while a few bounced back.

• Atom is mostly empty space with a dense, concentrated positive mass in the center.

• Electrons occupy space around the nucleus.

Modification of plum-pudding model

Prediction of what wouldoccur with plum-pudding model.

Explanation for why a few alphaparticles bounced back.

Rutherford’s Nuclear Model (1911)

James Chadwick and Neutron Experiment (1932)

Results

• Found new type of particle in the atom that was electrically neutral – the neutron.

• Determined mass of this particle.– Slightly larger than proton

• Mass of nucleus finally accounted for.

Chadwick Model (1932)

Structure of the Atom

+

-

Atomic Number = # of protons

Atomic Mass = weighted average of all isotopes measure in amu’s.

Subatomic Particles

• The proton is 1800 times bigger than the electron.

Particle SymbolRelative Electric

ChargeRelative Mass

(amu)Location in the

atom

Electron e- -1 0.0005486 Around nucleus

Proton p+ +1 1.007276 Part of nucleus

Neutron no 0 1.008665 Part of nucleus

Atomic Structure

• Atoms are on the order of 10-10 m.– 100 million atoms lined up would be 1 cm

long.

• The nucleus is very small compared to size of the atom.

• If the atom was the size of a football stadium…

• The nucleus would be the size of a marble.

Closer to scale model of atom

Isotopes of carbon

Isotopes

• Atoms of the same element with different masses.

• Atomic number = number of protons

• Mass number = number of protons + number of neutrons

Isotopes

Mass Number

Hyphen Notation

Nuclear SymbolAtomic Number

Atomic mass vs. molar mass

• Same number off of the periodic table but different units

  Atomic Mass Molar Mass

Mass of: 1 atom of an element 1 mole of an element

Measured in: amu (atomic mass unit) grams

Weighted averagesFind the average atomic mass of copper.

Cu-63, 69.15%, 62.93 amuCu-65, 30.85%, 64.93 amu

Solution:0.6915 x 62.93 amu = 43.52 amu0.3085 x 64.93 amu = 20.03 amu

43.52 amu + 20.03 amu

63.55 amu