Atomic Models:Developing the Structure

of the Atom

Atomic Models:Developing the Structure

of the Atom

The Greek ModelThe Greek Model Around 440 BC, Leucippus originated the

atom concept. He and his pupil, Democritus, refined and extended it in future years.

Five major points to their atomic idea. – All matter is composed of atoms, bits of matter too

small to be seen. These atoms CANNOT be broken apart

– There is a void (empty space) between atoms– Atoms are completely solid– Atoms of different materials are different in size,

shape, and weight

Around 440 BC, Leucippus originated the atom concept. He and his pupil, Democritus, refined and extended it in future years.

Five major points to their atomic idea. – All matter is composed of atoms, bits of matter too

small to be seen. These atoms CANNOT be broken apart

– There is a void (empty space) between atoms– Atoms are completely solid– Atoms of different materials are different in size,

shape, and weight

The Dalton Model (1803)The Dalton Model (1803) John Dalton of England introduced the

atomic idea to “Modern” chemistry The basic ideas in Dalton's atomic theory:

– Chemical elements are made of atoms – The atoms of an element are identical – Atoms of different elements have different

masses – Atoms only combine in small, whole number

ratios such as 1:1, 1:2, 2:3 and so on.

John Dalton of England introduced the atomic idea to “Modern” chemistry

The basic ideas in Dalton's atomic theory:– Chemical elements are made of atoms – The atoms of an element are identical – Atoms of different elements have different

masses – Atoms only combine in small, whole number

ratios such as 1:1, 1:2, 2:3 and so on.

Dalton’s Atom, 1807Dalton’s Atom, 1807

The atom is a solid indivisible sphere that can undergo chemical reactions but is unchanged by the reaction.

The Thomson Model,1903 The Thomson Model,1903 In 1897, J.J. Thomson discovered

the first subatomic particle, the electron, using a cathode ray tube.

First model showing an atom that is divisible (has smaller parts).

In 1897, J.J. Thomson discovered the first subatomic particle, the electron, using a cathode ray tube.

First model showing an atom that is divisible (has smaller parts).

Plum Pudding Model

The nucleus was discovered.

The Gold Foil Experiment

The Rutherford ModelThe Rutherford Model1. The atom contains a tiny dense center:

“nucleus” – the nucleus is about 1/10 trillionth the volume of

the atom

2. The nucleus contains almost the entire mass of the atom.

3. The nucleus is positively charged.

4. The electrons move around in the empty space (the void?) surrounding the nucleus

1. The atom contains a tiny dense center: “nucleus” – the nucleus is about 1/10 trillionth the volume of

the atom

2. The nucleus contains almost the entire mass of the atom.

3. The nucleus is positively charged.

4. The electrons move around in the empty space (the void?) surrounding the nucleus

Rutherford’s Atom1908

Rutherford’s Atom1908

3p+

e-

e-

e-

The Bohr Model (1913)The Bohr Model (1913) Niels Bohr applies

mathematics to Rutherford's model. This eventually led to the idea of energy levels for the electrons.

Led scientists to the understanding that valence electrons, those in the highest energy level, determine chemical reactivity.

Niels Bohr applies mathematics to Rutherford's model. This eventually led to the idea of energy levels for the electrons.

Led scientists to the understanding that valence electrons, those in the highest energy level, determine chemical reactivity.

Chadwick (1935)Chadwick (1935)

Hypothesized that the mass of the nucleus could not be explained by the masses of protons alone.

Be foil experiment discovered neutrons, particles that had mass but no charge.

Hypothesized that the mass of the nucleus could not be explained by the masses of protons alone.

Be foil experiment discovered neutrons, particles that had mass but no charge.

Subatomic ParticlesSubatomic Particles

3 protons

4 neutrons

3 electrons

Lithium

Particle Charge Mass (kg) LocationProton 1 1.67E-27 nucleus

Neutron 0 1.67E-27 nucleus

Electron -1 9.11E-31moving around the

outside of the nucleus

Chemical versus Nuclear ReactionsChemical versus Nuclear Reactions

Chemical NuclearStable atoms (Not radioactive)

Electrons exchanged between atoms

Atoms are conserved (unchanged but rearranged)

Reactions are affected by temperature and pressure

Radioactive nuclei = unstable, which decays

Nucleus ejects/absorbs neutrons, protons, electrons.

Atoms change into new elements/isotopes! Atomic # or mass # changes

Reactions NOT affected by temperature or pressure.