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The atomic theory

John Dalton (1776–1864).Democritus first suggested that matter may be made of indivisible particles = atoms. John Dalton provided the first scientific atomic theory (based on experiments) supporting the idea of "atoms" (1805).

Dalton's atomic theory stated that:

• Matter is made of solid "marble-like" particles (atoms).
• The mass of the atom decides what the properties of the atoms are.

Thanks to Dalton's new theory, elements can be said to be made of atoms. Jöns Jacob Berzelius compiles a table with the relative masses of the then-known atoms (1818), which are later used by Dmitri Mendeleev to organize the periodic table (1869).

Jöns Jacob Berzelius (1779–1848).

Dmitri Mendeleev (1834–1907).

Henri Becquerel

Studies uranium salts (1896).

• Discovers that a photographic plate is exposed by the uranium.
• Not caused by x-rays!

Henri Becquerel (1852–1908).

Marie Curie (1867–1934).

Marie Curie

Characterizes the radiation from e.g. radium (1896).

• Coins the term ”radioactivity”.
• The atom is not indivisible!

Nobel prize in physics 1903 (together with her husband Pierre, and Henri Becquerel).

J. J. Thomson

Uses a cathode ray tube to discover and characterize the electron (1898).

(a) Thomson's cathode ray tube experiment. (b) When a voltage is applied, a beam of particles produces shadow from the cross in the center. (c–d) A magnet bends the ray, indicating that the beam is made of charged particles.

Thomson's "plum pudding" model.

The "plum pudding" model

The electrons are spread throughout the atom, like plums in a plum pudding.

Ernest Rutherford

The gold foil experiment

Alpha particles beamed at a thin gold foil.

• Thomsons "plum pudding" model predicted: Particles should pass right through.
• Reality: About 1/8000 particles bounced of a very dense, solid nucleus.

Rutherford's gold foil experiment. A beam of alpha particles ③ was fired toward a thin gold foil ⑤. Most of the alpha particles went straight through the foil, hitting the fluorescent screen ④ at the back, but a few bounced off ⑦, indicating the atom had a solid nucleus.

According to Rutherford's atomic model, most of the mass is found in the atomic nucleus, and the electrons circulate around the nucleus like planets in the solar system.

Rutherford's atomic model

• A very small, dense, and positively charged nucleus.
• Electrons circle around, like ”planets”.

Niels Bohr

Problem with Rutherford's model: The electrons should immediately fall back into the nucleus.

The visible spectrum of hydrogen.

When an electron moves from an outer shell to an inner, light of a certain wavelength (energy) is emitted.

• Bohr studied the emission spectrum from hydrogen.
• Only certain colors (energies) are emitted ⇒ The electrons may only reside in certain energy levels (electron "shells").

Beyond Bohr's model

• In reality, the electron shells are not that simple.
• The electrons are not negatively charged ”marbles”, but rather ”smeared” in time and space.
• Electron clouds rather than electron shells.

Protons and neutrons

Protons discovered by Rutherford (1917).

• Problem: How can the protons stay in the nucleus?
• There must be some kind of ”glue”!

Neutrons discovered by Rutherford's student, James Chadwick (1932).

Summary: The building blocks of the atom

Electrons

• Charge: –1 (e^–)
• In different energy levels (shells) around the nucleus




Protons

• Charge: +1 (p⁺)
• In the nucleus

Neutrons

• Charge: 0 (n)
• In the nucleus

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