the history of atomic theory

The history of atomic theory

Antoine Levoisier(1753-1794)

Joseph Proust(1754-1826)

John Dalton(1766-1844)

J. J. Thomson(1856-1940)





J. J. Thomson(1856-1940)

Thomson’s Atomic Model

• Electrodes can be made out of many elements, so the atoms of elements must contain negatively charged particles.

• Matter is (normally) neutral overall, so there must also be positive charge somewhere inside atoms to balance them out.





J. J. Thomson(1856-1940)

Ernest Rutherford(1871-1937)

Alpha Particle





J. J. Thomson(1856-1940)

Ernest Rutherford(1871-1937)

• Most particles just pass right through.

• A few, however, are deflected at different angles.

After all this… the modern atomic model

• The core of an atom is called the nucleus.

• Subatomic particles called protons and neutrons stick together in the nucleus.

• A cloud of electrons surrounds the nucleus, and can interact with other atoms.

Proton (positive charge)

NeutronElectron (negative charge)

Understanding the scale within an atom

What makes one atom different from another?

Atomic number (Z) = number of protons in nucleus

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

= atomic number (Z) + number of neutrons

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

XAZ

Mass NumberAtomic Number

Element Symbol

H11Hydrogen

Hydrogen-1

H (D)21

DeuteriumHydrogen-2

H (T)31

TritiumHydrogen-3

What makes one atom different from another?

An element is defined by the number of protons its atoms have. Every element has a different number of protons, indicated by the atomic number.

A periodic table also almost always lists the atomic weight, which is a weighted average of all the isotopes of that element, according to their abundance.

The mass of 1 atom of carbon-12 is defined to be 12 atomic mass units (amu).

(measured in atomic mass units, amu)

Understanding where atomic weight values come from

carbon-12: 98.89% natural abundance 12 u

carbon-13: 1.11% natural abundance 13.0034 u

Why is the atomic weight of the element carbon 12.011 amu?

= 12.011 u

mass of carbon = (12 u)(0.9889) + (13.0034 u)(0.0111)

= 11.867 u + 0.144 u

Atomic weights: Practice

Copper, a metal known since ancient times, is used in electrical cables and pennies, among other things. The atomic masses of its two stable isotopes,

are 62.93 amu and 64.9278 amu, respectively. Calculate the atomic weight of copper.

= 63.55 amu

Cu6329 Cu65

29(69.09%) (30.91%)

the history of atomic theory

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