History of the Atom & Atomic Structure

Atomic Structure

History of the Atom

Democritus (460 BC – 360 BC) Ancient Greek philosopher▪ No experiments performed!

Major Contribution: The Atom▪ He proposed that everything was made of

these atoms and they were all indivisible Was his theory correct?▪ No! There are subatomic particles!

History of the Atom

John Dalton (1766-1844) Major Contribution: Atomic Theory

(1808)▪ This began the modern era of chemistry

Four Principles:▪ Elements are composed of indivisible

particles called atoms.▪ All atoms of a given element are identical.

History of the Atom

John Dalton▪ Compounds are composed of atoms of one or

more elements, and will form only in whole-number ratios.▪ This is called the Law of Multiple Proportions▪ i.e. H2O exists, while H2.35O does not

▪ A chemical reaction involves the combination, separation, or rearrangement of atoms, not their creation or destruction▪ This is called the Law of Conservation of Mass

Was his theory correct?▪ Mostly! Still thought atom indivisible!

History of the Atom

J.J. Thomson (1856-1940) Major Contribution: The Electron Cathode Ray Tube Experiment (1897)▪ Nobel prize (1906)

History of the Atom

J.J. Thomson

History of the Atom

Thomson’s Atomic Model

Also known as the Plum Pudding Model Was his theory correct?▪ No! Missing other parts of atom!

History of the Atom

Ernest Rutherford (1871-1937) Two Major Contributions:▪ The nucleus▪ The atom is mostly empty space

Gold Foil Experiment (1910)▪ Nobel prize in Chemistry (1908)

History of the Atom

Ernest Rutherford’s Gold Foil Experiment

History of the Atom

Rutherford’s Atomic Model

Was his theory correct?▪ Mostly! Missing neutrons and location of

electrons!

History of the Atom

Niels Bohr (1885-1962) Major Contribution: Planetary Model

of the Atom▪ Nobel Prize in Physics (1922) for

spectrum of hydrogen Atomic Line Spectra▪ Bohr observed that when light was

given off from an atom, there were only single lines visible

▪ Bohr proposed that each line represented an electron in a different orbit

History of the Atom

Atomic Line Spectra

History of the Atom

Bohr’s Atomic ModelElectrons

Nucleus

History of the Atom

Current Theory of the Atom Many scientists contributed to developing

quantum mechanics, which is the current model of the atom

Known as the electron cloud model▪ The cloud is an area of probability where the

electron is found▪ These electrons, moving at extremely high

speeds, effectively occupy the entire area of the cloud, the same way that moving fan blades effectively occupy the entire area through which they pass.

History of the Atom

Current Model of the Atom:Probability cloud where electrons found

Nucleus

Atomic Structure

Parts of the Atom Proton▪ Positive▪ Nucleus

Neutron▪ Neutral▪ Nucleus

Electron▪ Negative▪ Outside Nucleus

Atoms and the Periodic Table

Atoms are identified by their number of protons This is referred to as their atomic

number All atoms of the same element

have the same number of protons

Atoms & Charges

In atoms that have a neutral charge, the numbers of electrons equals the number of protons

Na Atom Na+ Ion

11 Protons 11 Protons

11 Electrons

10 Electrons

Cl Atom Cl- Ion

17 Protons 17 Protons

17 Electrons

18 Electrons

Mass of an Atom

The mass of an atom is the number of protons plus the number of neutrons This is also referred to as mass number

The mass of protons and neutrons are equal

Electrons and Mass of Atom Why is the

electron not part of the mass? It takes roughly

1800 electrons to equal the mass of 1 proton, so it is left out.

Think of electrons like flies buzzing around an elephant (the nucleus)

Mass Number and Periodic Table

The periodic table does not give the mass number, but always the atomic number

For simplicity, we round the number on the periodic table to get the mass number

Round this number to whole number

Isotopes

Isotopes are elements that have the same number of protons, but contains a different number of neutrons Example: carbon-12 and carbon-14▪ The number indicates the mass number▪ Both contain the same number of protons (6),

so carbon-14 must have two extra neutrons

Isotopes & Periodic Table

The masses given on the periodic table are an average of all the isotopes on the planet We refer to the masses on the periodic

table as the average atomic mass of an element

This explains why the atomic masses are not whole numbers – it is an average!