Transcript History of the Atom - Fall River Public Schools

Atomic Structure

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!

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.

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!

J.J. Thomson (1856-1940)
 Major Contribution: The Electron
 Cathode Ray Tube Experiment (1897)
▪ Nobel prize (1906)

J.J. Thomson

Thomson’s Atomic Model
 Also known as the Plum Pudding Model
 Was his theory correct?
▪ No! Missing other parts of 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)

Ernest Rutherford’s Gold Foil Experiment

Rutherford’s Atomic Model
 Was his theory correct?
▪ Mostly! Missing neutrons and location of electrons!

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

Atomic Line Spectra

Bohr’s Atomic Model
Nucleus
Electrons

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.

Current Model of the Atom:
Probability
cloud where
electrons found
Nucleus

Parts of the Atom
 Proton
▪ Positive
▪ Nucleus
 Neutron
▪ Neutral
▪ Nucleus
 Electron
▪ Negative
▪ Outside Nucleus

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

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
 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

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)


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 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

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!