This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago. He asked: Could matter be divided.

Historyand Progression

of Atomic Theory

Democritus

This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago.

He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?

400 BC

DEMOCRITUS’ ATOMIC THEORY 400BC

Democritus asserted that space contained an infinite number of particles

• Named atomos, or "indivisible”•Atoms are eternal and invisible; absolutely small, so small that their size cannot be diminished; totally full and incompressible.•Atoms are homogeneous, differing only in shape, arrangement, position, and number

Atomos

To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes.

Atoms were infinite in number, always moving and capable of joining together.

Dalton’s Theory (early 1800’s)

He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles.

Atoms of the same element are exactly alike.

Atoms of different elements are different.

Compounds are formed by the joining of atoms of two or more elements in specific ratios.

J.J. Thomson (English 1897)

Used Cathode Ray tube to determine the presence of – (electrons) and + (protons) particles.

J.J. Thomson (English 1897)

Plum Pudding Model -- the structure of an atom is something like pudding. He assumed that the basic body of an atom is a spherical object containing electrons & protons randomly confined in homogeneous jellylike material. Positive charges cancel the negative charges.

Nagaoka (Japanese 1904) The Japanese Nagaoka imagined

the atom to be like the planet Saturn. He thought it had a positively charged center, with electrons in orbit around it’s middle. There was a problem with this picture of the atom. A charged particle, such as an electron, moving in a circle, radiates electromagnetic waves. As it radiates, it loses energy. Unless it can get energy from some other source, it will slow down and follow a spiral path into the center of the circle. A Saturnian atom would have collapsed.

Ernest Rutherford (Eng.1911)

Atoms have a central positive nucleus surrounded by negative orbiting electrons.

This idea was the result of his famous Gold Foil Experiment(see next slide). This experiment involved the firing of radioactive alpha particles through gold foil.

This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space.

Most of particles passed straight through the foil but approximately 1 in 8000 were deflected.

Rutherford’s Gold Foil Alpha Scattering Experiment

Alpha scattering due to repulsion from the nucleus

Nucleus-Positive Charged Center The nucleus is far too large in this drawing.

Niels Bohr (Danish 1913)

The Bohr Model is probably familiar to us as the "planetary model" of the atom is used to symbolize atomic energy.

Electrons orbit the nucleus much like planets orbiting the Sun.

However, the orbits are not confined to a plane as is approximately true in the Solar System).

Bohr Model of the Atom

Bohr Atom used as logo for Atomic Energy Commission

Chadwick (English 1932)

James Chadwick discovered a third type of particle, which he named the Neutron. Neutrons help to reduce the repulsion

between protons and stabilize the atom's nucleus.

Neutrons always reside in the nucleus of atoms and they are about the same mass and size as protons.

Neutrons do not have any electrical charge; they are electrically neutral

Chadwick’s atom w/ P+, e- & N+/-

Cloud Theory (1920’-1930’s)Quantum

Mechanics

Cloud Theory

Based on the work of many scientists Based on the mathematical approach of

Quantum Mechanics Electrons are assigned regions of space

(Orbitals) not pathways (Orbits) Electrons are moving around the nucleus

rapidly in no predictable path producing a cloud of e-’s over time. Think of a rapidly moving fan blade.

Cloud Theory of Today

Electron Cloud

How to find Protons

Number of protons in an atom is ALWAYS equal to the Atomic Number

Number of Electrons (e-)

Atoms – Protons and electrons are equal

Ions Loss of electron makes positive ions

Gain of electrons makes negative ions

Loss or Gain of Electrons

Loss of Electrons Atom Ion

Na Na+1 + 1e- (1e- removed) +11 +11 -11 -10 0 net +1 NET CHARGE

Gain of Electrons

Cl2 + 2e- 2Cl- (2e- added to Cl2)

+17 +17 -17 -18 0 net -1 NET CHARGE

How to calculate Neutrons

Atomic Mass (rounded to integer) - Atomic Number ---------------------------------------------------- Number of Neutrons in the nucleus

Atomic Mass – Atomic Number = # Neutrons

Atom Contents

Protons (P+) – always equal to Atomic # Electrons (e-)

Atoms – Same as the # of Protons Ion – Net charge after e-’s have been lost or

gained in an attempt to become stable (HAPPY NUMBERS!) Loss of e-’s = Positive charge Gain of e-’s = Negative charge

Neutrons (N±) = Atomic Mass – Atomic #

Energy Levels Maximum K Shell = 2 e-’s L Shell = 8 e-’s M Shell = 8 e-’s N Shell = 18 e-’s

When you get to Chemistry, you will find that this gets more involved than shown here.

Oxygen Atom

How many protons(+) are present?

How many electrons(-) are present?

How many neutrons(±) are present?

In what energy levels are the e-’s found?

Oxygen Atom

How many protons(+) are present? Atomic number = 8 = protons

How many electrons(-) are present? In atoms, p+ = e- therefore 8 e-’s

How many neutrons(±) are present? At. Mass – At. # = 16 – 8 = 8 neutrons

In what energy levels are the e-’s found? 8e-’s total = 2 in K shell, 6 in L shell

Magnesium Ion

How many protons(+) are present?

How many electrons(-) are present?

What is the charge of the ion?

How many neutrons(±) are present?

In what energy levels are the e-’s found?

Magnesium Ion