Atoms The Development of Atomic Theory

Atoms An atom is the smallest piece of an element that still retains all of the properties of that element These are the pieces from which matter is built Different types of atoms represent different elements

An early theory of the Atom Democritus – ( B.C.) He felt that matter was made of empty space through which atoms move. Atoms were solid, homogeneous, indestructible, invisible (due to small size) and indivisible. 1.Different atoms would have different sizes and shapes 2.Different properties of substances are due to the size, shape and movement of atoms 3.Changes in matter result from changes in the grouping of atoms and not from changes in the atoms themselves.

Democritus’ Atom

In the late 1700s Scientists began to usher in in a new chemical era by making careful quantitative measurements which allowed the compositions of compounds to be determined with accuracy.

Antoine Lavoisier ( ): He found and termed both oxygen (1778) and hydrogen (1783) Helped construct the metric system Put together the first extensive list of elements Helped to reform chemical nomenclature. He was also the first to establish that sulfur was an element (1777) rather than a compound He discovered that, although matter may change its form or shape, its mass always remains the same. Line engraving by Louis Jean Desire Delaistre, after a design by Julien Leopold Boilly

John Dalton

John Dalton: ( ) An English schoolteacher who gathered ideas from others and performed experiments to test and correct his atomic theory of matter. Studied the ratios in which elements combine in chemical reactions, formulated hypothesis and theories to explain this observations (inductive or deductive?) and then tested those ideas through more experiments.

Dalton’s Atomic Theory (1808) 1.All elements are composed of tiny indivisible particles that are called atoms. These are solid spheres, like a billiard ball.  Today we know that atoms are divisible  In each atom there is a nucleus and electron cloud, and there are different particles that occupy those areas.

Dalton’s Atomic Theory 2.Atoms of the same element are identical. The atoms of any one element are different from those of any other element.  Today, we know that there are some differences between some particles of the same elements. Ex: Ions (particles with charges) Isotopes (particles of the same atom with different masses)

Dalton’s Atomic Theory 3.Atoms of different elements can combine with one another in simple whole number ratios to form compounds. This aspect of Dalton’s theory has stood the test of time

Dalton’s Atomic Theory Chemical reactions can occur when atoms are separated, joined, or rearranged. However, atoms of one element are not changed into atoms of another by chemical reactions.  Today, we know about nuclear reactions such as fission and fusion which can change elements’ identities.

Dalton’s Atomic Theory

What’s next? New experiments were performed which led to new discoveries. Scientists discovered the electron, the nucleus, and the different types of atoms that can exist for one atom.

J. J. Thomson (1897) Thomson conducted cathode ray tube experiments. This is where an electrical current is passed though a glass tube filled with a gas. A beam of light would cause the end of the tube to glow

J.J Thomson’s Experiment Thomson placed positive and negative plates along the path of the beam. The beam was deflected away from the negative plates. From his experiments, Thompson concluded that a cathode ray consists of a beam of negatively charged particles which we now know are electrons.

J. J. Thomson (1897) Thomson discovered electrons which are negative particles within the atom Thomson arrived at the Plum-pudding Model – positive sphere (pudding) with negative electrons (plums) dispersed throughout

Ernest Rutherford (1911) Rutherford performed his Gold Foil experiment Gold Foil experiment This is how he discovered the nucleus, which is a dense, positively charged region in the center of the atom

Rutherford’s Experiment Rutherford’s team directed a beam of alpha particles at a thin sheet of gold. Alpha particles are positively charged particles emitted from certain radioactive elements.

Rutherford’s Experiment Rutherford’s prediction was that the particles would pass directly through the gold foil.

Rutherford’s Results A large proportion of the alpha particles passed through the gold foil without deflection, as expected. A small fraction of the alpha particles bounced back at them. Rutherford said that this was as surprising as shooting a bullet at a piece of tissue paper and having it ricochet back at your face.

Rutherford’s Experiment Positively charged beam

Rutherford’s Results Explained

Rutherford’s Deductions Rutherford proposed that the mass of the atom and the positive change are concentrated in a small region. (nucleus). How did he come to these conclusions? 1. Small fraction deflected = small size 2. Deflection = high density 3. Deflection of alpha particles = positive charge

Ernest Rutherford (1911) Nuclear Model –dense, positive nucleus surrounded by negative electrons –Most of the atom is empty space. –The electrons are in that area, but their mass is so small, that they did not interfere with the movement of the alpha particles.

The Size of the Nucleus The nucleus’ size, relative to the rest of the atom is like a MARBLE in a FOOTBALL STADIUM

Rutherford’s Activity

James Chadwick (1932) Discovered neutrons – neutral particles in the nucleus of an atom – Chadwick did not perform his own experiments, but instead based his theory experimental evidence of two other scientists In 1920, Ernest Rutherford suggested that there were neutral particles in the nucleus of atoms. This conclusion arose from the fact that an element's atomic number (protons = electrons) was generally less than its atomic mass. In 1930 Joliot-Curie Experiments were conducted. Beryllium atoms were bombarded by alpha particles, and emitted radiation which was originally thought to be gamma radiation. However, further investigations into the properties of the radiation revealed they were not deflected upon passing through a magnetic field and were therefore neutral. In 1932, Chadwick proposed that this particle was Rutherford's neutron.

James Chadwick (1932) Neutron Model revision of Rutherford’s Nuclear Model

Niels Bohr (1913) Bohr was Rutherford’s student. Energy Levels – electrons can only exist in specific regions around the nucleus. – Bohr proposed that electrons orbited the nucleus as the planets orbit the sun.

Niels Bohr (1913) Planetary Model – Bohr deduced that electrons move in circular orbits within specific energy levels

Erwin Schrödinger (1926) Quantum mechanics – electrons can only exist in specified energy states Electron cloud model – orbital: region around the nucleus where e - are likely to be found

Erwin Schrödinger (1926) Electron Cloud Model (orbital)  dots represent probability of finding an e - not actual electrons