Defining the Atom
The Greek philosopher Democritus (460 - 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word, "atomos"). He believed that atoms were individsible and indestructible. His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method. It is just mainly Philosophy.
Dalton's Atomic Theory (Experimental Based)
1. All elements are composed of tiny indivisible particles called atoms.
2. Atoms of the same element are identical. Atoms of any element are different from those of any other element.
3. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
4. In chemical reactions, atoms are combined, separated or rearranged - but never changed into atoms of another element.
Sizing up the Atom
*Element are able to be subdivided into smaller and smaller particles - these are the atoms, and they still have properties of that element.
*If you could line up 100,000,000 copper atoms in a sing file, they would be approximately 1 cm. long.
*Despite their small size, individual atoms are observable with instruments such as Scanning Tunneling (electron) microscope.
Structure of the Nuclear Atom
One change to Dalton's atomic theory is that atoms are divisible into subatomic particles: electrons, neutrons and protons. They are the fundamental particles. There are many other types of particles like the mvon, meson, hyperon, positron and quarks.
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle, the electrn. The cathode ray tube passes electricity through a gas that is contained at a very low pressure.
Mass of the Electron
In !916, Robert Millikan determined the mass of the electron: 1/1840 the mass of a hydrogen atom. It has one unit of negative charge.
Conclusions from the Study of the Electron
a. Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.
b. Atoms are neutral, so there must be positive particles in the atom ot balance the negative charge of the electrons.
c. Electrons have so little mass that atoms must contain other particles that account for most of the mass.
Eugene Goldstein in 1886 observed what is now called the "proton" - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron). In 1932, James Chadwick confirmed the existence of the "neutron", a particle with no charge, but a mass nearly equal to a proton.
Electron has -1 charged, with a mass of 9.11 x 10 to the power of -28 g and it is located in the election cloud.
Proton is of +1 charged, with a mass of 1.67 x 10 to the power of -24 and it is found in the nucleus.
Neutron is of 0 charged, with a mass of 1.67 x 10 to the power of -24 and it is found in the nucleus.
Thomson's Atomic Model
Thomson believed that the electrons were like plums embedded in the positively charged "pudding" thus it was called the "plum-pudding model".
Ernest Rutherford's Gold Foil Experiment in 1911
Alpha particles are helium nuclei. The alpha particles were fired at a thin sheet of gold foil. Particles that hit on the detecting screen (film) are recorded.
Rutherford's Findings regarding the Atom
*Most of the particles passed through
*A few particles were deflected
*A very few particles greatly deflect
a. The nucleus is small.
b. The nucleus is dense or heavy
c. The nucleus is positively charged
The Rutherford's Atomic Model (Experimental Based)
*The atom is mostly an empty space.
*All the positive charge and almost all the mass is concentrated in a small area in the center. He called this as the nucleus.
*The nucleus is composed of protons and neutrons (they make the nuclei).
*The electrons distributed around the nucleus, and occupy most of the volume.
*His model was called a "nuclear model"