Defining the atom notes

Defining the AtomDefining the AtomChapter 2 Notes

Early Models of the AtomEarly Models of the Atom

Democritus, a Greek Philosopher, was among the first to suggest the existence of the atom.

He believed atoms were indivisible and indestructible.

Though true, these thoughts lacked experimental support.

John DaltonJohn Dalton

200 years later, John Dalton transformed Democritus’s ideas on atoms into scientific theory using experimental methods.

He studied the ratios in which chemicals combine in reactions.

The result was his atomic theory

Dalton’s Atomic TheoryDalton’s Atomic Theory

1. All elements are composed of tiny, indivisible parts called atoms.

2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element

Dalton’s Atomic Theory (cont)Dalton’s Atomic Theory (cont)

3. Atoms of different elements can physically mix or chemically combine in simple whole-number ratios to form new compounds.

4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

Subatomic ParticlesSubatomic Particles

One part of Dalton’s theory does not hold true today:◦Atoms are not indivisible; they can be

broken down into smaller, more fundamental particles.

Three types of subatomic particles.

ElectronsElectrons

Discovered in 1897 by J.J. Thomson

Electrons are negatively charged subatomic particles.

His discovery involved the use of the cathode ray tube.

Robert Millikan found the quantity of charge and mass of the electron to be 1 unit of negative charge

◦and 1/1840 the mass of a Hydrogen atom, respectively.

Cathode Ray TubeCathode Ray Tube

Involved passing electric current through gases at low pressure in a sealed glass tube.

Electrodes attached to a source of electricity were placed at each end of the tube.

The positive end was the anode and the negative end was the cathode

The result was glowing beam that moved from the cathode to the anode

Protons and NeutronsProtons and Neutrons

Eugene Goldstein found that in a cathode ray tube, there were rays moving in the opposite direction of the cathode ray

These positively charged particles are called protons

James Chadwick later discovered a final subatomic particle known as a neutron.◦A neutron is electrically neutral but has a mass

nearly equal to that of a proton.

Rutherford’s Gold Foil Expt.Rutherford’s Gold Foil Expt.

The Atomic NucleusThe Atomic Nucleus

The nucleus is the central core of the atom

It is composed of protons and neutrons

The nucleus is the region of the atom that has an overall positive charge.

The Atomic NucleusThe Atomic Nucleus

Most of the mass of the atom is concentrated in the nucleus.

The rest of the atom is assumed to be mostly empty space; other than small negatively charged electrons