Development & Structure of the Atom

Development of the Atom Dalton, Thomson, Rutherford Subatomic Particles Isotopes Average Atomic Mass

democritus (~450 bc)

greek philosopher

all matter around us is made of indivisible tiny particles-

"atomos”

john dalton (1803)

described atoms as indivisible, indestructible, solid, and uniform spheres

“billiard ball model”

john dalton’s atomic theory five postulates

1. Each element is composed of tiny, indestructible particles called atoms.

2. All atoms of a given element have the same mass and chemical properties.

3. Atoms of different elements are different.

4. Atoms combine in simple, whole-number ratios to form compounds.

5. Atoms of one element cannot change into atoms of another element. In a chemical reaction, atoms only change the way that they are bound together with other atoms.

john dalton’s theory summarized

1. elements are made of tiny particles called atoms.

2. atoms of one element are identical while atoms of different elements are different.

3. conservation of atom rearrangement in chemical reactions (lavoisier previously stated this in terms of the law of conservation of matter)

3. different atoms form compounds in constant ratios. (proust previously stated this in terms of the constant mass ratios)

j.j thomson (1896)

electrons are dispersed in a uniform positive charge

“plum pudding model” 1990

depicts the atom as a diffuse, positively charged sphere with negatively charged electrons embedded within it

j.j thompson’s experiment

to test the properties of the particles, thomson placed two oppositely-charged electric plates around the cathode ray. the cathode ray was deflected away from the negatively-charged electric plate and towards the positively-charged plate. this indicated that the cathode ray was composed of negatively-charged particles.

attracted to positive end = negative

thomson also placed two magnets on either side of the tube, and observed that this magnetic field also deflected the cathode ray. the results of these experiments helped thomson determine the mass-to-charge ratio of the cathode ray particles, which led to a fascinating discovery the mass of each particle was much, much smaller than that of any known atom. thomson repeated his experiments using different metals as electrode materials, and found that the properties of the cathode ray remained constant no matter what cathode material they originated from. from this evidence, thomson made the following conclusions:

• the cathode ray is composed of negatively-charged particles.

• the particles must exist as part of the atom, since the mass of each particle is only the mass of a hydrogen atom.

• these subatomic particles can be found within atoms of all elements.

j.j thompson’s discoveries

discovered atoms have negative particles (electrons) using a cathode ray tube

discovered electron's charge to mass ratio: 1.76 x 10 C/g

millikan’s oil drop experiment (1909)

measured the fundamental charge of an electron by suspending charged oil droplets in an electric field and observing their motion

• charge of an e- is 1.60 x 10-19C

ernest rutherford (1909)

an atom has a positive, heavy, tiny nucleus with electrons moving around it.

most of an atom is empty space

the gold foil experiment

rutherford shot alpha particles at a thin sheet of foil

conclusions:

• most of the alpha particles passed straight through the foil = the atom is mostly open space

• some of the alpha particles were deflected at large angles, and some were reflected = the atom contains a small, heavy, positive central nucleus.

james chadwick (1932)

discovered the neutron

proton

mass (kg) = 1.67262 × 10^-27

mass (amu) = 1.00727

charge (relative) = +1

charge (C) = +1.60218 × 10^-19

neutron

mass (kg) = 1.67493 × 10^-27

mass (amu) = 1.00866

charge (relative) = 0

charge (C) = 0

electron

mass (kg) = 0.00091 × 10^-27

mass (amu) = 0.00055

charge (relative) = -1

charge (C) = -1.60218 × 10^-19

all developments of atoms timelines + major discoveries