Atomic Theory and Structure
Democritus:
-Greek Philosopher (~480 BC)
-named the basic particle the “atom” (means “indivisible”)
Foundations of Atomic Theory
-developed 1700s – 1800s
-Elements combine to form different compounds
-Do they always combine in the same ratio?
Law of Conservation of Mass
-Mass is neither created nor destroyed
Law of Definite Proportions
-Chemical compound contains same elements in same proportions by mass regardless of size of the sample or source
-Ex. NaCl (table salt) … always 39.34% Na, 60.66% Cl by mass
-(same combination of atoms)
Law of Multiple Proportions
-If two or more different compounds are composed of the same 2 elements (or more) than the ratio of masses of the second element combined with certain mass of the 1st element is always a ratio of small whole #s
-CO2 always has twice as much O2 as C (1:2 ratio)… CO always has 1:1 ratio, C to O.
Dalton’s Atomic Theory
-1808 English schoolteacher
-Explains laws (from above, ↑) with a few statements
- All matter is composed of atoms (very) small
- Atoms of a given element are identical in size, mass, and other properties – atoms of different elements differ in properties
- Atoms cannot be subdivided, created, or destroyed
- Atoms of different elements combine in simple whole-number ratios to form chemical compounds
- In chemical reactions, atoms are combined, separated, or rearranged.
(3, 5 explain law of conservation of mass)
Modern Atomic Theory
Now we know [atoms can be divided into subatomic particles]
[elements can have atoms with different masses… Isotopes]
- All matter is composed of atoms
- Atoms of any one element differ in properties from atoms of another element
**An atom: the smallest particle of an element that retains the chemical properties of that element**
Atoms are made up of 2 regions: nucleus (protons and neutrons) & electron cloud (general area where the electrons are located)
Discovery of the Electron
-Came about with experiments dealing with electricity
Cathode Rays and Electrons
-Sent an electrical current through a cathode
2 obsrvations:
- Rays deflected by a magnetic field (just like current in wire, known to be negative)
- Rays deflected away from a negatively charged object
-Thomson measured the ratio of charge of particles to their mass- the ratio is ALWAYS the same!
-Therefore, cathode rays composed of electrons
From this information, Thomson proposed the “Plum Pudding Model” (electrons spread evenly throughout the atom)- This was DISPROVED shortly later
Discovery of the Atomic Nucleus
-Rutherford’s gold foil experiment
-Bombarded foil with alpha particles (4x mass of hydrogen atom)
-Most passed through, but some bounced back! Caused by a very densely packed bundle of matter with a positive electric charge- called it the nucleus.
-Volume of the nucleus is very small compared to the total volume of the atom
-Where were electrons?
Composition of the Atomic Nucleus
-All nucleus are made of protons and neutrons (except hydrogen, which is only a proton)
-Proton is positively charged (equal in magnitude to the electron’s negative charge), Neutrons are neutral- they have no charge.
-Atoms of different elements differ in their number of protons- Number of protons determine atom’s identity (determines which element it is)
Forces in the Nucleus
-Like charges repel each other, but when extremely close together they have a very strong attraction
-Short range proton-neutron, proton-proton, and neutron-neutron forces hold the nuclear particles together and are referred to as nuclear forces
The Sizes of Atoms
-Electrons exist in the electron “cloud” (no exact position at any one moment)
-Atomic radii- from the center of the nucleus to the outer portion of the electron cloud
-SMALL- measured in picometers (pm)
-Atomic nuclei are very dense as well