Atoms, Molecules, and Ions

Each element is composed of atoms
All atoms of a given element are identical, but the atoms of one element are different from the atoms of all other elements
Atoms of one element can’t be changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions
Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms

Dalton had no direct evidence for the existence of atoms but based his conclusions on chemical observations
* Mostly about conservation of mass
Today we have images of atoms from instruments like the scanning tunneling microscope
The modern view of atomic structure owes a great deal to the experiments of:
  * J.J.Thomson: credited with discovery of the electron
  * Robert Millikan: the charge on the electron
  * Ernest Rutherford: basic structure of the atom; existence of the nucleus
  * Marie Curie: radioactivity

A visible beam, the “cathode ray” appeared when high voltage applied across the electrodes and was deflected by an applied magnetic field.
Thomson measured the deflection of the ray at any particular magnetic field strength.
Thomson calculated the charge/mass ratio of the cathode ray particles to be 1.76 x 10^8 coulombs/gram (C/g)
Robert Millikan determined the charge on the electron in 1909:
* The electronic charge (e) = 1.602 x 10^-19C
* Mass of electron = 1.602 x 10^-19 C x (1kg/1.759 x 1011C) = 9.107 x 10-31 kg
Conclusions to this point:

  1. The atom is not indivisible as Dalton thought
  2. Charge and mass of electron established

Rapidly moving electrons occupy most of the volume of the atom
The nucleus is a tiny region that contains virtually all the mass of the atom
The proton, neutron and electron are the subatomic particles that affect chemical behavior

Structural formulas: show the order in which atoms are bonded.
Perspective drawings: show the three-dimensional array of atoms in a compound.

Only noble gases exist as isolated atoms
Most matter is composed of molecules or ions
Molecular compounds are composed of molecules and almost always contain only nonmetals.
The subscript to the right of the symbol tells the number of atoms of that element in one molecule of the compound.
Changing the subscript gives a totally different compound having completely different properties
Empirical formulas: give the lowest whole-number ratio of atoms of each element in a compound.
Molecular formulas: give the exact number of atoms of each element in a compound.

When atoms lose or gain electrons, they become ions.
Cations: positive and are formed by elements on the left side of the periodic chart.
Anions: negative and are formed by elements on the right side of the periodic chart.
Ionic compounds tend to be composed of metals bonded to nonmetals
Metals tend to lose electrons to form cations
Non-metals tend to gain electrons to form anions

Because compounds are electrically neutral, one can determine the formula of a compound this way:
  * The charge on the cation becomes the subscript on the anion.
  * The charge on the anion becomes the subscript on the cation.
  * If these subscripts are not in the lowest whole-number ratio, divide them by the greatest common factor.