All matter is made up of chemical elements, each of which is made up of particles called atoms. A rather crude, but useful, picture of an atom is that of a rigid sphere. The radii of these atoms are a few angstroms (where an angstrom is 10-10 cm. There are 92 so-called naturally occurring elements of which 90 can be detected on Earth at this time. What happened to the "missing 2" will be left for you to think about.
These spheres contain a central nucleus which contains two types of particles:
The atomic number is the number of protons in the nucleus of the atom. The atomic mass number is the number of protons plus neutrons in the nucleus. All atoms of an element have the same number of protons - hence the same atomic number; for example all atoms of hydrogen have one proton in the nucleus. Each element is assigned a symbol - H for hydrogen. You should learn the symbols for the eight most abundant elements in the Earth's crust [Oxygen (O), Silicon (Si), Aluminum (Al), Calcium (Ca), Iron (Fe), Magnesium (Mg), Sodium (Na), and Potassium (K)] .
If we could weigh individual atoms of calcium we would find some variation in their mass; this must be due to variations in the number of
neutrons in their nucleus as all must have the same number of protons - 20. Atoms which have the same atomic number but different mass numbers are isotopes.
For example, all calcium atoms have an atomic number of 20 but there are three isotopes of calcium - 39, 40, and 42. (How many neutrons does each isotope possess?)
In addition to the particles in the nucleus, there are negatively charged particles - electrons which "orbit" the central nucleus. Recognition that the distribution of electrons around the nucleus is not random was one of the great accomplishments of physicists in the 1920s - quantum mechanics.
Electrons occupy "levels" which are separated from each by some volume of space which electrons can pass through but not remain. In fact, much of the volume of an atom consists of empty space. [I remember that this is how Superman was able to pass through solid objects!] Electrons are "attracted" to the nucleus (positive and negative charges attract).
The primary division of these levels is called the principal quantum number. The first level (K) is closest to the nucleus.
As the principal number increases 1...2....3....4 the number of electrons that can reside in the level increases.
The maximum occupancy = 2n2 where n is the level ... 1, 2, 3, 4, etc.
| K|| 2
| L|| 8
| M|| 18
| N|| 32
In a neutral atom the number of negative particles equals the
number of positive particles (electrons = protons).
A useful exercise is to start with the first element (Hydrogen) with its one proton and one electron and add protons and electrons to build up the remaining chemical elements. The build-up process works as follows. Start with a single electron - it occupies the empty
level closest to the nucleus. The
second electron goes into the first level (atomic Helium) and the
shell K) is filled. Actually, there are sub levels within the main level (except for the
For element 11 (Sodium - Na) there are 11 electrons and 11 protons. Two electrons
are in the K shell, 8 in the L shell and one in the M shell. The outer most electron(s) are termed the valence electrons.
| Sub Shell|| Max Electrons
| s|| 2
| p|| 6
| d|| 10
| f|| 14
|Main Level|| Orbitals
| K|| 1s (max = 2) Total = 2
| L|| 2s (max = 2), 2p (max = 6) Total = 8
| M|| 3s (max = 2), 3p (max = 6), 3d (max = 10) Total = 18
ion is a charged atom; that is, there is an excess of positive (cation)
or negative charge (anion). If an electron is removed a cation is
formed. If an electron is added an anion is formed. Of the eight most abundant elements in the Earth's crust only Oxygen forms anions; the rest form cations by loosing one or more electrons.
When there are 8 outer most electrons in the s and p sub shells the
atom has a tendency to resist change and ions are formed with great difficulty if at all.