The Elements and Atomic Building Blocks

It is natural to try to simplify the characterization of pure compunds by considering of what building blocks each unique chemical substance is made. Luckily for Chemistry, the number of unique building blocks that we need to consider is actually only about 100. These building blocks are of course the ELEMENTS, and are arranged conveniently in a table:
Metals (in red) comprise most of the elements, and are characterized by their tendency to lose electrons when involved in chemical reactions. Metals make good electrical and thermal conductors because of this property. Non-metals (in blue) have a tendency to gain electrons when in combination with metals, and are thus oxidizing agents. (The rightmost column of the Periodic Table is shown in blue, but these elements are, in fact, inert) Metalloids, semi-metals, or semiconductors can swing both ways and are found in the region of the Table between the metals and non-metals.

The periodic table is labeled by its rows, which are called Periods,

which show a systematic variation of the properties of the elements across the period. The columns of the Periodic Table are called Groups or Families,
because the elements within a group share similar properties. Several of the Groups of the Periodic Table have special names because of the similarity of the chemical properties of the elements within the group.

Each element has a symbol associated with it. In fact, each isotope of each element has a symbol.

The atomic number, Z, of the atom is simply the number of protons it has in its nucleus, the number of electrons it has in the neutral atom, or the charge on the bare nucleus of the atom stripped of all its electrons (these are all the same number). The atomic symbol, X in the above diagram, is a shorthand for the name of the element and is uniquely defined by the atomic number and is therefore redundant. Since the element is defined by the number of electrons in the nuetral atom, isotopes of the same element may have the same chemical properties (determined by the ELECTRONS) but different numbers of neutrons. Some elements have many stable isotopes, and some have famous unstable ones. The number of neutrons in the nucleus (they are never anyplace else in an atom) is (A - Z).
Remember, Chemistry is entirely determined by what the electrons are doing in an atom or molecule. Therefore isotopes of the same element behave chemically the same, but if an atom is ionized (electrons are added or removed), then its' behavior is completely changed.
When elements combine to form a compound, the properties of the compound are determined from the properties of the MOLECULE of the substance. The molecule, then, is the building block of a compound, just like the atom is a building block of the element. The molecule is the smallest portion of a compound that can be isolated and still retain the properties of the compuond. A molecule is depicted in various ways; here are a few:
Common compounds should have familiar molecular structures. Structure, is first and foremost determined by the connectivity of the atoms in the molecule, i.e. what atoms have chemical bonds between them. Later, we will discuss in detail the lengths of the bonds and the angles between bonds to further describe the structure of molecules. These are a few structures you should be familiar with:

Some Common Molecules and Their Shapes
Ethanol
Ethanol (space filling)
Ethylene
Hydrazine
Ammonia
Cyclobutane
Cyclopentane
Cyclopropane
Methane
One of the properties that elements within a group share is the tendency to gain or loose electrons. We will call this Valence or Valence State. Here are some Valences of the elements that you should know.
The simple valences (or oxidation states) are obtained by the group number for metals or 8 minus the group number for non-metals (at least for the non-transition, or A, groups).

It seems like there are a lot of elements and a lot of things to know. Many times, a subset of the entire Periodic Table of the elements are of importance. For instance, in Biology, not all elements are found to be significant. The elements found in the Human body are colored in the following periodic table, so, when dealing with 'Human Chemistry', only a few elements are important. Here, the most abundant elements in biological systems are shown in red, lesser elements in blue, and 'trace' elements in green

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