Electrons and chemistry

The electron is denoted in physics with an e^-.

Normally, an atom has as many electrons (of - charge) as protons (of + charge). The atom is therefore overall neutral in terms of its electrical charge.

• If it loses one or more electrons, an atom becomes a positive ion or CATION.
• If it gains one or more electrons, it becomes a negative ion or ANION.

In the same way, a Carbon atom C can lose two electrons and become a cation with two surplus positive charges, this would be denoted by C^2-. In contrast, if it gained an additional electron, it would become an anion C⁺

Chemical bonds
 

Electrons are organised in successive layers around the nucleus. These layers are filled according to very strict rules defined by a quantum law which will be revealed later on (Pauli's law of exclusion).

The important thing to remember is that it is the outer most electrons which are responsible for all of the chemistry, because it's them that can join together two separate atoms creating chemical bonds. For example, two atoms can each share one of their electrons and then create what we call a covalent chemical bond. As a result of such bonds, atoms can assemble themselves into an infinite diversity and complexity of molecules.
 

Molecules and macromolecules
 

Molecules are the assemblies which are at the root of both inert and living matter.
When the number of molecules in an atom gets up to several hundred, we begin to speak of macromolecules. Certain polymers can be made up of several millions of atoms!
In biochemistry (or organic chemistry), molecules such as proteins, lipids, glucides and nucleic acids (constituants of DNA, which stores the genetic code) organise themselves into superstructures (cellular components); these are the constituants of the living cell, the basic building block of all living organisms.

The water molecule, one of the simplest, is made up of one atom of oxygen and two atoms of hydrogen.

How can we imagine the classical atom?
 

Imagine that the nucleus of hydrogen (a single proton) measured 1 millimeter:  it would then be millions of times bigger than it is in reality (enlarged by 1,000,000,000,000 x) !

• it would weigh 1.7 million metric tonnes !
• its single electron would have a diameter of less than 1 thousandth of a millimetre (= 1 micron) and a mass of 900 metric tonnes !
• this electron revolves at 50 metres from the nucleus, giving the atom a diameter of 100 metres.

The volume of the nucleus is a million milliard times smaller than that of the atom;
the volume of the atom is thus made up of at least 99,9999999999999 % empty vacuum!

(source: David Calvet of CNRS Marseille, labo in2p3 a reference web site on nuclear physics, which is extremely clear (-- translators note -- clear in french that is).
 

Can we "see" an atom?
 

Since very recently and indirectly yes, and with the aid of two ultra-powerfull microscopes:

• The MET or Microscope Electronique à Transmission permits the internal structure of matter to be studied at the scale of the nanometre (10 ^-9 m). Rows of atoms then become visible. For example, this photo of a crystal of formula  SrTiO3 permits us to admire the presence of Strontium (Sr) atoms in super brilliance.

• The tunnel effect microscope permits us to "feel" the bumps of a surface, atom by atom. One of its variants, the atomic force microscope, even permits us to deposit atoms one at a time onto a surface. It is in this manner that the IBM logo has been written with Xenon atoms

crazyflash: THE ATOMIC FAMILY: ménage à trois

The atom is a nuclear family made up of three turbulent members: The proton, with a very positive disposition, and the chief of the clan but he doesn't have it easy. Inspite of everything, it's him who gives the atom its character! The neutron, a little swiss (stays neutral) and is very attached to the proton but can wander and loves making Isotopes. Together, protons and neutrons form the nucleons of the nucleus: They normally huddle together in their families (up to 113 families) which group together with their clones.  The kids, or electrons, are hyperactive, always racing around the nucleus. They have a fairly negative disposition (in opposition to the father-proton) and often run away. This leaves their parents disillus-ION-ed. In spite of this their hippie tendancies push them to bond together all of the atoms in the world.