3. Atoms and Orbitals

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Atoms, which are now known to consist of even more fundamental particles, are the basic building blocks of matter. Every element on the Periodic Table of Elements has associated with it an atom. Every atom has a nucleus, or core composed of protons and neutrons (called nuclei), that is surrounded by electrons. Protons are positively charged particles, while neutrons have no charge, or are neutral (see “Charge Carriers and the Electric Field”). Protons and neutrons have approximately the same mass (although we now know they are not quite the same), 1.67x10^-27 kg. The protons in the nucleus are balanced (in a neutral atom) by the negatively charged electrons that orbit the nucleus. These electrons are only about 1/2000 the mass of a proton or neutron.

Above is an atom of Silicon depicted in two forms, one three dimensional and the other two-dimensional. Note that the concentric circles present a simplified model. Source: “Two ways of Illustrating The Silicon Atom”. Learnabout-electronics.org. April 1, 2012. <www.learnabout-electronics.or...ductors_02.php>

Some key points about atoms as we understand them today:

The nucleus is actually incredibly small compared to the size of the whole atom (about 1/100,000 the size of the whole atom), for the electrons orbit at a relatively large distance. In fact, if the nucleus were a marble, the electrons would orbit at a distance equivalent the distance across large sports stadium.
Though a useful visual tool, electrons in an atom don’t actually orbit the nucleus in concentric circles. Quantum mechanics (see Basics of Quantum Theory) tells us that electrons are quantum particles, and therefore we cannot precisely determine their position around an atom. They therefore have probability “clouds” called orbitals that give us a good idea of their location.
Electrons are bound to the atom by electromagnetic forces between the electrons and the positively charged protons in the nucleus. It is possible for incoming energy to be given to the electrons and change their energy state. In other words, an electron can be excited to different orbitals; the more energy an incoming photon has, the further away from the nucleus the electron will jump. Energy is conserved because the potential energy between an electron and the nucleus is smaller with distance; thus, in order to move back towards the nucleus (lower energy state), it must eject a photon (a quantum of energy).

If the incoming energy is high enough, it can completely strip away an electron from the outermost orbital of the atom. This is called ionization.
The nucleus (protons and neutrons) are not attracted by electromagnetic forces, but rather by the strong force, which is the reason that such enormous energy is released in nuclear reactions (see Nuclear Fusion). This force is rather complicated, but it stems from the interactions between the particles called quarks that make up nuclei.

Above is an illustration of various types of orbitals an electron can occupy around the nucleus of an atom, the simplest type being the top s₁ orbital. These represent the most likely places we would find an electron in an atom. Source: chemwiki.ucdavis.edu. 29 Feb. 2012. <chemwiki.ucdavis.edu/Physical...ronic_Orbitals>