From Dave's wiki
Jump to navigation Jump to search

Atoms are the building blocks of all substances and are made up of electrons, protons, and neutrons, whose properties are as follows:

Particle Relative charge Relative mass
Proton +1 1
Neutron 0 1
Electron -1 1/1840 of proton mass

Each atom consists of a nucleus containing the protons and neutrons. The electrons surround the nucleus in energy levels or shells at certain distances from the nucleus. Each shell is divided up into regions of space where the electrons can be found and each shell can contain up to a certain number of electrons.

The carbon atom has a diameter of 0.154 nm. Each atom is much larger than the nucleus, which is only about 10e-14 meters in diameter.

The chemical properties of an atom are a consequence of the arrangement of electrons surrounding the nucleus. In particular, the number of electrons in the outer shell is crucially significant.

The number of protons is equal to the number of surrounding electrons, so atoms have no overall charge.

The atomic number of a chemical element (also known as its proton number) is the number of protons found in the nucleus of an atom of that element, and therefore identical to the charge number of the nucleus. It determines the arrangement of electrons around the nucleus and therefore characterises the chemical properties of any given element.

Neutrons are uncharged and contribute solely to the mass of the atom and have no influence on chemical properties.

Elements within a vertical column in the Periodic Table are called a group and have similar properties.

The mass number of a nucleus equals the number of protons plus the number of neutrons. All atoms of a given element have the same atomic number, but there is variation in the mass number. For example, there are three types of carbon atoms: carbon-12, carbon-13, and carbon-14. They all have an atomic number of 6 but their mass numbers are 12, 13, and 14 respectively. These different types of atom of the same element are called isotopes.

In investigating metabolism, an atom in each molecule of a particular compound may be replaced by another isotope of the same element to label or tag it. The compound will behave in an identical manner chemically, but will have a slightly different mass. This mass difference can be detected by appropriate physical measurements (e.g. mass spectrometry or ultracentrifugation). Information on the way DNA is replicated was obtained by incorporating into it a heavy isotope of nitrogen (nitrogen-15) instead of the more abundant nitrogen-14. The heavy isotope oxygen-18 provided evidence on how photosynthesis occurs.

Certain isotopes of some elements are radioactive, that is, they emit particles (alpha or beta) or rays (gamma) from their nuclei as they break up. Such radiation can be detected by the blackening of photographic film or by using a Geiger Counter. The radioisotope carbon-14 has been used in photosynthetic studies to trace the path by which carbon from CO2 is converted into carbohydrate.

Electrons are in shells or energy levels surrounding the nucleus and the number of electrons in each shell obeys a set of rules. The first shell, the one closest to the nucleus, may contain up to 2 electrons; the second shell may contain up to 8 electrons; the third shell, though it may contain up to 18, is stable with 8 electrons. The maximum number of electrons that can occupy a specific energy level can be found using the formula 2n^2, where n is the shell number.

The electronic configurations are those of isolated atoms. However, an isolated atom is unstable unless it has a full outer shell. Therefore only the inert gases (group 0) are able to exist separately. Atoms of all other elements combine (form bonds) with each other so that, after combination, a stable state is reached, i.e., a full outer shell. There are two ways in which this occurs:

  1. Covalent bonding
  2. Ionic bonding

Atoms of non-metals bond to each other by sharing one or more pairs of electrons. Each pair constitutes a covalent bond. Electrons spin about their own axes and this makes them behave like tiny magnets. When they pair up each electron spins in the opposite direction to its partner with the result that they attract each other. This attraction compensates for the repulsion due to their having the same charge. The entity formed when a group of atoms covalently bond together is called a molecule.

Let us consider how the carbon atom is linked to the four hydrogen atoms in the methane molecule CH4. An isolated carbon atom has four outer electrons while an isolated hydrogen atom has only one. The carbon atom shares 4 outer electrons with 4 hydrogen atoms. The carbon atom, now having a share in 8 electrons has achieved stability with a complete outer shell. Each hydrogen also now has a full outer shell (2 electrons).