Sunday, December 27, 2009

Structure of matter

Structure of matter

All matter in the world about us consists of atoms. These are the basic building blocks of the elements such as hydrogen, carbon, oxygen, iron, and lead. Each atom contains a tiny central positively charged nucleus and a number of electrons. The electrons carry negative electric charge and move around the nucleus in clouds - or shells as they are called - with loosely defined boundaries. The nucleus is typically 10 000 times smaller than the electron clouds and the electrons themselves are even smaller. This means that the atom is mainly empty and difficult to depict except in diagrams, which are largely schematic.

The nucleus of the atom contains protons, which carry a positive charge equal to the electron's negative charge, and neutrons, which carry no charge at all. Each atom contains equal numbers of protons and electrons and is therefore electrically neutral. Atoms of the same or different elements can, combine to form larger, uncharged entities called molecules. For example, two atoms of oxygen form one molecule of oxygen, and two atoms of hydrogen combine with one atom of oxygen to form one molecule of water.

The number of electrons in the atom - and hence the number of protons in the nucleus, called the atomic number - gives an element its unique characteristics. The atomic number of carbon is 6, for instance, where as for Helium it is 2. Because protons and neutrons have the same mass, and are much heavier than electrons, most of an atom's mass is concentrated in ,the nucleus, and the total number of protons plus neutrons is called the mass number.

The carbon atom can be represented with a nucleus of 6 protons and 6 neutrons within 6 orbital electrons for hydrogen the nucleus has one proton and zero neutron within one orbital electron .

Since the number of electrons equals the number of protons in an electrically neutral atom, we can specify an atomic species by the number of protons and neutrons it contains.

Moreover, since the number of protons is unique to each element, we can simply use the name of the element together with the mass number to specify each spe­cies or nuclide. So carbon-12 is a nuclide with six protons plus six neutrons.
Nuclides of an element that have the same number of protons, but different numbers of neutrons, are called isotopes of that element. Hydrogen, for instance, has three isotopes: hydrogen-1 (common hydrogen with a nucleus of only one proton), hydrogen-2 called deuterium (one proton and one neutron), and hydrogen-3 called tritium (one proton and two neutrons).

Carbon has 3 isotopes , carbon-12 , carbon-13 and carbon-14 , all with 6 protons that characterize the element but with 6 , 7 and 8 neutrons respectively .

No comments:

Post a Comment