Atoms
Tuesday, January 26, 2010
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Definition:
An atom is the smallest unit of a substance that retains the properties of that substance.
The name atom comes from the Greek word átomos, which means uncuttable, or indivisible, something that cannot be divided further.
Atomic Structure
An atom consists of a dense central nucleus surrounded by a cloud of negatively charged electrons.
The nucleus is the positively charged central region of an atom .The nucleus of an atom is of 20,000 times smaller than the size of the atom. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons 
(except in the case of hydrogen-1, which is the only stable nuclide with no neutron).Both protons and neutrons are referred to as nucleons which are bound together by the strong force. Both protons and neutrons have the same mass. The nucleus contains almost all of the mass of the atom.
Neutrons provide a means to “glue” the protons in place. Without neutrons, the nucleus would split apart because the positive protons would repel each other.
Electrons orbit the nucleus very quickly. Electrons are negatively charged and have almost no mass. The electrons of an atom are bound to the nucleus by the electromagnetic force. Electrons occupy a lot of 'empty' space.
The number of electrons in a neutral atom = the number of protons in the nucleus, making the normal atom electrically neutral. Thus the constituent particles of an atom are the proton, electron and neutron.
A Summary of the Sub-atomic Particle Properties:
| Mass | Charge | Symbol | Location | |
| Proton | 1 | +1 | p+ | Found in the nucleus |
| Neutron | 1 | 0 | n0 | Found in the nucleus |
| Electron | 1/2000 | -1 | e- | Found orbiting the nucleus |
Atomic number is the number of protons present in the nucleus or the number of electrons present outside the nucleus. It is denoted by the letter Z.
Atomic number (Z) = Nuclear charge or number of protons (p)
= Number of electrons (e)
- Mass number
The sum of the number of protons and neutrons in the nucleus of an atom is called the mass number. It is represented by the letter A.
Mass number (A) = Number of protons (p) + number of neutrons (n)
Example (1):
| Protons (Z) | Neutrons (N) | Atomic Number (Z) | Mass Number (A) | |
| Hydrogen | 1 | 0 | 1 | 1 |
| Helium | 2 | 2 | 2 | 4 |
| Lithium | 3 | 4 | 3 | 7 |
| Sodium | 11 | 11 | 11 | 22 |
Number of electrons = Atomic number= 3
Number of protons = Atomic number = 3
Number of neutrons = A - Z = 7-3 = 4
An atom is represented by its symbol for the element (X) with the atomic number written on the lower side of the symbol and the mass number written on the upper side.
posted by Lekshmi @ 9:58 AM,
