 | The first of these two regions is the nucleus, or central core of the atom which is composed of positively charged protons and neutrons with a neutral charge. It is believed that the neutrons are needed to hold the positively charged protons together in the nucleus. The force that holds these particles together is termed the nuclear binding force and it is believed to be one of the strongest forces that exists in nature. The nucleus takes up a very small portion of the atom. If the atom was the size of a football field the nucleus would be the size of a household fly on the 55 yard line. |
 | The second region surrounds the nucleus and is termed an electron cloud. The cloud holds the third particle which is a negatively charged electron. The electrons in a many electron atom are arranged in energy levels about the nucleus. The electrons in their lowest energy state (termed ground state) occupy these energy levels from lowest (closest to nucleus) to highest energy. Only certain numbers of electrons can be placed in each energy level. |
Characteristics of the Atom
1. The number of protons in the nucleus of an atom is termed it's atomic number. This number is distinctive (characteristics) for the atoms of each element.Atomic Number = # of protons
Isotopes are atoms of the same element with different numbers of neutrons.3. Another characteristic of an atom of an element is it's atomic mass (mass #). Protons and Neutrons have approximately the same mass, while electrons have very little mass in comparison to either Protons or Neutrons. The atomic mass is therefore determined from the number of protons and neutrons. It is often called the mass number (#).
Mass # (Atomic Mass) = # of Protons + # of Neutrons.4. The number of electrons in a neutral atom (no charge) is equal to the number of protons (atomic #). Atoms may either gain or lose electrons during chemical interactions with other atoms. If they gain electrons they become negatively charged, if they lose electrons they become positively charged. We term these charged atoms ions.
Since atoms of an element always have the same number of protons, isotopes are atoms of the same element with different mass numbers.
For Example: If Magnesium loses two electrons it would have 12 + charged protons and only 10 - charged electrons and would become a +2 ion. If Chlorine gains one electron it would have 35 protons (+) and 36 electrons (-) and become a - 1 ion. The charge on the ion indicates the number of electrons gained or lost.
Representing Isotopes
Scientists like to represent these isotopes using a special notation. The following symbols are often used:| A Z | A = Mass Number X = Element Symbol Z = Atomic Number | Examine the isotope notation of the isotopes in the diagram above. 1. What is the mass # of tritium? 2. What is the atomic number of each isotope of hydrogen illustrated in the diagram? 3. What is different between the isotopes illustrated? |
Mass Number - Atomic Number = Number of Neutrons
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