# Atomic Structure

HideShow resource information
• Created by: Brooke
• Created on: 15-07-13 12:26
• Atomic Structure
• Atoms are made up of sub-atomic particles called protons, neutrons and electrons
• The electrons have virtually no mass compared with the masses of protons and neutrons; therefore nearly all of the mass of the atom is concentrated in the nucleus
• Atomic Number, Mass Number and Isotopes
• Atomic Number = number of protons in the nucleus of an atom of an element
• Mass Number = number of protons + number of neutrons in the nucleus of an atom of an element
• For any atom, this information can be shown simply as:
• This atom of chlorine contains 17 protons and 18 neutrons (17+18=35 =mass number)
• Isotopes are atoms that have the same atomic number but different mass numbers. This is because they contain the same number of protons but different number of neutrons
• The Electrons
• The number of electrons in an atom is the same as the number of protons
• Electrons exist around the nucleus of atoms in different energy levels called electron shells. There are several electron shells
• The first electron shell surrounds the nucleus. The second surrounds the first and is therefore bigger and further away from the nucleus.
• The third electron shell surrounds the second and is therefore bigger than both the first and the second electron shells and further away from the nucleus than both of these
• Each electron shell can accommodate a fixed number of electrons: *The first shell can take up to a maximum of 2 electrons *the second shell can take up to 8 electrons  *the third shell can take up to a maximum of 18 electrons
• When you are trying to work out in which shells the electrons of a particular atom will be, the following rules should be followed: 1)Electrons always occupy those shells that are CLOSEST to the nucleus, 2) When a shell becomes full, the next one is started
• The arrangement of electrons in shells is known as its ELECTRONIC CONFIGURATION
• Electronic configurations are often given in a shorthand notation. For Example: Carbon, C 2.4
• Relative Atomic Mass
• The relative atomic mass is calculated from the masses and relative abundances of all the isotopes of a particular element
• Chlorine has two isotopes: chlorine-35 and chlorine-37. A typical sample of chlorine will be 75% chlorine-35 atoms and 25% chlorine-37 atoms. The RAM is calculated as follows: Total mass of 100 atoms= (75x35)+(25x37)=3550. Mean mass of one atom=93550/100)=35.5. RAM of chlorine is 35.5