chemistry atomic structure

• All substances are made of atoms.
• A substance with only one sort of atom is an element.
• An atom is the smallest piece of an element that can exist.
• A molecule is formed when atoms join together by chemical bonds.
• The structure of an atom in terms of the positions, relative masses and relative charges of sub-atomic particles is: proton 1 +1 in the nucleus, neutron 1 0 in the nucleus, electron 1/1836 -1 in shells around nucleus.
• Atomic (proton) Number is the number of protons (= number of electrons if it’s an atom, because atoms are neutral).
• Mass (nucleon) Number is the number of protons + neutrons.
• Isotopes are different atoms of the same element containing the same number of protons but different numbers of neutrons in their nuclei.
• Relative atomic mass (of an element) is an average value that takes account of the abundance of the isotopes of the element.
• The relative atomic mass of an element (Ar) can be calculated from isotopic abundances.
• A sample of chlorine gas is a mixture of 2 isotopes, chlorine-35 and chlorine-37.
• These isotopes occur in specific proportions in the sample i.e. 75% chlorine-35 and 25% chlorine-37.
• The average mass, or R.A.M. of chlorine can be calculated using the following equation: R.A.M. = (mass of isotope-A x % of isotope-A) + (mass of isotope-B x % of isotope-B) 100.
• The relative atomic mass of chlorine in the sample is 35.5.