Science Chemistry - Atomic Structure Yr10

The atomic mass is the weight of and element. Calculated by the total mass of protons and neutrons(as electrons weight 2000 times less than either)

The atomic number is the neutron number.As the number of neutrons is equal to the number of protons, we therefore know the number of electrons. also, this means that: the atomic mass - the atomic number = number of neutrons

An isotope is where the element has the same electronic configuration as usual, but the mass has changed (due to a different number of protons)

The top number is always the bigger of the two - the atomic mass (this is what changes in different isotopes)the bottom number is the atomic number which remains the same

This is taking into account all of the isotopes f that element, and forming a new mass. E.g.To calculate the relative atomic mass we see:
. A_r Ag = 107 + 109 = 108 2 A_r Br = 79 + 81 = 80 2

THE RELATIVE ATOMIC MASS OF AN ELEMENT, IS THE MASS OF AN ATOM OF THIS ELEMENT, COMPARED WITH THE MASS OF ONE ATOM OF CARBON ISOTOPE 12 (¹²C). (WHICH HAS A MASS OF 12.000 EXACTLY).

Non-stable isotopes are radioactive. Nucleus will split up and give off subatomic energy.From element 84 in the periodic table, ALL ELEMENTS ARE THEN RADIOACTIVE.

The time taken for half the atoms in the sample to decay is a constant property of the isotope. This property is called the half life t_½.What it means is this. If I have a sample of ¹⁴C, then after t_½ , half the atoms present will have decayed. After a further t_½ half of the remaining atoms will have also decayed, and so on. This means that t_½ does not depend on the number of atoms in the sample. Whatever this is, half of them will have decayed after t_½. For example ¹⁴C has t_½ = 5730 years. This is very convenient for archeologists, who use ¹⁴C dating as a way of finding the age of objects they find. (For instance the Shroud of Turin was proved to be a medieval cloth by carbon dating).

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