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Atoms – The Basics

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Inside the Atom Atoms are made up of smaller particles These particles are found in different regions of the atom

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Atomic Number & Mass Number Atomic number = # p + For atoms atomic number also = number of e- Mass number = # p + + # n 0 Mass number is a whole number Number on the periodic table is average atomic mass (not a whole number) How can you solve for numbers of neutrons?

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How Atoms Differ: Ions Different elements have different numbers of p + Ions have charges the number of e- ≠ number of p+ charge = # p + - # e - Ions are the SAME element, but DIFFER because they have either a positive or negative charge. If an atom GAINS electrons, its overall charge becomes more negative. If it LOSES electrons, its charge becomes more positive

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Isotopes Atoms with the same number of p + but different number of n 0 are isotopes. Mass number of isotopes of the same element changes Isotopes are the SAME element, but DIFFER because they have different masses.

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EX:

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Symbols of Ions and Isotopes For ions chemists use the following notation: Or just:

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For isotopes chemists use the following notation: Name of element – mass number Examples: carbon-12 carbon-14 uranium-236

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Determine the Numbers of p +, n 0, and e - for the following symbols given: calcium - 46 nickel - 60

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Complete the following table ProtonsNeutronsElectrons Na + Bromine- 84 O 2- with an atomic mass of 13amu

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Average Atomic Mass The masses of p + and n o are the same and they are actually tiny (1.67 x 10-24 g). We round it to 1 amu. The mass of an e- is even smaller 1/1840 of a proton or neutron so we say it is approximately 0 amu.

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We can do this because scientists measure the mass of an atom relative to a standard mass and that is carbon-12. Scientists agreed that carbon-12 has a mass of EXACTLY 12 atomic mass units (amu).

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So, why do the elements on the Periodic Table have masses with decimals??? They are Average Atomic Masses

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Average Atomic Masses This is the weighted average mass of all the isotopes of an element. Examples of weighted averages: Semester and yearly grades Taxes Budget

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Calculating the Weighted Average Atomic Mass 1. Multiply the mass of each isotope by the % of the isotope 2. Add the products

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Example Calculate the atomic mass of magnesium. The three magnesium isotopes have atomic masses and relative abundances of 23.985 amu (78.99%), 24.986 amu (10.00%) and 25.982 amu (11.01%). 24.31 amu

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You can also estimate which isotope is the most abundant. Ex: Fluorine has an atomic mass of 18.99840 amu. It has several naturally occuring isotopes Fluorine-14, Fluorine- 16, Fluorine-19, Fluorine-21, Fluorine-22. Guess which one is the most abundant? Helium has two naturally occurring isotopes: helium-3 and helium-4. The atomic mass of helium is 4.003amu. Which isotope is more abundant in nature and why?

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