Lesson 12

Atoms by Numbers

Atomic Number and Atomic Mass

Atomic Number - the number of protons in an atom of an element; elements on the periodic table are arranged in order by atomic number

A neutral atom is one where the number of electrons and protons are equal, therefore balanced.

• The mass of an atom is made up of the protons and the neutrons
• Changing the number of protons in an atom Changes the identity of the element

Practice Problems

3) If you have a sample of atoms and each atom has 12 protons in its nucleus, which element do you have? Magnesium, Mg.
5) Why does carbon, C, gave a larger atomic mass than boron,B, even though they both have 6 neutrons? Carbon has a larger atomic mass because even though the neutron numbers are the same, it has more protons than boron.

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