1. Ch. 11 Section 2 The Atom

2. Vocabulary To Know  Proton- subatomic particle that has a positive charge and that is found in the nucleus  Atomic mass unit- a unit of mass that describes the mass of an atom or molecule  Neutron- a subatomic particle that has no charge and that is found in the nucleus of an atom  Atomic number- the number of protons in the nucleus of an atom

3. Vocabulary To Know Cont..  Isotope- an atom that has the same number of protons as other atoms of the same elements do but that has a different number of neutrons  Mass number- the sum of the numbers of protons and neutrons in the nucleus of an atom  Atomic mass- the mass of an atom expressed in atomic mass units

4. What is an Atom Made of?- Inside the Nucleus  The nucleus contains protons and neutrons.  Since the particles of an atom are so small scientists came up with a new SI unit of measurement know as AMU.  The atomic mass unit of proton is 1.  The atomic mass unit of a neutron is 1.  Protons and neutrons are the most massive particles in an atom.

5. What is an Atom Made of?- Outside the Nucleus  Electrons are negatively charged particles in atoms.  Electrons are located outside of the nucleus on the electron cloud.  The mass of an electron is so small that is has no AMU.

6. ION  An atom has an overall charge of zero.  When the protons and neutrons are not equal in an atom, the atom becomes a charged particle known as an ion.  An atom that loses one or more electrons becomes a positively- charged ion.  An atom that gains one or more electrons becomes a negatively- charged ion.

7. How do Atoms of Different Elements Differ?  Since there are more than 110 different elements, they all differ by the number of protons, neutrons, and electrons that they have.  The number of protons in the nucleus of an atom is the atomic number.  All atoms of the same element have the same atomic number.

8. Isotopes  Isotopes are atoms that have the same number of protons but have different number of neutrons.  Atoms that are isotopes of each other are always the same element since they have the same number of protons.  They have different number of neutrons, however which gives them different masses.  You can identify each isotope of an element by its mass number.  The mass number is the sum of the number of protons and neutrons in the nucleus of an atom.

9. Calculating the Mass of an Element  Most elements contain a mixture of two or more isotopes.  The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes of that element.  The atomic mass is the mass of an atom expressed in atomic mass units.

10. Forces in Atoms  Four basic forces are at work every where, even within the atom.  They are: gravitational force, electromagnetic force, strong force, and weak force.  These forces work together to give an atom its structure and properties.