1) An atom is the smallest particle of an element and is approximately 0.00000003 cm in diameter. 2) Atoms are made up of a nucleus containing protons and neutrons, surrounded by electrons. 3) The number of protons determines the element, while the number of neutrons distinguishes between isotopes of that element.

1. The Atom Physical Science Chapter 4.2

2. Objectives: Describe the size of an atom Namethe parts of an atom Describe the relationship between numbers of protons and neutrons and atomic number State how isotopes differ Calculate atomic masses Describe the forces within an atom Bellringer An atom is the smallest particle into which an element can be divided and still be that element. Now that scientists have learned that an atom is made up of even smaller particles, is this definition still accurate?

3. How small is an atom? Three One-Hundred-Millionths of a Centimeter! Scientists know that aluminum is made of average-sized atoms. An aluminum atom has a diameter of about 0.00000003 cm

4. What is an atom made of? The Nucleus Protons Positively charged particles in the nucleus Neutrons Particles of the nucleus that have no electrical charge Outside the Nucleus Electrons Negatively charged particles in atoms Found around the nucleus within electron clouds

6. How do atoms of different elements differ? Starting Simply The hydrogen atom has one proton and one electron Now for Some Neutrons The helium atom has two protons, two neutrons, and two electrons

7. Building Bigger Atoms For bigger atoms simply add protons, neutrons, and electrons Protons and Atomic Number Atomic number The number of protons in the nucleus of an atom All atoms of an element have the same atomic number

8. Isotope Atoms that have the same number of protons but have different numbers of neutrons

9. Properties of Isotopes An unstable atom is an atom with a nucleus that will change over time Radioactive isotope Telling Isotopes Apart You can identify each isotope of an element by its mass number Mass number Sum of the protons and neutrons in an atom

11. Naming Isotopes To identify a specific isotope of an element write the name of the element followed by a hyphen and the mass number of the isotope Carbon-12 6 protons, 6 neutrons Chlorine isotope has 17 protons and 18 neutrons SOchlorine-35, Mass number = protons + neutrons = 17 + 18 = 35 Calculating the Mass of an Element Atomic massof an element weighted average of the masses of all the naturally occurring isotopes of that element

13. Forces in the atom Four Basic Forces Four basic forces are at work everywhere even within the atom These forces are: gravitational force electromagnetic force strong force weak force These forces work together to give an atom its structure and properties