1. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. 4.4 Atomic Number and Mass Number All atoms of an element have the same number of protons and the same atomic number. Learning Goal Given the atomic number and the mass number of an atom, state the number of protons, neutrons, and electrons.

2. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Atomic Number The atomic number is a whole number specific for each element. is the same for all atoms of an element. is equal to the number of protons in an atom. appears above the symbol of an element in the periodic table. 11 Na Atomic number Symbol

3. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Atomic Number = Protons in an Atom Atomic number = number of protons—for example, the atomic number of H is 1; every H atom has one proton. the atomic number of C is 6; every C atom has six protons. the atomic number of Cu is 29; every Cu atom has 29 protons. Core Chemistry Skill Counting Protons and Neutrons

4. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Atomic Number = Protons in an Atom All atoms of lithium (left) contain three protons, and all atoms of carbon (right) contain six protons.

5. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Atoms are Neutral For neutral atoms, the net charge is zero. number of protons = number of electrons Aluminum has 13 protons and 13 electrons. The net (overall) charge is zero. 13 protons (13+) + 13 electrons (13–) = 0

6. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Study Check Use the periodic table to fill in the atomic number, number of protons, and number of electrons for each of the following elements: ElementAtomic Number ProtonsElectrons N Zn S

7. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Solution Use the periodic table to fill in the atomic number, number of protons, and number of electrons for each of the following elements: ElementAtomic Number ProtonsElectrons N777 Zn30 S16

8. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Mass Number The mass number represents the number of particles in the nucleus. is equal to the number of protons + the number of neutrons. is always a whole number. does not appear in the periodic table.

9. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Composition of Some Atoms of Different Elements

10. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Study Tips: Protons and Neutrons Number of protons = atomic number Number of protons + neutrons = mass number Number of neutrons = mass number – atomic number Note: Mass numbers are given for specific isotopes only.

11. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Study Check An atom of lead (Pb) has a mass number of 207. A. How many protons are in the nucleus? B. How many neutrons are in the nucleus? C. How many electrons are in the atom?

12. General, Organic, and Biological Chemistry: Structures of Life, 5/e Karen C. Timberlake © 2016 Pearson Education, Inc. Solution An atom of lead (Pb) has a mass number of 207. A. How many protons are in the nucleus? atomic number = 82; number protons = 82 B. How many neutrons are in the nucleus? mass number – number protons = number neutrons 207 – 82 = 125 neutrons C. How many electrons are in the atom? An atom is neutral, which means that the number of electrons is equal to the number of protons. An atom of Pb has 82 protons and 82 electrons.