1. Unit 13: The nucleus of an atom We know that atoms are composed of electrons, protons and neutrons. Protons and neutrons together (i.e. the nucleus) are called nucleons. Most of an atom’s mass is located in the nucleus. Radius of a nucleus is on the order of 10 -14 m

2. Unit 13: The nucleus of an atom

3. For Example: A= 27 Z = 13 This means that N = 14. There are 13 protons and 14 neutrons in an atom of Aluminum.

4. Unit 13: The nucleus of an atom Isotopes are atoms that have the same number of protons (or the same atomic number) as other atoms of the same element but that have a different number of neutrons and thus, a different atomic mass. Examples of Carbon isotopes:

5. Unit 13: The nucleus of an atom A nucleus is very dense. Nucleons combine to form a nucleus as though they were tightly packed spheres.

6. Unit 13: The nucleus of an atom Because the mass of a nucleus is extremely small, the unified mass unit, u, or the atomic mass unit, amu, is often used. 12 amu is defined as the mass of the nucleus of 1 carbon-12 atom. 1 amu = 1.66053886x10 -27 kg.

7. Unit 13: The nucleus of an atom The mass of one proton = 1 amu The mass of one neutron = 1amu The mass of one electron = 5x10 -4 amu

8. Unit 13: The nucleus of an atom A particle has a certain amount of energy, called rest energy associated with its mass.

9. Unit 13: The nucleus of an atom Energy is not conserved in many nuclear processes. For this reason, we use a particle’s mass in terms of its energy equivalent in the equation:

10. Unit 13: The nucleus of an atom This energy is measured in electron- volts (eV). Formal definition of eV: An eV is the amount of energy gained or lost by a charge of a single electron moved across an electric potential difference of 1 V. ◦ 1 eV = 1.6x10 -19 J

11. Unit 13: The nucleus of an atom Strong force is the interaction that binds nucleons together in a nucleus. ◦ Strong force is independent of electric charge ◦ Strong force has very short range, ~10 -15 m

12. Unit 13: The nucleus of an atom In order to break a nucleus apart into separated protons and neutrons, energy must be added to overcome this force’s attraction. This is the same amount of energy released when unbound nucleons come together to form a stable nucleus. This is called binding energy.

13. Unit 13: The nucleus of an atom This is called binding energy.

14. Unit 13: The nucleus of an atom The mass of the nucleons when unbound minus the mass of the nucleons when bound is called the mass defect. When nucleons bind together to form a nucleus, they lose a small amount of mass. There is a change in mass to stay bound. This mass change is released as various types of photons or other particle energy such as electromagnetic waves, gamma radiation, etc.

15. Unit 13: The nucleus of an atom Binding energy = mass defect * (speed of light) 2