Structure of an Atom 4.2

1. Structure of an Atom 4.2

2. Objectives • Identify 3 subatomic particles and compare their properties • Distinguish the atomic number of an element from the mass number of an isotope and use these numbers to describe the structure of atoms

3. Properties of Subatomic Particles • What are the building blocks of matter? • What are atoms made of? • Protons, electrons and neutrons are subatomic particles. • Rutherford found that the positive charge varies among elements and that the nucleus must contain at least one particle with a positive charge. With his gold foil experiment. • He name these particles protons. • A proton is a positively charged subatomic particles found in the nucleus of an atom. • Proton’s have a charge of +1

4. Protons, Neutrons and Electrons • J.J. Thomson detected in his cathode ray tube experiments were name electrons. • An electron is a negatively charged subatomic particle found in the space outside the nucleus. • Electrons has a charge of -1 • James Chadwick designed an experiment to show that neutrons existed. (Rutherford predicted them) • He found they had a neutral charge • A neutron is a neutral subatomic particle found in the nucleus of an atom (charge =0) • Neutrons have a mass almost equal to a proton

5. Comparing Subatomic Particles Protons, electrons and neutrons can be distinguished by mass, charge, and location in at atom

6. Atomic Number & Mass Number • Atoms of any given element always have the same number of protons. • Ex: one proton in the nucleus of each hydrogen atom. Hydrogen has been assigned the atomic number 1. • The atomic number of an element equals the number of protons in an atom of that element. • Atoms if different elements have different numbers of protons. • Sulfur has an atomic number of 16 because it has 16 protons

7. Atomic Number and Mass Number • Each positive charge in an atom is balanced by a negative charge. • The atomic number is then also equal to the number of electrons. • If hydrogen has an atomic number of one, how many electrons does it have?If Sulfur has 16 protons, how many electrons does it have?

8. Mass Number • How can you find the number of neutrons in an atom? • The mass number is the sum of the protons and neutrons in the nucleus of that atom. • An atom of aluminum with 13 protons and 14 neutrons has a mass number of 27. • If you know the atomic number and the mass number, you can find the number of neutrons by subtracting. Number of neutrons= mass number – atomic number

9. Isotopes • In Dalton’s atomic theory, all the atoms of a given element are identical. • We now know that every atom of a given element does not have the same number of neutrons • Isotopes are atoms of the same element wwhtdifferent numbers of neutrons and therefore different mass numbers. • Still have the same atomic number

10. Isotopes of helium Helium - 3 Helium - 4 Isotope Example • Ex: Oxygen has 8 protons. Some oxygen atoms have 8 neutrons and a mass number of 16. Other atoms have 9 neutrons and a mass number of 17. • Uranium-238 has a mass number of 238 and 146 neutrons in the nucleous • Uranium-235 has 143 neutrons. What is the atomic number of uranium? • 92

11. Isotopes • Most isotopes have the same properties as other variations of the same element. • Hydrogen however, is an exception. • Hydrogen-1 has No neutrons; Hydrogen-2 has one neutrons; Hydrogen-3 has two neutrons. • Since hydrogen-1 has only one proton, adding a a neutron doubles it’s mass. • Water with hydrogen-2 atoms is called heavy. • Heavy water has different properties than Hydrogen-1

12. Compare and Contrast • How are the compositions of heavy water and ordinary water similar?What type of hydrogen atoms does ordinary water contain? • What type of hydrogen atoms does heavy water contain? • Compare the properties of heave water and ordinary water. See figure 12 page 112