Structure of an Atom 4.2

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

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 Properties of Subatomic Particles

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

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

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

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?

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

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 wwht different numbers of neutrons and therefore different mass numbers. –Still have the same atomic number

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 Helium - 4 Helium - 3 Isotopes of helium

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

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