Unit C: Atoms, elements, and Compounds 6.7: Explaining the Periodic Table pg. 234 Key Concepts: 3. Elements are organized according to their atomic number and electron arrangement on the periodic table. 5. Atoms contain protons and neutrons in a central core surrounded by electrons. 6. Elements can be both beneficial and harmful to humans and to the environment. Atomic Number Atomic Number: the number of protons in an atom s nucleus. - Dalton s Atomic Theory states that the atoms of each element are different from atoms of other elements. - Atoms are unique because of the number of protons they have in their nucleus. - The number of protons in the nucleus is known as the Atomic Number. - Elements are arranged on the Periodic Table according to increasing Atomic Number. - You can not combine different atoms to make new elements. Copper (Cu 29P + ) + Tin (Sn 50P + ) Gold (Au 79P + ) - Protons are tightly contained within the Nucleus of an atom. They can not combine with other atom s nucleus to form new atoms.
Figure 2: the atomic number is given in the top left-hand corner of each element on the periodic table. Mass Number and Atomic Mass Mass Number: the number of protons and neutrons in an atom s nucleus. Isotope: an atom with the same number of protons but a different number of neutrons. Atomic Mass: the mass of an atom in atomic mass units (u). - An atom is made up of mostly empty space. Electrons are found in this empty space and make up very little mass. _ the mass of an atom is found within its nucleus, yet the nucleus takes up very little space. - Example, Lithium: Lithium has the atomic number of 3. It has 3 protons in the nucleus. - It has an Atomic Mass of 6.94 u. - Atomic Mass relates to the mass of the sub atomic particles (Protons and Neutrons) within the nucleus. - The most common form of Lithium has an Atomic mass is 6.0 u and there are 3 Protons in the nucleus the rest of the mass must come from neutrons which are also found in the nucleus. 6u 3 = 3 neutrons are found in the nucleus.
- The mass of a proton is equal to the mass of a neutron. 1:1 ratio. - An isotope is an atom with the same number of protons (3) for lithium, but a different Atomic Mass, 7u for Lithium. - Mass Number of 7 u 3 (Protons) = 4 neutrons in the nucleus. - The True Atomic Mass of Lithium is not a whole number but a combination of Lithium 6u and Lithium 7u, mass number of 6.94 u. This is the average weight of the two isotopes. - When determining the number of neutrons; round the Atomic Mass to the nearest whole number and then subtract the number of protons Figure 4: A Lithium atom contains 3 protons and 4 neutrons, giving it a mass number of 7. Figure 5: One Lithium isotope contains 3 protons and 4 neutrons, giving it a mass number of 7. The other lithium isotope contains 3 protons and 3 neutrons, giving it a mass number of 6.
Figure 6: The element lithium has an Atomic number of 3 and an atomic mass of 6.94 u. Sample Problem 1: Finding the Number of Neutrons Find the number of neutrons in the most common isotope of Aluminum. Bohr-Rutherford Diagrams of an Atom Bohr-Rutherford diagram: a simple drawing that shows the numbers and location of protons, neutrons, and electrons in an atom. - Bohr-Rutherford diagram shows the number of protons and neutrons found within the nucleus, and the number of electrons orbiting around the nucleus. - The number of protons equals the Atomic Number. - The number of protons always equals the number of electrons for neutrally charged atoms. - The number of neutrons is equal to the Atomic Mass subtract the Atomic Number.
Sample Problem 2: Drawing a Bohr-Rutherford Diagram Step 1: Determine the amount of protons and neutrons Determine the number of Electrons. Step 2: Draw circles to represent the nucleus and electron shells surrounding the nucleus. Step 3: Draw the electrons in their electron shells. There are a maximum of 2 electrons in the first shell, 8 electrons in the second shell and 8 electrons in the third shell. Identify the number of protons and neutrons found in the nucleus. The Periodic Table Meets Bohr-Rutherford - The Bohr-Rutherford model is used to explain the pattern of families of elements in the periodic table, first 20 elements.
Patterns in the Periodic Table - As you go down each family, the number of electron shells increase by one. Each row increases by one orbit. - Elements in the same family have the same number of electrons in their outer shell. - Elements found in the same family undergo similar chemical reactions, because they have the same number of electrons in their outer electron shell or orbit. Figure 11: These elements have similar properties because they each have one outer electron.
Table 1: Theory Meets Evidence Evidence Elements have unique properties Same objects can attract each other In Rutherford s gold foil experiment, positively charged particles were fired at a sample of gold atoms. * Most particles passed through the foil. * Some particles were deflected at large angles. Elements of the same chemical family have similar properties. Each Element gives off unique pattern of coloured lines when it is excited by energy. Theory - each element is made of different atoms - All atoms contain three subatomic particles: protons, neutrons, and electrons. - Atoms are electrically neutral because they have an equal number of protons and electrons. - Matter is positively charged if it contains more protons than electrons. - Matter is negatively charged if it contains more electrons than protons. - Objects that attract each other are oppositely charged. - Moat of the volume of the atom is empty space. - The centre of the atom is very small, dense, positively charged core called the nucleus. - Electrons exist in specific orbits around the nucleus. - The number of electrons in the outermost orbit determines many elemental properties. - Elements of the same chemical family have the same number of electrons in their outermost orbits. - Each electron orbit has a definite amount of energy. - Electrons that absorb energy jump to higher orbits. - Electrons emit energy when they return to lower orbits. - The colour of light observed corresponds to the energy difference between the two electron orbits.
Evidence of Learning: Students can - describe and relate atomic number, atomic mass, and mass number. - find the number of neutrons in an atom from its atomic number and atomic mass. - draw a Bohr-Rutherford diagram for a small atom. - relate Bohr-Rutherford diagrams to the placement of atoms on the periodic table. Check Your Learning Questions 1 10, page 240 Summary: - Atomic number represents the number of protons in an atom. This number is unique to each element. - In a neutral atom, the number of electrons equals the number of protons. - Atomic mass is the mass of an atom measured in atomic mass units, (U). - Mass number represents the total number of protons and neutrons in an atom. - The maximum number of electrons in the first, second, and third orbits is 2, 8, and 8, respectively. Electrons fill lower orbits before filling higher orbits. - Bohr-Rutherford diagrams of atoms show the number and location of protons, neutrons, and electrons in the atom. - All atoms within each family of elements have the same number of electrons in their outermost orbits.