Atomic Structure Section 3.3 Distinguishing Between Atoms OBJECTIVES: Explain how the atomic number identifies an element. Use the atomic number and mass number of an element to find the numbers of protons, electrons, and neutrons. Explain how isotopes differ, and why the atomic masses of elements are not whole numbers. Calculate the average atomic mass of an element from isotope data. represents the number of protons and neutrons in the nucleus of an atom. The Atomic Number, Z, represents the number of protons in the nucleus. The Atomic Number, Z, also represents the number of electrons in a neutral atom. The Mass Number, A, represents the total number of protons and neutrons in the nucleus of an atom. The Atomic Number is 11: # p + = 11 protons, # e - = 11 electrons The Mass Number is 23: # n 0 = 23 11 = 12 neutrons Find the Mass Number 1 F Find the number of electrons Mass Number 80 Br 35 1
if an element has an atomic number of 3 and a mass number of 78 what is the if an element has 1 protons and 10 neutrons what is the Mass number if an element has 78 electrons and 117 neutrons what is the Mass number What are Isotopes? Think of them as cousins Dalton was wrong. Atoms of the same element can have different numbers of neutrons. These atoms of the same element would have different mass numbers. These cousins of the same element are called isotopes. Isotopes Atoms of the same element with different number of neutrons in the nucleus are called isotopes. Isotopes have the same atomic number but different mass numbers. All atoms of the same element have the same number of protons. Most elements occur naturally with varying number of neutrons. 1 1 H 2 1 H 3 1 H Hydrogen Isotopes Hydrogen has isotopes 1 proton and 0 neutrons, protium 1 proton and 1 neutron, deuterium 1 proton and 2 neutrons, tritium radioactive 2
Naming Isotopes Isotopes To name an isotope properly, we put the mass number after the name of the element. carbon- 12 carbon -1 uranium-235 Examples: Cont d Isotopes 1. Write the symbol for Cobalt-60. 2. How many protons and neutrons does an atom of mercury-202 have? 3. How many electrons are present in an atom of Copper-63? Cont d Isotopes Qs. If we look at the Periodic Table, all of the elements have a Mass Number that is in decimal form, or not a whole number why?? Ans.The Mass Number reported on the periodic table is a Weighted Average of the Isotopes that are naturally occurring in nature. Let s look at this a bit further Measuring Atomic Mass The Mass Number = the Atomic Mass, or Atomic Weight. This number not only tells us: The mass of one mole of an element (Unit 2) about the number of neutrons and protons present It also tells us the mass of one atom of an element relative to an atom of another element. Unit is the Atomic Mass Unit (amu). The standard for the amu scale is carbon: Carbon = 6 p + and 6 n 0 = mass standard of 12 amu All other atoms are designated relative to Carbon. i.e. hydrogen is 1/12 th the mass of carbon, or an Atomic Mass of 1 amu. One More Visit to the Properties of Subatomic Particles Particle Symbol Relative Charge Mass in Amu s Actual Mass (g) Electron e - 1-0.11 x 10-28 Proton p + 1+ 1 1.67 x 10-2 Neutron N 0 0 1 1.67 x 10-2 3
Mathematics of Isotopes Mathematics of Isotopes A simple average assumes that each object has the same number of occurrences. A weighted average means that the numbers of all of the object are not equal, but occur differently. A weighted average is calculated by multiplying the percent of the object (as a decimal number) by its mass for each object and adding the numbers together. Mathematics of Isotopes Some Natural Isotopes Isotope Examples Magnesium has three isotopes. 78.% magnesium 2 with a mass of 23.850 amu, 10.00% magnesium 25 with a mass of 2.858 amu, and the rest magnesium 26 with a mass of 25.826 amu. What is the atomic mass of magnesium? Boron is 20% B-10, and 80% B-11. What is the Atomic Mass of Boron? What are Ions? Result from the loss or gain of electrons by an atom
What are Ions? Result from the loss or gain of electrons by an atom What are Ions? Result from the loss or gain of electrons by an atom + 2e - + 1 e - Be Be +2 A Positive Ion: A CATION 1 1 - F F A Negative Ion: An ANION Determine the number of p +, n o, and e - for the following: Ion p + n o e - F - Cl - Na + Ca +2 Al +3 O 2- Br - Element Let s Complete the Table Symbol Atomic Number (Z) protons neutrons electrons charge Oxygen 0 16 Ru 58 2+ Mg +2 Mass # (W) 26 23 56 17 1-35 Nitrogen 8 10 18 2-3 Let s Review 1. Atoms are composed of,, and. 2. The & are located in the nucleus, while the are outside of the nucleus. 3. Atoms are most empty space, or dense clouds of + & - charges?. The number of protons is referred to as the for the atom. 5. All atoms of the same element have the same number of. 6. Isotopes have the same number of, but a different number of. 7. The mass number of an element is the total number of &. 8. Why is the atomic mass of an element given in decimal form in the periodic table? 5