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How do atoms differ? Section 4.3 REVIEW: THE ATOM The nucleus REVIEW Particle Location Electron In the space surrounding nucleus Charge Relative Mass (amu) Actual Mass (g) 1- 0 (1/1840) 9.11 x 10-28 Proton Nucleus 1+ 1 1.673 x 10- Neutron Nucleus 0 1 1.675 x 10- 24 24 If all atoms are composed of protons, neutrons and electrons, how do elements differ? –What makes an atom of oxygen an oxygen atom? ATOMIC NUMBER (Z) • An element’s atomic number (Z) represents the number of protons in its nucleus • The number of protons in an atom’s nucleus determines the identity of the atom – If an atom has 9 protons it’s fluorine, if it has 20 it’s calcium • In a neutral atom: – number of protons = number of electrons, so: atomic number = # protons = # THE MODERN PERIODIC TABLE Hydrogen Atomic number (Z): number of protons Atomic mass: weighted average of element’s isotopes masses Element name 1 H Symbol: one or two letters 1.008 *Elements are arranged according to atomic number – An element’s atomic number determines its position on the periodic table • The periodic table is organized left-to-right, topto-bottom by increasing atomic number TAKE A LOOK AT A PERIODIC TABLE: • Complete the following table: Element Number of protons Number of electrons sodium (Na) Copper (Cu) Boron (B) 66 14 MASS NUMBER (A) • An atom’s mass number (A) represents the total number of neutrons and protons in the nucleus of an atom Mass number = # protons + # neutrons or Mass number = atomic number + # neutrons • How do you determine the number of neutrons in an atom? # neutrons = mass number (A) - atomic number (Z) • It is always true that: – A carbon atom has 6 protons in its nucleus – A neutral carbon atom has 6 electrons • A carbon atom also contains neutrons, but unlike electrons and protons the number of neutrons can change from carbon atom to carbon atom • Some carbon atoms have 6 neutrons, others have 7 neutrons and still others have 8 neutrons ISOTOPES • Dalton’s atomic theory states that all atoms of a given element are identical. This is mostly true • Atoms of the same element can differ in the number of neutrons • most elements have two or more isotopes • Isotopes are atoms of the same element with different numbers of neutrons (and therefore different masses) – atoms have the same atomic number but different mass numbers IDENTIFYING ISOTOPES • To distinguish one isotope from another an atom’s mass number is added after the element’s name: – carbon-12; carbon-14; uranium-235 • Remember the mass number of an isotope represents the sum of the neutrons and protons in the nucleus Potassium-39 Potassium-40 Potassium-41 protons 19 19 19 neutrons 20 21 22 electrons 19 19 19 isotope Hydrogen–1 (protium) Hydrogen-2 (deuterium) Hydrogen-3 (tritium) protons electrons neutrons 1 1 0 1 1 1 1 1 2 WRITING SYMBOLS FOR ATOMS MASS NUMBER = # PROTONS + # NEUTRONS A X SYMBOL ATOMIC NUMBER = # PROTONS Z PRACTICE 26 protons 56 Fe 26 26 electrons 30 neutrons PRACTICE: FILL IN THE BLANKS symbol atomic number Fe mass number 56 60 144 102 59 Al number number of number of of protons electrons neutrons 27 45 45 31 235 92 U uranium-235 used for nuclear reactors and atomic bombs 238 92 U uranium-238 TURN TO THE PERSON NEXT TO YOU: • You have an atom each of two isotopes of carbon: – What is the same about the two atoms? – What is different about the two atoms? – How can we tell the two atoms apart? IMPORTANT TO NOTE: • Isotopes with more neutrons have greater mass, but are chemically the same (under normal conditions) • The number of protons determines the identity of the element and electrons are responsible for an element’s chemical properties THE MODERN PERIODIC TABLE Hydrogen Atomic number (Z): number of protons Average atomic mass: weighted average of element’s isotopes masses Element name 1 H Symbol: one or two letters 1.008 *Elements are arranged according to atomic number ATOMIC MASS • The mass of an atom depends on the number of electrons, protons and neutrons it contains • because atoms are extremely small particles (even the smallest speck of dust can contain 10,000,000,000,000,000 atoms) it’s impractical to measure the mass of atoms in grams Particle Actual Mass (g) Electron 9.11 x 10-28 Proton 1.673 x 10-24 Neutron 1.675 x 10-24 MEASURING ATOMIC MASS • Because the actual masses of protons and neutrons are very small chemists have developed a way to measure atoms based on the Carbon-12 atom as the standard • Instead of grams, the unit we use is the Atomic Mass Unit (amu) – Carbon-12 is exactly 12 amu – 1 amu is exactly 1/12 of the mass of a carbon-12 atom • Protons and neutrons have a mass slightly greater than 1 Particle Mass (amu) amu Electron 0.000549 Proton 1.007276 Neutron 1.008665 AVERAGE ATOMIC MASS • When one consults the periodic table, the atomic mass of carbon is not 12.00 amu but 12.011 amu • This is because most elements have more than one naturally occurring isotope and the atomic mass on the periodic table is the weighted average of the mass of each of the element’s isotopes AVERAGE ATOMIC MASS • In nature elements exist as a mixture of their isotopes, for example chlorine: • 75% chlorine-35 • 25% chlorine-37 • Atomic mass of chlorine is 35.453 amu • So the average atomic mass is calculated by taking the weighted average of the isotopes’ masses – This is why an element’s atomic mass is not a whole number AVERAGE ATOMIC MASS OF CARBON Isotope Mass of isotope carbon12 carbon13 12.00000 amu 13.00335 amu carbon-12: carbon-13: Abundance of isotope (%) 98.90% 1.10% mass x abundance 12.000 x 0.9890 = 11.868 amu 13.00335 x 0.0110 = + 0.143 amu 12.011 amu PRACTICE: CALCULATE ATOMIC MASS FOR ELEMENT X. IDENTIFY THE ELEMENT Isotope Mass (amu) % abundance X-6 6.015 7.5% X-7 7.016 92.5% Isotope Mass (amu) % abundance 6X 6.015 7.5% 7X 7.016 92.5% 1. Mass contribution = (mass)(% abundance) 6X Mass contribution = (6.015)(.075) = 0.451 amu 7X Mass contribution = (7.016)(.925) = 6.490 amu 2. Sum of the mass contributions: 0.451 amu + 6.490 amu = 6.941 amu Which element is this?