Do Now: If a student’s grade is weighted per the table below, what would their grade be? Tests Classwork Homework Weight 50% 30% 20% Average 80 95 85 YWBAT • Explain what makes elements and isotopes different from each other and how isotopes of an element differ. • Calculate the atomic mass of an element. Distinguishing Among Atoms • An atom is the smallest particle of an element that still has the chemical properties of that element. • But what makes atoms of one element different from another? Atomic Number • Atomic Number: the number of protons in the nucleus of an atom of that element. It’s like an ID number for an element Atomic Number • Atoms are electrically neutral so Protons = Electrons Mass Number • Mass Number: total number of protons and neutrons in an atom Mass Number = protons + neutrons Mass Number • You can also refer to atoms by using the mass number and the name of the element. Atoms If you know the atomic number and mass number, you can determine the atom’s composition. # Protons = Atomic Number # Electrons = # Protons # Neutrons = Mass Number – Atomic Number Practice How many protons, neutrons, and electrons are in each of the atoms? Isotopes Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Hydrogen has 3 isotopes The correct way to represent an isotope is to write the chemical name followed by the mass number (protons + neutrons) Hydrogen has 3 isotopes 1 0 1 1 1 1 1 2 1 The correct way to represent an isotope is to write the chemical name followed by the mass number (protons + neutrons) Isotopes Just because the number of neutrons is changing does not change which element it is because the atomic number does not change. Isotopes • In nature, most elements occur as a mixture of two or more isotopes. • Each isotope of an element has a fixed mass and a natural percent abundance. Average Atomic Mass • The mass that is listed on the periodic table is an average atomic mass. • It is a weighted average of the atomic masses of naturally occurring isotopes. Calculating Average Atomic Mass Chlorine-35 has an amu of 34.9689 with an abundance of 75.771% while Chlorine-37 has an amu of 36.9659 and an abundance of 24.229% What is the average atomic mass of chlorine? Carbon-12 (12.00 amu) makes up 98.93% of all of the carbon atoms, while carbon-13 (13.003 amu) is about 1.07% abundant. What is the average atomic mass of carbon? Calculating Average Atomic Mass Chlorine-35 has an amu of 34.9689 with an abundance of 75.771% while Chlorine-37 has an amu of 36.9659 and an abundance of 24.229% What is the average atomic mass of chlorine? 35.453 Carbon-12 (12.00 amu) makes up 98.93% of all of the carbon atoms, while carbon-13 (13.003 amu) is about 1.07% abundant. What is the average atomic mass of carbon? 12.011 Review How are the atoms of one element different from the atoms of another element? How are isotopes of the same element different? Electrons and the Structure of the Atom • Atoms of the same element have the same number of protons, which is equal to an atom’s atomic number. • But atoms of the same element can have different numbers of neutrons. • Atoms of the same element with different numbers of neutrons are isotopes. Atomic Mass Units • The actual masses of individual atoms are inconveniently small. • So instead, we compare the relative masses of atoms using a reference isotope as a standard. • The reference isotope chosen is carbon-12. • The isotope of carbon has been assigned a mass of exactly 12 atomic mass units. Atomic Mass Unit (AMU) • An atomic mass unit is defined as one-twelfth of the mass of a carbon-12 atom.