Counting Subatomic Particles Now that scientists have discovered that atoms can be subdivided into subatomic particles, there was a new problem. How do we count subatomic particles? We use terms like atomic number and mass number to do so. Atomic Number Atoms are composed of identical protons, neutrons, and electrons. How are atoms of one element different from those of another element? Each element contains a particular number of protons. The atomic number of an element is the number of protons in the nucleus. # protons in an atom = # electrons (if the atom is neutral!) Mass Number The mass number of an element is the average atomic mass of an element rounded to a whole number. Element # p # e # n Mass no. Carbon 6 Nitrogen 7 14 Mass number = 195 This number is equal to the number of protons and neutrons in the nucleus. Mass number = p + + n 0 Sodium 11 12 Uranium 92 238 Radon 136
Nuclear Symbols Contain the symbol of the element, the mass number, and the atomic number. Nuclear Symbols Find each of these a) number of protons b) number of neutrons c) number of electrons d) atomic number e) mass number Nuclear Symbols Nuclear Symbols If an element has an atomic number of 34 and a mass number of 78, what is the a) number of protons b) number of neutrons c) number of electrons d) complete nuclear symbol If an element has 91 protons and 140 neutrons, what is the a) atomic number b) mass number c) number of electrons d) complete nuclear symbol
Isotopes Naming Isotopes Frederick Soddy 1877-1956 Dalton was wrong about all atoms of elements of the same type being identical. Atoms of the same element can have different numbers of neutrons. Thus, different mass numbers! These atoms are called isotopes. 1912: English radiochemist Frederick Soddy proposes the idea of isotopes. 1921: Soddy wins Nobel Prize in Chemistry for this work When referencing isotopes of an element, it is important to indicate which mass number the particular isotope has. We typically name isotopes using their element name along with their mass number. Ex. carbon-12, carbon-14, hydrogen-1, hydrogen-2 Isotope Protons Neutrons Electrons Mass number Average Atomic Mass 8 10 Elements occur in nature as a mixture of isotopes. 33 42 15 31 29 63 6 8 The percentage of each isotope in the naturally occurring element on Earth is nearly always the same, no matter where the element is found. This percentage is taken into account when determining the average atomic mass of a particular element. The average atomic mass is the weighted average of the atomic masses of all naturally occurring isotopes of an element.
Calculating Average Atomic Mass To calculate the average atomic mass of an element, two pieces of information will be needed: percent abundance of each isotope and the atomic mass of each isotope. AAM = (mass1 x %A1) + (mass2 x %A2) + *Percent abundance (%A) must be in decimal form in order to conduct calculation. Calculating AAM Ex. 1 Oxygen has three naturally occurring isotopes, oxygen-16, oxygen-17, and oxygen-18. Oxygen-16 has a percent abundance of 99.762%, oxygen-17 has an abundance of 0.038%, and oxygen-18 has an abundance of 0.200%. The atomic masses of the three isotopes are 15.995 amu, 16.999 amu, and 17.999 amu, respectively. Calculate the average atomic mass of oxygen. Calculating AAM Ex. 2 There are three naturally occurring isotopes of neon. Their percent abundances and atomic masses are: neon-20, 90.51%,19.99244 amu; neon-21, 0.27%, 20.99395 amu; neon-22, 9.22%, 21.99138 amu. Calculate the weighted average atomic mass of neon. Calculating AAM Ex.3 Naturally occurring strontium consists of the following isotopes. Isotope Atomic mass, amu Percent abundance Strontium-84 83.913 0.56 Strontium-86 85.909 9.86 Strontium-87 86.909 7.00 Strontium-88 87.906 82.58 Calculate the weighted average atomic mass of strontium.
Calculating AAM Ex.4 The two naturally occurring isotopes of nitrogen are nitrogen-14, with an atomic mass of 14.003074 amu, and nitrogen-15, with an atomic mass of 15.000108 amu. What are the percent abundances of these isotopes? Relating Mass to Number of Atoms How can we determine the number of atoms in a particular number of grams of a substance? We can use three concepts: the mole, Avogadro s number, and molar mass. These three concepts allow us to relate atoms and mass. The Mole Avogadro s Number (il numero d Avogadro) Remember, that the mole is the SI base unit for measuring amount of substance. In particular, one mole is equal to the number of particles as there are atoms in exactly 12 grams of carbon-12. The mole is simply a counting unit like a dozen. Italian chemist Count Amedeo Avogadro devised a way to count the number of representative particles of a substance. Avogadro s number is the number of representative particles in exactly 1 mole of a pure substance. Avogadro s number = 6.022 x 10 23 particles
Molar Mass We can also define a mole in terms of the amount of substance that contains Avogadro s number of particles. The mass of one mole of a pure substance is called the molar mass of that substance. Molar mass is measured in grams/mole. Molar Conversions We can convert between particles, moles, and grams. atoms x NA moles x molar mass grams The molar mass of an element is numerically equal to the atomic mass of the element. NA molar mass