1 Elements each element has a unique number of protons in its nucleus the number of protons define the element the number of protons in the nucleus of an atom is called the atomic number the elements are arranged on the Periodic Table in order of their atomic numbers Each element also has a corresponding mass number. Protons + neutrons.

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3 Elements Each element has a unique name and symbol. The symbol is either one or two letters  One capital letter or one capital letter + one lower case letter.  H = Hydrogen = “water-former”  Br = Bromine = ‘stench’ Liquid Bromine

4 The Periodic Table of Elements Atomic number Atomic mass Element symbol

Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons X A Z U Mass Number Atomic Number Element Symbol 2.3 Elemental & Isotope Notation Isotope Notation = Uranium-238

Example: How many protons, electrons, and neutrons are in an atom of ? for most stable isotopes, n 0 > p + Check: Z = 24 = # p + # e - = # p + = 24 Solution: in neutral atom, # p + = # e - mass number = # p + + # n 0 Concept Plan: Relationships: therefore A = 52, Z = 24 # p +, # e -, # n 0 Given: Find: symbol atomic number # p + # e - symbol atomic & mass numbers # n 0 A = Z + # n 0 52 = 24 + # n 0 28 = # n 0 6

Isotopes: Isotopes = atoms of the same element with different masses. Isotopes have different numbers of neutrons C 12 6 C 13 6 C 14 6 C 7 Carbon-11Carbon-12 Carbon-13 Carbon-14

Neon 9.25%221210Ne-22 or 0.27%211110Ne-21 or 90.48%2010 Ne-20 or Percent natural abundance A, mass number Number of neutrons Number of protonsSymbol 8

Mass Spectrometer Tro: Chemistry: A Molecular Approach, 2/e 9

Mass Spectrum A mass spectrum is a graph that gives the relative mass and relative abundance of each particle Relative mass of the particle is plotted in the x-axis Relative abundance of the particle is plotted in the y-axis 10

By definition: 1 atom 12 C “weighs” 12 amu 1 amu = x g On this scale 1 H = amu 16 O = amu Atomic mass is the mass of an atom in atomic mass units (amu) 3.1 Atomic Mass

Average Atomic Mass Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations. Average atomic mass is calculated from the isotopes of an element weighted by their relative abundances. 12

Natural lithium is: 6 Li (6.015 amu) = 7.42% 7 Li (7.016 amu) = 92.58% (7.42 x 6.015) + (92.58 x 7.016) / 100 = amu Average atomic mass of lithium: 13

14 Charged Atoms The number of protons determines the element. All sodium atoms have 11 protons in the nucleus. In a chemical change (aka: a reaction), the number of protons in the nucleus of the atom doesn’t change, however the number of electrons may. Atoms in a compound that gain or lose electrons and have a charge, these are called ions.

Types of Ions Cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. Anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. Na 11 protons 11 electrons Na + 11 protons 10 electrons Cl 17 protons 17 electrons Cl - 17 protons 18 electrons 2.5

16 Ions of Hydrogen

Ions 17 A monatomic ion contains only one atom Na +, Cl -, Ca 2+, O 2-, Al 3+, N 3- A polyatomic ion contains more than one atom OH -, CN -, NH 4 +, NO 3 - Nitrate Polyatomic Ion

18 Practice—Fill in the Table.

19 Practice—Fill in the Table, Continued.

Example —Find the Number of Protons and Electrons in Ca 2+. For cations, p + > e −, so the answer is reasonable.Check: Z = 20 = #p + Solution: ion charge = #p + − #e − Solution Map: Relationships: Ca 2+ # p +, # e -, # n 0 Given: Find: symbol atomic number # p + # e - ion charge = #p + − #e − +2 = 20 − #e − ─18 = ─ #e − 18 = #e − 20

21 Review What is the atomic number of boron, B? What is the atomic mass of silicon, Si? How many protons does a chlorine atom have? How many electrons does a neutral neon atom have? Will an atom with 6 protons, 6 neutrons and 6 electrons be electrically neutral? Will an atom with 27 protons, 32 neutrons, and 27 electrons be electrically neutral? Will an Na atom with 10 electrons be electrically neutral?