Chem-Phys, Chapter 17 Page 1 Chapter 17 – Properties of Atoms and the Periodic Table

Chem-Phys, Chapter 17 Page 2 Scientific Shorthand Symbols are used to identify elements: –OOxygen –HHydrogen –AlAluminum –AgSilver (Latin name: Argentum) Symbols can be arranged in formulas: –H 2 ODihydrogen monoxide (water!) –CO 2 Carbon dioxide

Chem-Phys, Chapter 17 Page 3 Symbols of Some Elements ElementSymbolElementSymbol AluminumAlIronFe CalciumCaMercuryHg CarbonCNitrogenN ChlorineClOxygenO GoldAuPotassiumK HydrogenHSodiumNa

Chem-Phys, Chapter 17 Page 4 Atomic Components An atom is the smallest piece of matter that still retains the properties of the element. Atoms are composed of smaller particles: –Nucleus – the positively charged center of the atom –Inside the nucleus are protons (positively charged, +1) and neutrons (neutral charge) –Surrounding the nucleus are negatively charged (-1) electrons

Chem-Phys, Chapter 17 Page 5 Quarks – Even Smaller Particles Protons and Neutrons are composed of even smaller particles called Quarks. Electrons are one of the basic types of particles and are not composed of smaller particles. There are six different types of quarks which have been found.

Chem-Phys, Chapter 17 Page 6 Finding Quarks Evidence for quarks was found using particle accelerators like the Tevatron at the Fermi National Laboratory in Batavia, Illinois.

Chem-Phys, Chapter 17 Page 7 Model Models are used to represent things that are difficult to visualize - or picture in your mind. Examples: –Map – a model of a city or state. –Globe – a model of the earth. –Toy train – a model of an actual train

Chem-Phys, Chapter 17 Page 8 Atomic Models In 400 B.C. Democritus proposed that matter is composed of tiny particles called atoms. He had no evidence. Around the same time, Aristotle, another Greek philosopher, proposed that matter was uniform throughout and NOT composed of smaller particles. He also had no evidence! In the 1800’s John Dalton offered proof that atoms exist.

Chem-Phys, Chapter 17 Page 9 Modern Electron Cloud Model of the Atom An electron cloud is the area around the nucleus of an atom where its electrons are most likely to be found. The positions of the electrons can not be pinpointed. The electron cloud is 100,000 times bigger than the nucleus. However, each electron is much smaller than a proton.

Chem-Phys, Chapter 17 Page 10 Chapter 17, Section 1 Review 1.Identify the names and symbols of common elements. 2.Identify subatomic particles such as quarks, protons, neutrons, and electrons. 3.Describe the electron cloud model of the atom. 4.Explain how electrons are arranged in an atom.

Chem-Phys, Chapter 17 Page 11 Atomic Mass The nucleus of the atom contains over 99% of the mass of the atom. An electron weighs about 1/2000 th as much as a proton. The mass of a proton and a neutron are almost equal.

Chem-Phys, Chapter 17 Page 12 Atomic Mass One amu (atomic mass unit) is 1/12 th of the mass of a carbon atom with 6 protons and 6 neutrons. The mass of one proton or the mass of one neutron is approximately 1 amu.

Chem-Phys, Chapter 17 Page 13 Atomic Number The atomic number is equal to the number of protons in the nucleus of an atom of that element. All atoms with 1 proton in the nucleus are considered to be hydrogen. All atoms with 2 protons in the nucleus are considered to be helium. Etc.

Chem-Phys, Chapter 17 Page 14 Mass Number The mass number for an atom is the sum of the protons in the nucleus and the neutrons in the nucleus. If you know the atomic number and the mass number, you can calculate the number of neutrons in the nucleus: # of neutrons = mass # – atomic #

Chem-Phys, Chapter 17 Page 15 Isotopes All atoms of a particular element have exactly the same atomic number (number of protons in the nucleus). However, not all atoms of a particular element have the same mass number (the same number of neutrons in the nucleus.) Atoms of an element with different numbers of neutrons in the nucleus are called isotopes.

Chem-Phys, Chapter 17 Page 16 Isotopes of Boron All Boron atoms have 5 protons. One isotope of Boron has 5 neutrons (mass number = 5 amu + 5 amu = 10 amu). Another isotope of Boron has 6 neutrons (mass number = 5 amu + 6 amu = 11 amu).

Chem-Phys, Chapter 17 Page 17 Average Atomic Mass The average atomic mass of an element is the weighted-average mass of the mixture of its isotopes. It is the black number in the box for the element on the periodic table. Naturally occurring Boron is 4/5 ths of the isotope with 11 amu and 1/5 th of the isotope with 10 amu. The average atomic mass of Boron = 4/5 x 11 amu + 1/5 x 10 amu = 10.8 amu

Chem-Phys, Chapter 17 Page 18 Radioactive Isotopes Some isotopes of some elements change, over time, into another element and give off radiation in the process. This radiation is why they are called radioactive. The time that it takes ½ of the atoms to change is called the “half-life.”

Chem-Phys, Chapter 17 Page 19 Half-Lives of Radioactive Isotopes Radioactive Isotope Changes to this element Half-Life Uranium-238Lead-2064,460 million years Potassium-40Argon-40, Calcium-40 1,260 million years Rubidium-87Strontium-8748,800 million years Carbon-14Nitrogen-145,715 years

Chem-Phys, Chapter 17 Page 20 Chapter 17, Section 2 Review 1.How do you compute the atomic number, average atomic mass, and mass number of an atom? 2.Identify the components of isotopes. 3.What is the interpretation of the average atomic mass of an element

Chem-Phys, Chapter 17 Page 21 Organizing the Elements In the Late 1800’s, a Russian chemist by the name of Demitri Mendeleev discovered that the elements could be grouped by their properties. This formed the first periodic table. Mendeleev’s table had blanks in it where he predicted that new elements would be discovered. He was correct!

Chem-Phys, Chapter 17 Page 22 Mendeleev’s Periodic Table

Chem-Phys, Chapter 17 Page 23 Mendeleev’s Predictions Predicted Properties of “Ekasilicon” (“Es”) Actual Properties of Germanium (Ge) Predicted in 1871Discovered in 1886 Atomic mass = 72Atomic mass = High melting pointMelting point = 938 o C. Density = 5.5 g/cm 3 Density = g/cm 3 Dark gray metalGray metal Den. of EsO 2 = 4.7g/cm 3 Den. GeO 2 = 4.23 g/cm 3

Chem-Phys, Chapter 17 Page 24 Improving the Periodic Table Henry Moseley, a young English scientist, discovered that Mendeleev’s arrangement of the elements was actually in order of atomic number. This is the ordering we use today.

Chem-Phys, Chapter 17 Page 25 The Atom and the Periodic Table In the modern periodic table, the elements are arranged in columns called groups, or families. The groups are arranged in 18 columns. Elements within a family have similar properties.

Chem-Phys, Chapter 17 Page 26 Electron Cloud Structure Floor (Nucleus) Step 1 – Energy Level 1 Energy Level 2 Energy Level 3 2 electrons 8 electrons 18 electrons Energy Level 4

Chem-Phys, Chapter 17 Page 27 Energy Levels The energy levels in an atom are numbered from 1 to 7. 1 is the lowest energy level and 7 is the highest. Row 1 on the periodic table fills level 1. Row 2 fills level number 2. Row 3 partially fills level number 3.

Chem-Phys, Chapter 17 Page 28 Electron Dot Diagrams In 1920, an American chemist, G. N. Lewis proposed a way of representing the outer electrons of an atom. Since all elements in column #1 have one outer electron, the electron dot diagram for these elements looks like this: HLiNaKRb

Chem-Phys, Chapter 17 Page 29 Electron Dot Diagrams Elements in column 17 have seven outer electrons. So the electron dot diagram for these elements looks like: FClBrI Elements in column 18 have eight outer electrons. So the electron dot diagram for these elements looks like: HeNeAr

Chem-Phys, Chapter 17 Page 30 Regions on the Periodic Table The horizontal rows on the periodic table are called periods. The first row is period 1, the second row is period 2, etc. Metals are located left of the “zig-zag line.” Non-metals are located to the right of the “zig-zag line.” Metaloids contact the “zig-zag line.”

Chem-Phys, Chapter 17 Page 31 Man-made Elements Scientists are continuing to try to synthesize new elements beyond 103. Element 118 was recently discovered. The search continues.

Chem-Phys, Chapter 17 Page 32 Elements in the Universe Hydrogen was the only element created during the “big bang.” Hydrogen combines in stars to make helium and other elements. When a star explodes (goes “supernova”) these elements are scattered around the galaxy. Many of the elements found on earth can be detected in other parts of the galaxy and universe.

Chem-Phys, Chapter 17 Page 33 Chapter 17, Section 3 Review 1.Explain the composition of the periodic table. 2.What does the periodic table tell us about the atomic mass? Atomic number? Chemical properties? 3.Explain what the terms metal, non-metal, and metaloid mean. 4.Explain how electrons are arranged in an atom.