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Mrs. Pesko Carl Sagan and Atomic Structure Let’s see what Carl Sagan has to say about the modern view of the atom and its structure! "Lives of Stars" - Cosmos Episode A Summary of Atomic Structure The atom consists of positive, negative, and neutral entities called protons, electrons, and neutrons Protons determine element’s identity # of protons is unique for each element Electrons determine an element’s chemical properties Neutrons act as a “glue” for the protons to minimize charge repulsions Protons and neutrons are located in the nucleus of the atom, which is small Therefore, the nucleus is positively-charged Electrons are located outside of the nucleus at an average distance of 10-8 cm Therefore, the electron cloud is negatively-charged Nucleus Size vs. Electron Cloud Size • Most of the volume of the atom is due to electrons Masses of the Proton, Neutron, Electron • Most of the mass of the atom is due to the protons and neutrons within the nucleus Element Symbols, Atomic Number, and Mass Number Atomic number (Z) Represents the number of protons in the nucleus Mass number (A) Represents the total number of nucleons in the nucleus (i.e., protons and neutrons) Mass Number = # of protons + # of neutrons Mass number is NOT the number on the Periodic Table Although numerically similar, this is the average atomic mass – more on this later! Isotopes There can be a variable number of neutrons for the same number of protons Isotopes have the same number of protons but different numbers of neutrons “Different” versions of the same element Masses are different due to the different number of neutrons So, isotopes have the same Z but different A! Isotopes of Carbon An Element’s Charge All elements on the periodic table are electrically neutral Positive charge of the protons equals negative charge of the electrons In order for an element to be neutral, the number of protons must equal the number of electrons So, atomic number indirectly equals number of electrons on the Periodic Table! Calculating # of Protons, Electrons, and Neutrons Atomic # Na Mass # 22 Hg 200 # of P+ # of N # of E- The Elements and the Periodic Table The Periodic Table is used to organize the 114 elements in a meaningful way Arranged by increasing atomic number Organization of the Periodic Table Columns in the periodic table are called groups Numbered from 1 to 18 Atoms with similar properties appear in groups They are similar because they all have the same number of valence (outer shell) electrons, which governs their chemical behavior Group numbers represent the number of valence electrons Group 1 has 1 valence electron Group 2 has 2 valence electrons Groups 13 – 18 must subtract 10 to get # of valence electrons Families of the Periodic Table Some of the groups in the periodic table are given special names, and are called families These names indicate the similarities of chemical properties between group members as a result of same number of valence electrons: Group 1:Alkali metals Group 2: Alkaline earth metals Groups 3-12: Transition Metals Group 17: Halogens Group 18: Noble gases Organization of the Periodic Table Rows in the periodic table are called periods Numbered from 1 to 7 Elements of the same period have the same number of energy levels As you move across a period, the number of electrons and protons increases, leading to increase in atomic number Elements within the same period do not generally show similarity in properties, except dblock and f-block (lanthanides) elements Review of the Periodic Table Organization of the Periodic Table Metals are located on the left hand side of the Periodic Table Most of the elements are metals Non-metals are located in the top right hand side of the Periodic Table Elements with properties similar to both metals and non-metals are called metalloids and are located at the interface between the metals and non-metals Non-metals above the staircase Metals below the staircase Properties of Metals and Non-Metals Metals Shiny All but gold and copper are silver or grey in color Conduct electricity Ductile Malleable Lose electrons Non-metals Not shiny Most are colored Do not conduct electricity Not malleable Gain electrons Metals Non-Metals More About the Elements Some elements are found in nature in their elemental form Most elements combine naturally with each other to become more energy-stable materials Atoms of an element gain, lose, or share valence electrons with other atoms to become chemically stable Atoms of elements are not chemically stable until they have 8 electrons (octet rule) Hydrogen is an exception – only needs 2 electrons! Formation of Positive Ions When an atom or molecule loses electrons, it becomes positively charged For example, when Na loses an electron it becomes Na+ Positively charged ions are called cations Formation of Negative Ions When an atom or molecule gains electrons, it becomes negatively charged For example when Cl gains an electron it becomes Cl Negatively charged ions are called anions An atom / molecule can lose or gain more than 1 electron! Oxidation Numbers Defined as the overall charge of an ion after it has lost or gained electrons Metals lose electrons, so they form cations Non-metals gain electrons, so they form anions Can be determined by looking at the Periodic Table Since ions are formed by losing or gaining valence electrons, the group number also represents the charge the elements in that group like to form! Predicting Oxidation Numbers
