The Atom. The Atom and The Periodic Table of Elements. Evolution of Atomic Theory

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1 The Atom The Atom and The Periodic Table of Elements An atom is the smallest particle of matter. It cannot be divided chemically An atom is very small: A sheet of paper is about a million atoms thick Because atoms are too small to be examined directly, scientists have devised various models to represent them The models have adapted over time and continue to change. The most current atomic theory found that the atom is made up of 3 subatomic particles: Protons (positively charged particles) Electrons (negatively charged particles) Neutrons (particles with no charge) Atoms have a nucleus or center that holds the protons and neutrons The relative mass of the atom is determined by the protons and neutrons (as they have the most mass) Atomic mass = # protons + # neutrons Shells or orbits hold revolving electrons Evolution of Atomic Theory Dalton s model Thomson s model Rutherford s model Bohr s model Simplified model Current model

2 The Periodic Table The Periodic table organizes elements into groups by their chemical and physical properties arranges elements according to their atomic number Each square on the Periodic table represents an element Inside the square certain information is given: The atomic number = the number of protons The chemical symbol The atomic mass = the number of protons + the number of neutrons Sometimes, the name is written as well Atomic mass Atomic number Name Symbol # of protons Protons + neutrons Groups/Families The periodic table is also arranged according to groups and periods Families or groups are different columns of elements that have similar chemical and physical properties. Groups/Families Elements in a particular group all have the same number of valence electrons Valence electrons are the electrons in the outermost shell The number of the column tells us the number of valence electrons in each group. Example: the elements of group 1 (such as sodium) all have one electron in the outer shell while the elements of group, like chlorine, all have valence electrons

3 Periods/Rows Each row is called a period The periods are numbered 1- goes across the table (from left to right) at the left the atoms are bigger and get smaller as they move across the period (Ex. Lithium is bigger than oxygen) The number of the period tells us the number of shells that element has REPRESENTS # OF SHELLS REPRESENTS # OF VALENCE ELECTRONS Major Categories of Periodic Table The Periodic table is separated into 3 major categories: metals, non metals and metalloids. A staircase shaped line separates and identifies the categories 1) Metals: found on the left hand side of the staircase Generally good conductors of heat and electricity Ductile and malleable can be shaped into wires or flattened into sheets Usually shiny Except mercury, they are all solid React with acids Non-metals on the right hand side of the staircase (except hydrogen) poor conductors of heat and electricity Many are gases at room temperature

4 Metalloids a cross between metals and non metals called semimetals have some properties of each group Sometimes good conductors of electricity, sometimes poor conductors of electricity depending on the condtions used to make semi conductors Metalloids include the following Boron (B), Silicon (Si), Geranium (Ge), Arsenic(As), Antimony (Sb), Tellurium (Te) and Polonium (Po) They form a staircase line that divides the metals and nonmetals Metals 1) Alkali Metals Special Families/Groups of the Periodic Table Column 1A(includes Li --> Fr) has an outer electron number of 1 Soft, low melting point, shiny metals, conduct heat and electricity Very reactive 2)Alkaline Earth metals Column 2A (includes Be --> Ra) has two outer electrons Harder, higher-melting metals, conduct heat and electricity Not as reactive as alkali metals

5 3) Halogens column A(includes F--> At) has outer electrons often reacts with metals Reacts well with hydrogen to produce acids Some are used as disinfectants Some are gases (Cl 2 and F 2 ), liquid (Br 2 ) and solids (I 2, At 2 ) at room temp. No other family is found in all three states at room temperature. Poor conductors Noble Gases Last column 8A (He Rn) All gases at room temperature Poor conductors often called inert gases because they are unreactive full outer shell (no free electrons) Hydrogen a group of 1 only 1 electron in outer shell very reactive but not an alkali metal colorless, odorless, tasteless and highly flammable Representing Atoms 3 types of models that you need to know: Lewis Notation Bohr-Rutherford model Ball and Stick model In general, the number of electrons is equal to the number of protons (the atomic number)

6 Lewis Notation Simplified representation of the atom Shows only the symbol and valence electrons represented by dots Valence electrons are placed one by one around the symbol (at the top, sides and the bottom), once these four places are filled, the electrons are doubled to form pairs Examples 1)B (boron) How many valence electrons does boron have? 3 Draw the symbol, then start adding the electrons: B B 2) Cl (chlorine) How many valence electrons does chlorine have? Lewis notation for Chlorine Cl Bohr-Rutherford Model: You must know the following: the period tells you the number of shells The group tells you the number of valence electron (electrons in the outer shell) The atomic number tells us the total number of electrons and of protons in the atom In general, electrons will fill the shells closest to the nucleus first before occupying a farther shell For the first 20 elements the following rules apply ( rule): The 1 st shell can hold a maximum of 2 electrons The second shell can hold a maximum of 8 electrons The 3 rd shell will hold 8 electrons (for the first 20 electrons only) The 4 th shell will hold 2 electrons (for the first 20 electrons only)

7 Drawing a Bohr Rutherford model Ex. Nitrogen (N) Atomic number = Protons = electrons = 2 Period # = # shells = 5 Group # = valence electrons = Remember that the first shell can only hold two electrons Try Silicon (Si) Atomic number = 14 Protons = electrons = 14 3 Period # = # shells = 4 Group # = valence electrons = Another way of drawing R-B Diagrams e - 8 e - 4 e - The Ball and Stick model Atoms are represented as a ball and its bonds with other atoms as sticks. The size of the ball is related to the number of electron shells it has Hydrogen would be smaller that Magnesium H H H O Na O Hydrogen Oxygen Sodium Water (H 2 O)