2.3 – The Periodic Table and Atomic Theory

Key Ideas - Outcomes You will need to know how to draw a Bohr diagram for the first 20 elements on the periodic table Electrons are arranged in a specific pattern The electrons in the outermost shells are called valence electrons Determine the difference between an atom and an ion Use atomic structure to explain why elements behave differently

Periodic Table The periodic table is full of patterns Elements that share properties line up in columns because they share the same arrangement of electrons One way to show the arrangement is with a Bohr diagram

Bohr Diagrams A Bohr model shows how many electrons are in each shell surrounding the nucleus The regions surrounding the nucleus of an atom are often called electron shells

Bohr Diagrams Niels Bohr described how the electrons are arranged in the shells around a nucleus in an atom His theory has 3 ideas: Electrons move around the nucleus in shells Each shell is a certain distance away from the nucleus and can hold a definite number of electrons After the shell closest to the nucleus is full, electrons start filling the next shell

How to Draw an Atom: Draw a circle and put the symbol and number of protons (and neutrons if possible) inside of it Add the appropriate number of shells (this is determined by the period number) EX: Magnesium is in the 3rd period so it will have 3 shells Mg 12p+ 12no There are now 3 shells in total

Filling up Electron Shells Maximum of 2 electrons in the first shell (1st shell) Maximum of 8 electrons in the next two shells (2nd and 3rd shells) Maximum of 18 electrons in the 4th shell and all shells after that (we won’t be making these!!) Electrons occur in pairs There must be 4 single electrons before the electrons will occur in pairs Remember the pattern: 2, 8, 8, 18 This pattern applies to all atoms although not all atoms have that many electrons

Magnesium example EX: Magnesium has 12 electrons (from atomic number) So, 2 electrons in the first shell, 8 in the second and 2 in the last Watch how the electrons are added (up, down, side to side)

Electron Shells and Reactivity: The way an element chemically reacts depends on the number of electrons in its outer shell. Atoms are stable when their outer shells are full of electrons. If a shell is not full, the atom is reactive, and it either wants to fill its outer orbital or get rid of it altogether. Shell (orbital) Maximum # of e- 1 2 8 3 4 18 Just use this table for our purposes in grade 9

Examples when outer shell is not full Look at the outer shell of each: Outer shells not full. Reactive! Lithium Lithium will donate 1 e- and Fluorine borrow 1 e- Fluorine

Bohr Models of the First 20 Elements

Valence Electrons This is a fancy way of saying “outer shell electrons” If you are ever asked for the number of valance electrons, they are just asking how many electrons are found in the last, outermost, shell only EX: Magnesium atom has valence electrons 2

Valence Electrons Most elements in the same family have the same number of valence electrons Examples: Alkali Metals = 1 valence electron Alkaline Earth Metals = 2 valence electrons Halogens = 7 valence electrons Noble Gases - are stable and non-reactive because they are full in the outer most valence shell.

Atoms VS Ions Atoms are elements as you see them on the Periodic Table: with equal amounts of p+ and e- EX: sodium atom has 11 p+ and 11 e- so it is neutral Ions are elements that have gained/lost electrons and now have a charge EX: sodium ion has 1+ charge because it has lost 1 e-

Practice! State if the following are Atoms or Ions Na K H+ Cl- Ca O2- Ne Be2+ Al N3- Na+ K+ Be Cl Ca2+ Hint: Anything with a charge is an ion, all the others are atoms

Ions and Electrons To determine the number of electrons in an ion you need to subtract the ion charge from the atomic number EX: Calcium ion has an atomic number of 20 and a +2 ion charge.......so, its ion has 20 - 2 = 18 electrons EX: Fluorine ion has an atomic number of 9 and a -1 ion charge.......so, its ion has 9 – (-1) = 10 electrons (remember subtracting a negative is like adding!)

Element Behaviour GOLDEN RULE: all atoms want a full outer shell (it makes them more stable) – they will gain/lose electrons to achieve this! Thus, elements that have the same number of valence electrons will behave the same way as they need to gain/lose the same number of electrons!

THINK ABOUT IT! Why do sodium and cesium behave the same way? Why do oxygen and sulphur behave the same way? Why are the noble gases un-reactive (inert)? Why is this statement true? “All atoms are trying to become like their nearest Noble Gas”

Bohr Models for Ions Draw your normal atom Bohr model (neutral – no charge) Place square brackets around the entire Bohr model Decide the easiest way for the outer shell to become full by either adding electrons or subtracting electrons If you add electrons, write a negative charge outside the brackets equal to the number of electrons you added Your ion charge should match the periodic table if you did it correctly!! -1 Fluorine Ion

Bohr Models for Ions Add electrons to non-metals to make full outer shells!!! (forms negative ions) -1 Fluorine Ion

Bohr Models for Ions Draw your normal atom Bohr model (neutral – no charge) Place square brackets around the entire Bohr model Decide the easiest way for the outer shell to become full by either adding electrons or subtracting electrons If you subtract electrons, write a positive charge outside the brackets equal to the number of electrons you lost (erase the outer shell that just lost electrons too!) Your ion charge should match the periodic table if you did it correctly!! +1 Sodium Ion

Bohr Models for Ions Subtract electrons from metals to make full outer shells!!! (forms positive ions) +1 Sodium Ion

Summary All atoms want full outer electron shells All metals lose electrons All non-metals gain electrons