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 18 elements on the periodic table • Electrons are arranged in a specific pattern • 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

Valence Electrons • We will go into more detail but you will need to know: • • • •

Alkali metals have one valence electron Alkaline Earth Metals have 2 valence electrons Halogens have 7 valence electrons Noble gases have filled valence shells (8 valence electrons)

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: 1. 2. 3.

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 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 1. 2. 3. 4. 5.

Maximum of 2 electrons in the first shell Maximum of 8 electrons in the next 2 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

Electron Shells: 3-D Outside the Nucleus • The way an element chemically reacts depends on the number of electrons in it’s 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

2

8

3

8

4

18

Just use this table for our purposes in grade 9

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)

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 eFluorine

Bohr Models of the First 18 Elements

Valence Electrons • Valence electrons are found in the outermost shell. • Most elements in the same family have the same number of valence electrons Examples: Halogens = seven valence electrons Alkali = one valence electron Alkali Earth metals = two valence electrons Noble Gases - are stable and non-reactive because they are full in the outer most valence shell.

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: in the previous slide, the magnesium atom has 2 valence electrons

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-

• 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-

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 charge to the atomic number ◦ EX: Calcium has an atomic number of 20 and a 2+ charge.......so, its ion has 20 - 2 = 18 electrons ◦ EX: Fluorine has an atomic number of 9 and a 1- charge.......so, its ion has 9 – (-1) = 10 electrons (remember subtracting a negative is like adding!)