Ions
Atoms have an equal number of protons and electrons.
• The net charge for an atom is 0
• When an atom loses or gains an electron, it becomes an ion.
•
Atoms that have lost electrons become positive and are called cations
•
A positive number is added to the atomic symbol to indicate how much more
positive it is
• Na+ has lost 1 electron. It has one proton more than electrons so it is 1+.
• Mg2+ has lost 2 electrons
• Al3+ has lost 3 electrons
• Atoms that have gained electrons become negative and are called anions
• A negative number is added to the atomic symbol to indicate how much more
negative it is
• Cl– has gained 1 electron. It has one electron more than protons so it is 1• O2– has gained 2 electrons
• N3– has gained 3 electrons
Orbits vs. Orbitals
Where do the electrons go?
• Electron configuration – the distribution of electrons in an atom
• Bohr model: electrons are in orbits called energy levels
• Modern Model: It’s….complicated.
• Each principal energy level is broken into orbitals
• There are 4 orbitals
• “s” orbital is the lowest energy orbital
• Can hold up to 1 pair of electrons – 2 total
• “p” orbital is the next highest
• Can hold up to 3 pairs of electrons – 6 total
• “d” orbital can hold up to 5 pairs of electrons – 10 total
• “f” orbital can hold up to 7 pairs of electrons - 14 total
Orbits vs. Orbitals
• 1st principal energy level has only the ‘s’ orbital
• It is the lowest energy level
• 2nd principal energy level has the ‘s’ and ‘p’ orbtal
• 3rd energy level has ‘s,’ ‘p’ and ‘d’
• 4th energy level has ‘s,’ ‘p,’ ‘d’ and ‘f’
• 5th energy level has ‘s,’ ‘p,’ ‘d’ and ‘f’
• 6th energy level has ‘s,’ ‘p,’ and ‘d’
• 7th energy level has ‘s,’ and ‘p’
(theoretically there are more energy levels but we
don’t have that many elements … yet)
• Looks kinda like this:
• But there are rules about how the orbitals are filled.
Electron Configuration
• There are three rules:
• 1. Aufbau Principle: each electron fills the lowest energy orbital
available 1st.
• 1s is the lowest energy level, always filled first
• 2. Pauli's Exclusion Principle: a maximum of two electrons may
occupy a single sub-orbital, but only if they have opposite spins.
• Electrons fill in each sub-orbital before pairing up.
• 3. Hund’s Rule: single electrons with same spin must occupy each suborbital before additional electrons with opposite spins can occupy the
same orbitals.
• Electrons will only pair up once each sub-orbital has an electron.
Electron Configuration
• Electron Configuration and the Aufbau Principle:
• Determine the number of electrons
• Fill lowest energy level first
• Use superscripts to show how many electrons are in the
orbital.
• Examples:
• Hydrogen (H) has 1 electron
1s1
• Electron Configuration:
• Boron (B) has 5 electrons
• Electron configuration:
1s2 2p3
• Neon (Ne) has 10 electrons
• Electron configuration: 1s2 2s2 2p6
• Aluminum has 13 electrons
• Electron configuration: 1s2 2s2 2p6 3s2 3p1
• Iron has 26 electrons: 1s2 2s2 2p6 3s2 3p6 4s2 3d6
Electron Configuration
Variations on Electron Configuration
• Electron configurations for the ground state always put electrons in the lowest
possible energy level.
• Excited state E.C.’s will have 1 or more electrons moved to a higher energy level.
• Ions will have more or less electrons in their E.C. depending the charge of the ion
Examples
# Electrons Ground State
Helium
2e-
Boron
5e-
Aluminum
13e-
Copper
29e-
Excited State
Ions
The Electron Hotel
•
•
•
But what about the other two rules(Pauli’s Exclusion Principle and Hund’s
Rule)?
Electrons normally repel each other because they have the same charge
•
Pauli’s Exclusion Principle says two electrons can occupy the same
orbital if they have opposite spins
•
Hund’s Rule says one electron must fill each suborbital before electrons
pair up (with opposite spins)
It’s like an oddly shaped hotel.
•
Each floor is a principal energy level and each room can hold one pair
of electrons.
•
The s building has 1 room on each floor
•
The p building has 3 rooms on each floor
•
The d building has 5 rooms on each floor
•
The f building has 7 rooms on each floor
Electron Configuration
Average Isotopic Mass(Honors)
• The Atomic masses in the Periodic Table are not mass numbers.
• Most are decimals. Can you have .45 of a proton?
• They are an average of all the naturally-occurring isotopes of
that element multiplied by their abundance.
• The formula for Average Isotopic Mass
 Isotopic Mass1  Percent Abundance  Isotopic Mass2  Percent Abundance
Average Isotopic Mass  
  
  etc.
100
100

 

• What is the average atomic mass of silicon given the following
abundance information on the isotopes of silicon?
Mass number
Si-28
Si-29
Si-30
Abundance
92.21 %
4.70 %
3.09 %
Ave.Iso.Mass =(28amu x 92.21) + (29amu x 4.70) + (30amu x 3.09) =
100
100
100
Average Isotopic Mass(Honors)
• What is the average atomic mass of hafnium given the following
abundance information on its isotopes?
Mass number
Hf-176
Hf-177
Hf-178
Hf-179
Hf-180
Abundance
5%
19 %
27 %
14%
35%