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+. Mg 2+ has lost 2 electrons Al 3+ 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- O 2– has gained 2 electrons N 3– 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 3 rd 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 sub- orbital 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 Electron Configuration: Boron (B) has 5 electrons Electron configuration: Neon (Ne) has 10 electrons Electron configuration: Aluminum has 13 electrons Electron configuration: Iron has 26 electrons: 1s 1 1s 2 2p 3 1s 2 2s 2 2p 6 1s 2 2s 2 2p 6 3s 2 3p 1 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6

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# ElectronsGround StateExcited StateIons Helium2e - Boron5e - Aluminum13e - Copper29e -

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 What is the average atomic mass of silicon given the following abundance information on the isotopes of silicon? Ave.Iso.Mass =(28amu x 92.21) + (29amu x 4.70) + (30amu x 3.09) = Mass numberAbundance Si % Si % Si %

Average Isotopic Mass(Honors) What is the average atomic mass of hafnium given the following abundance information on its isotopes? Mass numberAbundance Hf-1765 % Hf % Hf % Hf-17914% Hf-18035%