Nuclear Symbols Nuclear Symbol - used to represent atoms and their isotopes Isotopes – atoms with the same atomic number but different atomic masses due to different number of neutrons Nuclear symbols tell us: The element (ex. uranium) top number is the mass number # protons + # neutrons bottom number is the atomic number # protons You can determine the number of neutrons by subtraction mass number → atomic number → 238 (mass #) - 92 (atomic #) = 146 (# of neutrons)

Atomic Mass & Isotopes **The Atomic masses in the Periodic Table are not mass numbers. ** They are an average of all the isotopes of that element, weighted by abundance. · Rounding the ave. atomic mass in the P.T. usually gives you the most common isotope. For iron (mass amu in P.T.) the most common isotope is iron- _________ 56

Ions Ions – atoms that have lost or gained electrons Positive ions have lost electrons. Positive ions are called cations. Examples: Na + lost one electron Mg 2+ lost two electrons Al 3+ lost three electrons Negative ions have gained electrons Negative ions are called anions. Examples: Cl - gained one electron O 2- gained two electrons N 3– gained three electrons

Orbits vs. Orbitals ► Bohr model: energy levels are “orbits” ► Modern Model: energy levels are broken into orbitals There are 4 orbitals: - “s” orbital – holds 1 pair of electrons – 2 total - “p” orbital – holds 3 pairs of electrons – 6 total - “d” orbital – holds 5 pairs of electrons – 10 total - “f” orbital – holds 7 pairs of electrons - 14 total 1st principal energy level has only one orbital: s 2nd principal energy level has two orbitals: s and p 3rd has three: s, p and d 4th has four: s, p, d and f 5th has four: s, p, d and f 6 th has three: s, p, and d 7 th has two: s and p

The Electron Hotel Electron configuration: The arrangement of electrons of an atom in its ground state into various orbitals around the nucleus. “Electron Hotel” Electrons fill orbitals like people filling rooms in a hotel Like a hotel, there are rules: 1.Electrons fill the lowest energy level available first. (Aufbau Principle) Electrons fill in the 1 st hotel level first, before filling up the 2 nd level 2.Single electrons with same spin must occupy each sub-orbital before additional electrons with opposite spins can occupy the same orbitals.(Hund’s Rule) Only one electron per room until all the rooms in that orbital have at least one electron 3.No more than 2 electrons may occupy a single suborbital, and they must have opposite spins. (Pauli’s Exclusion Principle) No more than 2 electrons in a room, and they must have opposite spins

Electron Configuration Where do these electrons go? Not sure, they're moving fast! but we can say where they probably are... Electron configuration: The arrangement of electrons of an atom in its ground state into various orbitals around the nucleus. There are three rules: 1. Aufbau Principle: each electron fills the lowest energy orbital available 1st. 2. Pauli's Exclusion Principle: a maximum of two electrons may occupy a single sub-orbital, but only if they have opposite spins. 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.

Electron Configuration Determining Electron Configuration:  Step 1: Determine how many electrons the atom has  Step 2: Fill the lowest energy level first.  Step 3: Use superscripts to show how many electrons are in the orbital. - Example: Helium has an atomic number of 2. This means it also has 2 electrons. - The lowest energy level is 1s - Helium's 2 electrons fit into the 1s orbital like so: 1s 2 - Therefore, the electron configuration for Helium is: 1s 2 Carbon has ___ electrons:6 1s 2 2s 2 2p 2 Add up the superscripts, you get 6! Once you run out of electrons, stop. Iron has ___ electrons:26 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 Add up the superscripts, you get 26!

Electron Configuration Remember: The 1st number is the energy level The letter is the orbital Bonus: What element is this? 1s 2 2s 2 2p 3...and the superscripts are the number of electrons!

Ground State Electron Configuration Examples: Helium – Boron – Aluminum – Copper – 2 e - 5 e - 13 e - 29 e - Ground State 1s 2 1s 2 2s 2 2p 1 1s 2 2s 2 2p 6 3s 2 3p 1 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 9 Excited State 1s 1 2s 1 1s 2 2s 1 2p 2 1s 1 2s 2 2p 6 3s 2 3p 1 4s 1 1s 2 2s 2 2p 5 3s 2 3p 6 4s 2 3d 10 - 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.