Chem - Lecture 3

Structure of the atom – Nucleus – Determines mass of atom • Proton – Positive – Determines element • Neutron – Neutral – Determines isotope

Atomic Mass • Atoms are tiny, we need units that are useful for describing the… – … mass of a single atom • Atomic Mass Units (amu) – … number of atoms in a given sample • Mole (mol)

Measuring Atoms and 12C • Definition 1 • A single carbon-12 atom has a mass of exactly 12amu 1 amu = (1/12) mass of 12C nucleus • It’s a tiny mass, for our tiny atoms

Measuring Atoms and 12C • Definition 2 • Exactly 12g of carbon-12 contains exactly 1 mole of atoms 6.022 x 1023 (Avogadro’s number) • A large number, so we can count all of our tiny atoms 5

Measuring Atoms Example • Now, we can provide a single number and it will convey two meanings – The mass of a single isotope – The mass of a mole of that isotope 6.022 x 1023 isotope atoms

Average Atomic Mass • Each isotope has a natural abundance • Some isotopes are rare, while others are common » True • Abundances are similar world-wide » Are abundances similar world-wide?! (Answer: close enough)

Average Atomic Mass • The periodic table provides the average mass, weighted by isotopic abundance – Goal: To know how many atoms are in a sample just by kn – Plain English: • You multiply how heavy an isotope is with how abundant it is, then add up all the numbers – Math/technical • Take the product of frequency and mass for every isotope, take the sum of those products over all naturally occurring isotopes

Periodic Table • Elements are arranged by atomic number – Number of protons • Magnitude of positive charge in the nucleus • Rows are called “periods” • Columns are called “groups” – Elements in the same column have similar properties

The Chemical Bond • Sometimes atoms stick together to form a new compound – Example: two hydrogen atoms will react to spontaneously form H2 (molecular hydrogen) • Separation distance between nuclei ≈ 0.8 Å

The Chemical Bond • Remember: – Like charges repel, opposite charges attract • Let’s build up our H2 molecule: • Start by adding the ______ (0.8 Å apart) • How do these two ______ interact? • Add an ______to the system • How will the nuclei interact with the ______?

The Chemical Bond • Electron density between the nuclei pulls the nuclei together – Binding region • Electron density on either far edge of the nuclei will pull the nuclei apart – Antibinding region

Ionic vs Covalent • Ionic and covalent bonds are concepts that allow us to quickly predict how different compounds distribute electrons in the binding region

– One atom (metal) donates electron(s) to another atom (nonmetal) • The nonmetal holds-on to the charge while the now positive metal atom is attracted to the nonmetal’s overall negative charge

– Two atoms (nonmetals) share electrons roughly equally • You’ll have to wait until Ch 9 and 10 to predict when/why you get a covalent bond

Ionic Compounds • Composed of cations and anions • Often singly charged atoms, but some polyatomic ions do exist – Cation = positive thing • Often a metal (almost always) – Anion = negative thing • Often/always nonmetal atom(s)

• Electrically neutral – Cation and anion charges must balance

Ionic Naming • Cations • Determine if the metal can form more than one charge a) If the metal has only one charge: use element name b) If the metal can more than one charge: indicate the magnitude of charge in parenthesis using Roman numerals • Nonmetal cations, add –ium suffix • Example: NH3 is ammonia →NH4+ is ammonium

Ionic Naming • Anions 1. Monatomic anions: replace suffix with -ide • Cl which is chlorine becomes Cl- which is chloride • N which is nitrogen becomes N3- which is nitride • Oxyanions (polyatomic anions)