Lab 3: Chemistry

History of the atom Democritus years ago If you take a stick of chalk and break it in half, is it still chalk? What if you break it in half again? Thought there had to be a limit Called this the ATOM Means “unable to be cut”

Atom Anatomy Protons and Neutrons Protons- positively charged particles (+) Neutrons- carry no charge (0) Held inside the nucleus Electrons Electron- negatively charged particles (-) In constant motion in the space surrounding the nucleus

Elements and Isotopes Elements Pure substance that consists entirely of one type of atom More than 100 named elements (92 found naturally) Only about 2 dozen are found commonly in living organisms Represented by one- or two- letter symbols C- Carbon Na- Sodium Hg- Mercury

Atomic Number An element’s atomic number tells you a lot about the element! The atomic number tells you the number of protons and electrons The atomic mass tells you the total number of protons and average number of neutrons If an element gains or loses a proton, it is no longer the same element!

Practice! The element Tungsten is pictured below. How many Protons does this element have? How many Electrons? How many Neutrons?

Isotopes Sometimes elements have different numbers of neutrons. All atoms of Carbon have 6 protons and 6 electrons, but some have 6 neutrons, or 7 neutrons. When elements differ in the number of neutrons, they are called isotopes

Carbon Isotopes IsotopeNumber of Protons Number of Electrons Number of Neutrons Carbon- 12 (nonradioactive) 666 Carbon- 13 (nonradioactive) 667 Carbon- 14 (radioactive) 668

Why Care About Isotopes? Radioactivity The nuclei of certain isotopes are unstable, and break down into a more stable form Some radiation the isotopes give off can be dangerous, but radioactive isotopes have a number of important uses Dating Geologists determine the ages of rocks and fossils by analyzing the isotopes found in them

Bonding! Most elements are found combined with other elements! A COMPOUND is a substance formed by the combination of two or more elements Example: Water (H 2 O) Contains two Hydrogen atoms and one Oxygen atom

Types of Bonding When compounds form, the chemical properties usually change from their elemental properties Example: Sodium Chloride (NaCl) Sodium Silver-colored metal that reacts violently with water Chlorine Poisonous yellow-green gas used in battles of WW1 Sodium Chloride Table salt

Ionic Bonds Formed when one or more electrons are transferred from one atom to another Ions Atoms that gain or lose electrons Atoms become negatively charged when they GAIN electrons Atoms become positively charged when they LOSE electrons

Ionic Bonds Positively and negatively charged ions are attracted to each other

Ionic Bonding

Covalent Bonding Electrons are shared instead of being transferred Bond Number Share two electrons  Single bond Share four electrons  Double bond Share six electrons  Triple bond

Covalent Bond

Van der Waals Forces Some atoms have a stronger attraction for electrons Produces an uneven charge between atoms Can hold molecules together (like water)

Polarity Uneven electron sharing in covalent bonds Creates partial positive and partial negative charges on atoms

Hydrogen Bonding Covalent bonds are very strong When H atoms are partially positive, it allows them to share attractions with other electronegative atoms. This is called a Hydrogen bond. Each H 2 O molecule can bond up to four times this way Important in how proteins form their shapes

Cohesion Hydrogen bonds last for only a short amount of time before they switch molecules. Cohesion occurs when molecules of the same type are attracted to each other and “stick together”. Water is incredibly cohesive because of its tendency to be attracted to other water molecules

Cohesion Importance Transports water and nutrients from the roots to the leaves Evaporation from leaves pulls water from the roots through the veins Surface tension Measure of how difficult it is to stretch or break the surface of a liquid Hydrogen bonds give water high surface tension

Water Striders take advantage of cohesion

Adhesion Water can “stick” to other molecules as well. This is called adhesion

High Heat Capacity Water heats slower than many metals Hydrogen bonding resists temperature changes When heat is added to the water, the hydrogen bonds break, and water absorbs and stores that heat, while slowly raising the temperature When heat is removed, more hydrogen bonds form. Heat energy is released and the cooling process is slowed

Density Because of hydrogen bonding, Ice becomes less dense than liquid water When water freezes, the molecules move slower, giving more opportunity for Hydrogen bonding to occur. Causes the atoms to be held at “arms length” giving less mass in an equivalent volume

Universal Solvent Solution Liquid consisting of a uniform mixture of two or more substances Dissolving agent is the solvent The substance being dissolved is the solute Water’s polarity helps it dissolve other polar and ionic compounds

Salt in Water NaCl is dissolved in water + charged sodium is attracted to the – charged oxygen - charged chlorine is attracted to the + charged hydrogen The ionic bond is broken and the salt dissolves!