The Atom and the Periodic Table Chemistry The Atom and the Periodic Table

A Picture of an Atom Most of an atom is empty space! The nucleus contains most of the mass of the atom The # of protons = the # of electrons (in a neutral atom) The # of protons determines which element the atom is The number of neutrons can change – this is called an isotope Some elements can lose an electron(s) (become positively charged) or gain an electron(s) (become negatively charged) – charged atoms are called ions

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Atomic Mass = # of protons + average # of neutrons

Family or Group Period # of valence electrons

Electrons are placed in specific electron shells Electrons are placed in specific electron shells. Each period number equals the number of electron shells in the atom, which stays the same for all the elements in each period

Elements in a group or family have similar properties (characteristics) and undergo the same type of chemical reactions (due to having the same number of valance electrons) Elements across a period change in a regular way (the # of protons increases as does the #of electrons and the atomic mass), but the number of electron shells stays the same

Properties of Matter

All Pure Matter has Physical Characteristics Can observe without changing into a new substance Examples: boiling point, freezing/melting point, hardness, color, texture, & state at room temperature (gas, liquid or solid), density, solubility (if possible) in water or non-polar solvent If a metal: luster, malleability, conductivity of electricity and heat, attraction to magnet

If matter undergoes a physical change, then it is the same substance with the same physical and chemical properties

Example: an ice cube, liquid from the faucet, and steam from a boiling pot are all water. Each state has the same chemical formula, freezing/melting and boiling points, clear color, and is the universal solvent.

All Pure Matter has Chemical Characteristics Can’t be observed just by looking at it Describes ability to turn into a new substance To observe the property, the substance MUST react with another and form other pure substance(s), SO there must be a chemical reaction Examples: flammability, reacts with gas (iron to form rust, copper turns green, silver tarnishes), reacts with metals (acids)

If matter undergoes a chemical reaction, there is a new substance with its own physical & chemical properties

Sodium (Na) - soft, silvery metal that explodes in water Chlorine (Cl) - a poisonous yellow-green gas.

Chemical Reaction: sodium chloride (NaCl) a white solid dissolves in water without exploding safe to eat – table salt melting point of 801°C (Na 97.85°C) boiling point of 1413°C (Cl -34.6°C).

Mixtures

Compounds vs. Mixtures A compound is a pure substance made of 2 or more atoms (molecule) that reacted chemically and has unique chemical properties of its own. A mixture is made of 2 or more substances that do no react chemically, and retain their chemical properties.

Types of Mixtures Heterogeneous – no matter how well mixed, you can still see the separate parts Example – salad: lettuce, tomatoes, croutons, etc.; can pick out the cucumber slices if you don’t like Homogeneous – parts are so evenly mixed, you can’t see the different parts; looks like one substance Example – teaspoon of sugar stirred into hot tea; can’t see sugar but the tea tastes sweet.

What is another name for homogeneous mixture? Solution – a well-mixed mixture At least one substance dissolves into another Solute – substance dissolved into the 2nd substance (smaller amount) Solvent – substance that surrounds the 1st substance (larger amount) Solubility is a measure of how much solute goes into solvent at a given temperature – this is a physical property

Are all solutions a solid dissolving into a liquid? Liquid into liquid Gas into gas Gas into liquid Solid into solid Anti-freeze: ethylene glycol into water Oxygen in air Oxygen in water, CO2 in soda water Alloys: metal in metal such as bronze (Sn in Cu), brass (Zn in Cu), 14 kt gold (Ag, Cu, or Zn in Au)

Factors That Affect Solubility Temperature Pressure Solvent compatibility

Temperature Affects Solubility ↑ solid’s solubility by ↑ temperature, genetally generally

↑ gas’ solubility by ↓ temperature

Pressure Affects Solubility ↑ gas’ solubility by ↑ pressure over the solvent

Solvent Compatibility - Like Dissolves Like Polar solvent + ionic compound Water is a polar molecule: (+) & (-) sides Salt is an ionic compound The sodium ion (Na+) is attracted to water’s (-) side The chlorine ion (Cl-) is attracted to water’s (+) side.

Polar Solvent + Polar Solute Polar ammonia molecules dissolve in polar water molecules. The +H of the water molecule is attracted to the –N of the ammonia molecule AND… The +H of the ammonia molecule is attracted to the –O of the water molecule

Non-polar + non-polar Even if a substance is soluble, it may not dissolve in water Non-polar: no (+) & (-) parts of molecule Acetone is a solvent that is partially polar The components in nail polish are non-polar chemicals (resins) dissolved in acetone Letting your nails dry means allowing the solvent to evaporate leaving the resin behind. To remove, the polish needs to dissolve This is the same reason you can’t clean oil-based paint from a paint brush using water. You must use turpentine. - -

Describing Acids and Bases

Chemical Properties of Acids Tastes sour (if in food) Reacts with metals and carbonates Turns blue litmus paper red pH < 7 Separate into H+ and (-) ions in water

Never Taste a Chemical in a Lab Acids in foods taste sour or tart: citrus fruits, tomatoes, apples, vinegar

Acids React with Metals & Carbonates Acids corrode metals H+ react with the metal Chemical reaction producing H2 – gas Acids react with carbonates to produce CO2

Strong Acid versus Weak Acid A strong acid releases more H+ into solution. A weak acid only partially dissolves in water

Chemical Properties of Bases Sodium Hydroxide - NaOH Potassium Hydroxide - KOH Ammonium Hydroxide - NH4OH Calcium Hydroxide - Ca(OH)2 Magnesium Hydroxide - Mg(OH)2 Barium Hydroxide - Ba(OH)2 Aluminum Hydroxide - Al(OH)3 Tastes bitter (if in food) Feels slippery Turns red litmus paper blue pH > 7 Causes OH- and (+) ions in water

Bases in food taste bitter: radish, cabbage, Brussels sprouts, turnip, olives, coffee, unsweetened cocoa, quinine, Tums®

Indicator paper call tell you if a substance is acid (< 7), base (> 7) or neutral (= 7) The closer to 0, the stronger the acid The closer to 14, the stronger the base (or more alkaline) The closer to 7, the weaker the acid or base Strength determines safety

What Happens If An Acid & Base Mix? Neutralization = reaction between acid and base Resulting substance is less acidic, less basic than the original substances acid + base = a salt + water How close to pH 7 depends on the concentrations and amounts of the originals