Elements, Compounds, and Mixtures Notes

Elements, Compounds & Mixtures

Learning Objectives

• Define element, molecule, compound, and mixture.

• Describe the Periodic Table as a method of classifying elements and predicting their properties.

• Explain the basis of the Periodic Table using proton number and simple atomic structure, with reference to the first 20 elements.

• Describe the relationship between group number, number of outer-shell electrons, and metallic/non-metallic character.

• Describe lithium, sodium, and potassium (Group I) as soft metals reactive with water.

• Describe chlorine, bromine, and iodine (Group VII) as diatomic non-metals.

• Describe noble gases (Group VIII or 0) as unreactive, monoatomic gases and explain this in terms of electronic structure.

• Use the Periodic Table to retrieve information about specific elements.

• Describe the formation of ions by electron loss or gain.

• Determine the charge on an ion of a metallic or non-metallic element.

• Describe the formation of ionic bonds between a metal and a non-metal.

• Deduce the formula of an ionic compound from the charges on the ions present.

• Use the symbols of elements and write the formulae of simple ionic compounds.

• Deduce the formula of an ionic compound from its name.

• Describe differences between elements, compounds, and mixtures, and between metals and non-metals.

• List the general physical properties of metals.

• Construct word equations and simple balanced chemical equations.

What is an Element?

• A substance made of only one type of atom that cannot be broken down into simpler substances by ordinary chemical means.

• All atoms of an element have the same number of protons.

• Currently, there are 118 elements; the first 94 occur naturally.

• Different elements have different atoms, therefore different physical and chemical properties.

• Different elements can have certain properties in common.

The Periodic Table

• A specific arrangement of all chemical elements in a table format.

• Arrangement allows predictions about chemical properties of undiscovered or newly discovered elements.

• Helps to derive relationships between elements.

How to Read the Periodic Table

• Element name

• Atomic number (number of protons)

• Element symbol

• Relative atomic mass (mass of one atom)

Periods and Groups

• Elements are arranged in horizontal rows called periods and vertical columns called groups.

• Some groups have special names:

  • Group I: Alkali metals

  • Group II: Alkaline earth metals

  • Group VII: Halogens

  • Group VIII (0): Noble gases

• Each row is numbered and called a Period.

• Each column is numbered and called a Group.

• Group number = number of electrons in the outermost electron shell

• Period number = number of electron shells that an atom uses

Examples of Group and Period Relationships

• Sodium (Na) and Potassium (K) belong to Group I.

  • Sodium: 2, 8, 1

  • Potassium: 2, 8, 8, 1

  • Both have 1 electron in the outermost electron shell.

  • Group number = number of electrons in the outermost shells.

• Potassium (K) and Calcium (Ca) belong to Period 4.

  • Potassium: 2, 8, 8, 1

  • Calcium: 2, 8, 8, 2

  • Each atom uses 4 electron shells.

  • Period number = number of shells that an atom uses to hold its electrons.

• All elements in Group 1 have 1 electron in the outermost shell.

• All elements in Period 3 have electrons in the first three shells.

• Examples of electronic configurations:

  • H: 1

  • He: 2

  • Li: 2, 1

  • Be: 2, 2

  • B: 2, 3

  • C: 2, 4

  • N: 2, 5

  • O: 2, 6

  • F: 2, 7

  • Ne: 2, 8

  • Na: 2, 8, 1

  • Mg: 2, 8, 2

  • Al: 2, 8, 3

  • Si: 2, 8, 4

  • P: 2, 8, 5

  • S: 2, 8, 6

  • Cl: 2, 8, 7

  • Ar: 2, 8, 8

  • K: 2, 8, 8, 1

  • Ca: 2, 8, 8, 2

Elements with Similar Properties

• Alkali metals

  • Reactive metals that do not occur freely in nature, but exist in combination with other elements.

  • Soft, malleable, ductile, and good conductors of heat and electricity.

  • React explosively with water, forming an alkali.

  • Too reactive to be found in nature as an element.

• Alkaline earth metals

  • Harder, less reactive than alkali metals.

  • Oxides of these metals form an alkali when dissolved in water.

  • Have higher melting points than alkali metals.

• Halogens

  • "Halogen" means "salt-former."

  • When a halogen is combined with a metallic element, it is called a “salt.”

  • Halogens can exist in any of the three states of matter at room temperature (e.g., chlorine – gas, iodine – solid, bromine – liquid).

• Noble gases

  • Do not combine with other elements readily (i.e., inert).

  • Stable to exist as an element in nature.

• Non-metals

  • Do not conduct electricity or heat very well.

  • Very brittle, not shiny, and cannot be rolled into wires or pounded into sheets.

  • Exist as a solid (e.g., carbon) or a gas (e.g., oxygen).

Properties of Alkali (Group 1) Metals

• All are soft (can be cut by knife), silvery solids when freshly cut and show similar chemical properties.

• Examples: sodium, potassium.

Properties of Noble Gases (Group 8)

• Colorless

• Unreactive (do not react with other elements)

• Examples: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn).

What is a Compound?

• A substance made of two or more elements (i.e., two or more types of atoms) held together by a chemical bond (attraction between atoms).

• The chemical properties of a compound are different from those of the individual elements.

  • e.g., carbon dioxide (CO_2) has different properties from carbon and oxygen.

Two Types of Compounds

• Ionic compound

  • One metallic atom + one or more non-metallic atoms.

  • Held together by an ionic bond.

  • e.g., sodium chloride (NaCl), calcium carbonate (CaCO_3).

• Molecular compound

  • Two or more non-metallic atoms.

  • Held together by a covalent bond.

  • e.g., ammonia (NH3), carbon dioxide (CO2).

  • Not studied in Year 7.

Why Noble Gases Are Unreactive

• 8 outermost shell electrons (i.e., octet structure).

  • e.g. Ar, Ne

• Other elements do not have 8 outermost shell electrons and are unstable.

  • e.g. Na

Why Form Compounds?

• Two methods to attain octet structure:

  1. Donating and receiving electrons.

  2. Sharing electrons (not studied in Year 7).

Ionic Compound Formation

• An ionic compound is electrically neutral overall.

• When forming an ion, the atom loses or gains enough electrons so that its outermost electron shell is fully occupied (i.e., octet structure).

• Example: Forming an ionic bond between sodium and chlorine.

  • Sodium (Na) has the electronic arrangement of 2, 8, 1. It loses one electron to get the stable octet 2, 8, forming a Na^+ ion.

  • Chlorine (Cl) has the electronic arrangement of 2, 8, 7. It gains one electron to get the stable octet 2, 8, 8, forming a Cl^- ion.

Ionic Bonds

• Ionic bonds are formed by the transfer of one or more electrons from a metal atom to a non-metal atom.

• Ionic bond is the electrostatic attraction between oppositely charged ions ( + and - ).

Electron Transfer

• Sodium atom (Na) loses one electron, becoming a sodium ion (Na^+).

• Chlorine atom (Cl) gains one electron, becoming a chloride ion (Cl^−).

• This results in the ionic compound sodium chloride (NaCl), which is electrically neutral.

What is a Molecule?

• A molecule is one particle of a substance made of two or more non-metallic atoms held together by a chemical bond.

• A molecule can be an element (e.g., H2) or a compound (e.g., CO2).

• Atoms of a molecule can be all the same (e.g., H_2) or different from each other (HCN).

• A particle made of two identical atoms is a diatomic molecule (H2, O2, N_2, all halogens).

Classifying Substances

• Element: Metal or Non-metal

• Molecular Substance:

  • Molecular Substance (e.g., H2, O2, N2, Cl2)

• Compound:

  • Ionic compound (made of metal + non-metal)

  • Molecular compound (made of non-metals only)

• Molecular Substance: (e.g., HCN, H2O, CO2, C6H{12}O_6)

Considerations for Classification

1. How many elements are found in the symbol? One? More than one?

2. If there are two or more elements in the symbol, is it a mix of metallic and non-metallic elements? Or just non-metallic elements?

Substance Classification Examples

• H_2O: Molecule, Compound

• CO_2: Molecule, Compound

• CO: Molecule, Compound

• Co: Element

• CH_4: Molecule, Compound

• CCl_4: Molecule, Compound

• H2SO4: Molecule, Compound

• NH_3: Molecule, Compound

• HCl: Molecule, Compound

• KOH: Compound

• Fe: Element

• FeSO_4: Compound

• NaOH: Compound

• HNO_3: Molecule, Compound

• H_2: Molecule, Element

What is a Mixture?

• Two or more different substances present together that are not chemically bonded.

• 3 types of mixtures:

  • Element-element

  • Element-compound

  • Compound-compound

• The chemical properties of the individual substances remain unchanged.

Classifying Matter

• Pure substance

  • Element (e.g., Fe, N2, O2)

  • Compound (e.g., H_2O, NaCl)

• Mixture

  • Element-element mixture (e.g., air)

  • Element-compound mixture (e.g., sea water)

  • Compound-compound mixture

Examples of Mixture Classification

• Pure substance (element)

• Mixture of 2 elements

• Mixture of 1 compound and 1 element

• Pure substance (compound)

• Mixture of 1 compound and 1 elemental molecule

• Pure substance (elemental molecule)

Examples of Classifying Substances

• Seawater: Mixture of compounds (H_2O, NaCl) and elements

• Sugar (Sucrose): Compound (molecule of Carbon, Oxygen, Hydrogen)

• Glucose: Compound

• Fructose: Compound

Naming Chemicals and Writing Formulas

• A capital letter means a new element.

  • e.g., KF; 2 elements, K (potassium) & F (fluorine)

• If there is a number written in front of a chemical symbol, the number refers to the number of particles as a whole.

  • e.g., 2KF means 2 particles of KF

• If a number is written within a chemical symbol as a subscript, the number refers to the number of atoms of the preceding element.

  • e.g., K2O, where the subscript “2” means two potassium atoms in one particle of K2O

Rules for naming compounds

• #1 When a compound is made from a metal and a non-metal (i.e ionic compound), the name of metal comes first and keeps its name and the non-metal changes its ending to “-ide”

• e.g. FeO: Iron oxide

• Recall that an ionic compound contains one metal and one non-metal.

• Determine the type of ion that each element will be (i.e., positive vs. negative) and its charge, and write the appropriate symbol of the ion.

  • Write positive ion on the left, negative ion on the right

• Use the trick on the right to write the complete symbol

• Why does the trick work?

• Hint: what should the overall charge of the compound be?

• Reduce the subscripts to their lowest terms for the final symbol

• Name the compound based on its naming rule

• For the example on the right, its name is lithium oxide

Relationship Between Group Number and the Type of Ion Formed

Class activity

Lets play a matching game

• Li+ and Cl- form Lithium chloride (LiCl).

• Na+ and Br- form Sodium bromide (NaBr).

• K+ and O2- form Potassium oxide (K2O).

• Mg2+ and F- form Magnesium fluoride (MgF2).

• Ca2+ and N3- form Calcium nitride (Ca3N2).

• Al3+ and S2- form Aluminum sulfide (Al2S3).

• Fe2+ and P3- form Iron(II) phosphide (Fe3P2).

Comparing Properties Among Elements, Compounds & Mixtures

Metallic vs Non-Metallic Element

Melting and Boiling Points

Impurities in Compounds

• Impurities in a compound can affect its freezing/melting point and boiling point.

• Sprinkle salt on the icy road to decrease water’s freezing point so ice does not form easily.

• If water contains 10% salt, it freezes at -6°C.

• If water contains 20% salt, it freezes at -16°C.

Differences in Properties Between Mixtures and Compounds

How to Represent Chemical Reactions

What is a Chemical Reaction?

• A process in which one or more substances, the reactants, are converted to one or more different substances, the products.

• Substances are either elements or compounds.

• The constituent atoms of the reactants are rearranged to create different substances as products.

Examples of Synthesis Reaction in Everyday Life

• Sodium + Chlorine → Sodium chloride

• Zinc + Oxygen → Zinc oxide

Word Equations

• A representation of a chemical reaction using words only.

  • e.g., Sodium + Chlorine → Sodium chloride

• NOT Sodium + Chlorine → NaCl

• In a word equation, we do not know the quantity of the reactants and products.

(Balanced) Chemical Equation

• A symbolic representation of a chemical reaction in the form of chemical formulae.

• Word equation: Sodium + Chlorine → Sodium chloride

• Chemical equation: Na + Cl_2 → NaCl

• Balanced chemical equation: 2Na + Cl_2 → 2NaCl

Balancing Chemical Equations

• Reactants: sodium (Na) + chlorine (Cl_2)

• Product: sodium chloride (NaCl)

• Balanced: 2Na + Cl_2 → 2NaCl

Conservation of Mass

• No atoms are lost or made in a chemical reaction!

• Reactants → Products

• 1 carbon atom → 1 carbon atom

• 2 oxygen atoms → 2 oxygen atoms

Steps in Balancing a Chemical Equation

• DO

  1. Write down the word equation for the reaction.

  2. Replace names of reactants and products with their correct formulae.

  3. Balance the equation with respect to the number of atoms of each kind of element/compound on each side of the equation by changing the coefficient.

• The types and number of atoms on the reactant side (LHS) must equal to those on the product side (RHS)!

• DON'T Never change the subscript of a chemical formula.

  • e.g. 2H2 + O2 → 2H_2O

Example 1

• When fluorine gas comes in contact with calcium metal, calcium fluoride is made.

• Word equation (step 1): Calcium + Fluorine → Calcium fluoride

• Replacing each element and compound with the correct chemical formulae (step 2): Ca + F2 → CaF2

Check if Equation is Balanced

• Look at the number of atoms of each element/compound on the reactant side (LHS) and see if they are equal to those on the product side (RHS) (step 3).

• Ca + F2 → CaF2

• LHS: 1 Ca atom, 2 F atoms

• RHS: 1 Ca atom, 2 F atoms

• The equation is balanced.

A Helpful Strategy to Balancing Chemical Equations

• As applicable, balance the number of atoms on each side in this order:

  1. Metals

  2. Polyatomic ions (ions consisting of two or more atoms)

  3. Non-metals (other than hydrogen or oxygen)

  4. Oxygen

  5. Hydrogen