Isotopes, Ions, and Chemical Compounds

Isotopes and Ions

Isotopes

• Isotopes are neutral atoms of a single element.
• Identified by the number of protons.
• In a neutral atom, the number of protons equals the number of electrons.

Ions

• Ions occur when the number of electrons differs from the number of protons.
• The number of protons cannot be changed, but the number of electrons can.
• Ions are formed by losing or gaining electrons due to chemical properties.

Cations

• Cations are formed when an atom loses an electron.
• Losing an electron results in a positive charge because electrons carry a negative charge.
• Mnemonic: Cations have paws, so they are positive ions.

Anions

• Anions are formed when an atom gains electrons.
• Gaining electrons (negativity) results in a negative charge.
• Mnemonic: Anions are negative ions.

Key Differences

• Isotopes have an equal number of protons and electrons.
• Ions have a different number of protons and electrons.

Charge and the Periodic Table

• The charge of an ion depends on its chemical properties, determined by its location on the periodic table.
• Groups 1A, 2A, and 3A (main groups) have set charges.

Set Charges for Main Groups

• Group 1A: +1 charge (always).
• Group 2A: +2 charge (always).
• Group 3A: +3 charge (always).
• Group 4A: +4 charge (most of the time).

Metals vs. Nonmetals

• Metals tend to be positively charged.
• Nonmetals tend to be negatively charged.

Groups 5A, 6A, 7A, and 8A

• Group 5A: -3 charge (mostly nonmetals).
• Group 6A: -2 charge (mostly nonmetals).
• Group 7A: -1 charge (all nonmetals).
• Group 8A: 0 charge (noble gases).

Charge Trend

• Start with +1, +2, +3, +4, then decrease to -3, -2, -1.
• The first four groups increase positively; the last three decrease negatively.

Example - Oxygen

• Oxygen is in Group 6A (nonmetal).
• Therefore, it has a -2 charge.

Example - Magnesium

• Magnesium (Mg) is a metal.
• It's in Group 2A, so it has a +2 charge.

Middle Rows (Transition Metals)

• Transition metals follow their own rules and do not have set charges.

Compounds and Chemical Reactions

• The universe comprises only 118 elements; many are man-made.
• Different compositions affect properties.

Compounds

• Compounds are two or more elements chemically bonded together.
• Mixtures are physically bonded and not considered compounds.

Types of Compounds

• Ionic Compounds.
• Molecular Compounds.
  *Note: Metal alloys, not discussed.

General Rules for Writing Compounds

• Elements are represented by symbols (e.g., E).
• Subscripts indicate the number of atoms of each element in the chemical formula.
• Example: H_2O (2 hydrogen atoms and 1 oxygen atom).
• If no subscript is present, it is assumed to be 1.

Example 1

• CH_4 has 1 carbon and 4 hydrogen atoms.

Example 2

• CO has 1 carbon and 1 oxygen atom.

Polyatomic Ions

• Ions made of multiple atoms or elements.
• Six main polyatomic ions to know:
  • Carbonate
  • Hydroxide
  • Sulfate
  • Ammonium
  • Nitrate
  • Phosphate

Limestone

• The main ingredient is bicarbonate.
• Can neutralize stomach acid (like TUMS).

Writing Polyatomic Ions

• The subscript outside the parenthesis indicates how many of the polyatomic ions are present.
• Example: Al(NO3)3 (3 nitrate ions).
• Recognize polyatomic ions in formulas. E.g., NaNO_3 ; "3" here is not to be mistaken with "3 NOs"

Example

• In Al(NO3)3, the number "3" indicates that there are three nitrate (NO_3) ions.
• If in doubt use parenthesis to avoid mistakes.

Ionic Bonds

• Formed when a cation and an anion transfer electrons.
• Cations lose electrons and become positive.
• Anions gain electrons and become negative.

Ion Pair

• A cation gives its electrons to an anion.

Naming Ionic Compounds

• The cation name goes first, followed by the anion name.

Example

• Magnesium cation and nitrate anion become magnesium nitrate.

Crisscross Method

• A method to find the chemical formula by crisscrossing the charges of the ions.
• Example: Magnesium (Mg) with a +2 charge and Nitrate (NO_3) with a -1 charge.
• The resulting formula is Mg(NO3)2.

Polyatomic Ions in Formulas

• When there are multiple polyatomic ions, enclose them in parentheses and add the subscript outside.
• Example: Magnesium cation and phosphate anion becomes Mg3(PO4)_2.

Transition Metals

• Charges vary and are not set.
• Two naming systems:
  • Old school naming system (ferric, ferrous).
  • Modern naming system (element name with charge in Roman numerals in parentheses).

Modern Naming System

• Use Roman numerals to indicate the charge of the transition metal.
• Example: Iron with a +2 charge is iron (II).
• Example: Iron with a +3 charge is iron (III).

Example - Iron and Chlorine

• Iron (II) chloride contains Fe^{2+} and Cl^−.
• Iron (III) chloride contains Fe^{3+} and Cl^−.
• If not indicated, there's a chance of not distinguish/differentiate from each other.

Practice Points

Example 1

• Calcium cation and chlorine anion.
• Calcium (Ca) is in Group 2A, so it has a +2 charge.
• Chlorine (Cl) is in Group 7A, so it has a -1 charge.
• The resulting formula is CaCl_2.

Example 2

• Ammonium cation (NH_4) and sulfur anion.
• Ammonium has a +1 charge, and sulfur has a -2 charge.
• The resulting formula is (NH4)2S.

Example 3

• Copper (II) cation and bromine anion.
• Copper has a +2 charge, and bromine has a -1 charge.
• The resulting formula is CuBr_2.

Finding Ions from Formulas

• Reverse the crisscross method to find the charges of the ions.
• Example: Fe2O3. Reverse crisscross makes it iron three plus and oxygen two minus.

Example

• Given Mg3N2, magnesium will have a +3 charge, and nitrogen will have a -2 charge.

Naming Ionic Compounds

• Cations go first, then anions.
• The cation name stays the same unless it is a transition metal.
• Anions keep the first syllable of their name and replace the ending with "ide."
• If dealing with a transition metal write the charge in parenthesis e.g. iron (II) or (III)
• If it has a polyatomic ion slap polyatomic ions name at the very end. I.e. do not the "ide"

Example - Sulfur

• Sulfur becomes sulfide.

Example - Carbon

• Carbon becomes carbide.

Example - Hydrogen

• Hydrogen becomes hydride.

Example - Copper

• What is the element of Cu? Cu - Copper.
• In the element CuCl_2, what happens with chlorine? Chlorine becomes chloride, hence Copper Chloride.

Main Group Elements

• In the tall collumn we don't worry if to use parenthesis due to the transition metal.

Example - Barium Bromide

• Since in the tall collumn (main group) barium name will just be written Barium.
• What does Br stands for? For Bromide sounds a lot like Bromine. Since it's in the Group 7A
• What element's are in that group? 7a stands for minus one.
• So after barium: barium bromide!

Example - NaOH

• Na stands for Sodium Hydroxide
• Hydroxide from water hydro - oxygen! H + O - hydroxide is here. So as for element? Polyatomic ionic
• Where do we find sodium in the table? It is an element so NaOH is our rule."