Atomic Masses and Ionic Compounds

Atomic Masses, Ions, and Ionic Compounds

• Atomic mass unit (u) is a unit used to describe the masses of individual atoms and molecules, instead of grams or kilograms
• Defined as one-twelfth the mass of a carbon-12 atom which has 6 protons and 6 neutrons.
• Proton mass: approximately 1.00728 u
• Neutron mass: approximately 1.00866 u
• Electron mass: approximately 0.000549 u

Atomic Mass for Atoms

• Equal to the mass number of the atom
• Carbon-12 has a mass of 12 u
• Oxygen-16 has a mass of 16 u

Atomic Mass for Elements (Periodic Table)

• Most elements exist as a mixture of isotopes
• Atomic mass of an element is the weighted average of the masses of its isotopes

Atomic Mass of an Element

• Weighted average of the masses of the isotopes that make up an element
• Example: 50% of atoms with a mass of 10 u, 50% with a mass of 11 u, weighted average is 10.5 u

Atomic Mass of Boron

• 20% of boron atoms are Boron-10
• 80% of boron atoms are Boron-11
• Weighted average mass of Boron is 10.8 u

Atomic Mass for Molecules

• Sum of the masses of the atoms in a molecule
• Example: N₂O₃; N = 14.007 u, O = 15.999 u, Molecular mass of N₂O₃= 76.001 u

Questions

• Calculate the molecular mass of each substance using the periodic table: H₂O, O₂, Cl₂, CO₂, H₂S, SO₃, CO, PF₃, Se₂Br₂, SF₄, SF₆, and P₂S₅
• Calculate the molecular mass of each substance using the periodic table: carbon tetrachloride, silicon dioxide, trisilicon tetranitride, disulfur difluoride, iodine pentabromide, nitrogen trichloride, diphosphorus pentoxide

Questions

• Estimate the mass of each isotope in whole numbers: hydrogen-1, hydrogen-3, iron-56, phosphorus-31, carbon-14, americium-241

Questions

• Calculate the atomic mass of each element, given the isotopic composition: lithium-7 at 92.4%, lithium-6 at 7.6%, oxygen-16 at 99.76%, oxygen-17 at 0.038%, oxygen-18 at 0.205%, neon-20 at 90.48%, neon-21 at 0.27%, neon-22 at 9.25%, uranium-238 at 99.27%, uranium-235 at 0.720%

Recap

• Atomic number = number of protons
• Mass number = number of protons + number of neutrons
• Isotopes are atoms of the same element that have different mass numbers
• Molecules form when two or more atoms combine

Ions and Ionic Compounds

• Ions are electrically charged particles (atoms)
• Cations are positively charged ions
• Anions are negatively charged ions
• Ionic compounds form from positively and negatively charged ions

Electronic Configuration of Atoms

• Electrons are arranged around the nucleus in shells
• Lower energy configurations are more stable
• Noble gases have full valence electron configurations, making them stable
• Each shell can only hold a specific number of electrons

Ions and Ionic Compounds

• Elements in other groups have incomplete valence electron configurations, making them unstable
• Atoms tend to achieve a full outer electron shell (octet) to become stable
• The tendency of an atom to move toward having eight valence electrons is called the octet rule

Ions and Ionic Compounds

• Ionic bonding involves the transfer of electrons between atoms to achieve octets
• Positive ions (cations) lose electrons, negative ions (anions) gain electrons
• Covalent bonding involves the sharing of electrons between atoms to achieve octets

Ions and Ionic Compounds

• Methods to Draw Electron Configuration (EC) for elements, determining possible charges, identifying those that lose or gain electrons, and writing the ion (number before superscript charge)
• Naming Ions
• Cations are named by appending "ion" to the name of the element. Example: Sodium ion (Na⁺)
• Anions are named by adding "-ide" to the root element name. Example: Nitride ion (N³⁻)

Questions

• Naming species: O₂⁻, Co, Co²⁺, P³⁻, Sr²⁺