number of constituent particles/formula units per mole of the substance
average mass (atomic mass units/amu) of one particle (atom/molecule) or formula unit of a substance will always be equal to the molar mass of the substance (g)
mass spectra of elements
mass spectrum of a sample containing a single element can be used to determine:
identity of the isotopes of that element
relative abundance of each isotope in nature
average atomic mass: weighted average of the isotopic masses using the mass of each isotope and its relative abundance
elemental composition of pure substances
composition can be:
individual molecules
atoms or ions held together in fixed proportions (formula unit)
law of definite proportions: ratio of masses of constituent elements in any pure sample of that compound is always the same
empirical formula: chemical formula listing the lowest whole number ratio of atoms in the elements in a compound
composition of mixtures
pure substance: atoms, molecules, or formula units of a single type
mixture: atoms, molecules, or formula units of two or more types (relative proportions can vary)
elemental analysis can be used to determine the relative number of atoms in a substance and to determine its purity
atomic structure
positively charged nucleus
protons (positive charge)
neutrons(no charge)
Coulomb’s law
force between two charged particles
electron configuration
negatively charged electrons
electrons in shells/energy levelsandsubshells/sublevels
described by ground-state electron configuration
core electrons: inner electrons
valence electrons: outer electrons
explained by quantum mechanics
aufbau principle: as protons are added to the nucleus to build up the elements, electrons are similarly added from the lowest energy level up
ionization energy: relative energy required to remove an electron from different subshells of an atom or ion or from the same subshell in different atoms or ions
qualitative application of Coulomb’s law
weaker if farther from nucleus
stronger if stronger bond
photoelectron spectroscopy (PES)
experimental measurement of the energies of electrons in a given shell
position of each peak in the PES spectrum is related to the energy required to remove an electron from the corresponding subshell
relative height of each peak is proportional to the number of electrons in that subshell
periodic trends
periodic table organized by patterns of recurring properties of the elements
patterns of ground-state electron configurations
presence of completely or partially filled shells (and subshells) of electrons in atoms
trends can be predicted by the position of the element on the periodic table
trends can be qualitatively understood using:
Coulomb’s law
shell model
shielding: electrons block the positive charge from the nucleus (stronger with more energy levels)
effective nuclear charge
ionization energy: energy required to remove an electron from a gaseous atom or ion (assumed to be in ground state)
across a period: increases (more protons)
down a group: decreases (less attracted to nucleus)
atomic/ionic radius: obtained by measuring the distance between atoms/ions in chemical compounds
across a period: decreases (more attracted to nucleus)
down a group: increases (more shells)
electron affinity: energy change associated with the addition of an electron to a gaseous atom
more negative energy results in a greater quantity of energy released
across a period: generally more negative (more protons)
down a group: generally more positive (less attracted to nucleus)
exception: fluorine has low electron affinity (small atom causes repulsion)
electronegativity: the ability of an atom in a molecule to attract shared electrons to itself
across a period: generally increases (losing electrons to gaining electrons)
down a group: generally decreases (atomic radius increases)
valence electrons and ionic compounds
the likelihood that two elements will form a chemical bond is determined by the interactions between the valence electrons and nuclei of elements
elements in the same column of the periodic table tend to form analogous compounds
typical charges of atoms in ionic compounds are governed by the number of valence electrons and predicted by their location on the periodic table
