Mass Spectroscopy

  • Why are atomic masses not whole numbers? Because they are weighted averages of isotopes
  • Mass spectroscopy: study of the manner in which substances interact with electromagnetic radiation
  • Isotopes: atoms with the same number of protons (same element) but different number of neutrons
    • Have almost the same properties because chemistry comes from the number of electrons
    • Elements occur in nature as mixtures of isotopes
  • Elemental Analysis: its purpose is to determine the formula unit of each element

Fundamental Chemical Laws

  • Law of Conservation of mass: mass is neither created nor destroyed [in a reaction]
    • So in a reaction, the total amount of mass will always be the same
  • Law of Definite proportion: A compound always contains exactly the same proportion of elements by mass
  • Law of Multiple Proportions: If diff compounds have the same smallest whole number ratio of atoms, the composition by mass of those compounds is the same

  • Mass percent of an element:

Dalton’s Atomic Theory

  1. Each element is made up of tiny particles called atoms.
  2. The atoms of a given element are identical; the atoms of different elements are different
  3. Chemical compounds are formed when atoms of different elements combine with each other.
  4. Chemical reactions involve reorganization of the atoms—changes in the way they are bound together.

                     The atoms themselves are not changed in a chemical reaction.

Characterizing the Atoms


  • Nucleus: protons and neutrons

○ Nucleus is very small but accounts for almost all of an atom’s mass

○ Protons are positive; neutrons are neutral

  • Protons (+) = Electrons (-)

              So that atom is stable and electrically neutral

The Electron

Mostly energy, negligible mass

  • Electrons have potential energy that increases the further away they are from the nucleus → The energy an electron has depends on its distance from the nucleus

Valence electrons in outermost s and p shell is where the chemistry of an atom takes place

  • Determines how atoms react with other atoms

Electrons (-) orbit the nucleus

Adding energy to element excites electrons

  • Electron moves to higher energy lvl
  • To fall back electron releases energy as light, heat, sound etc 


The number of protons determines the type of element an atom is and the number of electrons determines how the atom will react

  • Gaining/losing protons changes element

Atomic Number: number of protons

Atomic Mass/Mass Number: total number of protons and neutrons and average of isotopes, expressed in atomic mass units (u)

  • Also called mass-to-charge ratio

Molar Mass: atomic mass of an atom expressed in grams, is equal to one mole (units: g/mol)

  • The molar mass of diff elements are different bcuz constituent particles are different but all equal one mole

Mole: 6.022× 10^23 of some chemical unit

  • Can be atoms, particles, people etc

Electrons are repelled by other electrons, an electron between a valence and nucleus causes the valence to be weaker (called shielding)

Octet Rule: in order to be stable an atom must have 8 electrons in its outermost s and p shells

Stable: unreactive, lowest energy lvl → everything in nature wants to be stable

J.J Thompson

His model of the atom had a spherical cloud of positive charge with negative electrons randomly embedded in it

One Model of The Nuclear Atom

Results could be explained only in terms of a nuclear atom—an atom with a dense center of positive charge (the nucleus) with electrons moving around the nucleus at a distance that is large relative to the nuclear radius.

Mass Spectroscopy

Three Types of Questions on the AP exam

1. Calculate avg atomic mass from mass spectrum (might be fictitious element)

        ● avg AM = (abundance of 1st isotope × its atomic mass) + (abundance of 2nd isotope × its atomic mass) / 100

2. Identify element from mass spectrum

       ● Identify isotopes masses and approximate abundances from the graph → estimate the avg atomic mass → compare estimate to elements on periodic table

3. Identify isotope from mass spectrum

      ● Determine the element represented by the graph using technique above → determine the number of neutrons by subtracting atomic number (protons) from mass number (protons + neutrons)

Graph Analysis

Each bar represents a different isotope

The height of the bars represents the relative abundance

X-axis may be labeled mass, m/z, mass charge, or atomic mass