Democritus (460‐370 B.C.) First to propose the ultimate particle: “atom”

“fathers of modern chemistry” Antoine- Laurent de Lavoisier John Dalton Jöns Jakob Berzelius

Dimitri Mendeleev Developed the first periodic table in 1869 Arranged by increasing atomic mass with gaps for elements yet to be discovered

Henry Moseley -discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each element in the periodic table. He proposed that the relationship was a function of the positive charge on the nucleus in This rearranged the periodic table by using the atomic number instead of atomic mass to represent the sequence of the elements

J. J. Thomson 1904 Established the charge- to-mass ratio of the charged particle Plum Pudding Model

Ernest Rutherford (1871‐1937) Showed that atoms have a nucleus Gold Foil Experiment – Shot a beam of alpha (α) particles at a thin sheet of gold foil –Because of the plum pudding model, he expected the α particles to pass through with no or minor alterations in their paths –He set up a Zinc sulfide screen around the foil that flashed when the particles hit it.

What he was expecting

What actually happened

Rutherford’s Experiment

James Chadwick Credited for the discovery of neutrons

Robert Millikan Calculated the charge of an electron with an oil drop experiment

Niels Bohr Theorized that electrons moved around the nucleus of an atom in orbits

John Dalton (1766‐1844) Proposed atomic theory: – Atoms of different elements can combine in simple, whole number ratios to form chemical compounds. Atom: the smallest unit of an element that can exist either alone or in combination with other atoms

Structure of Atoms Two Parts: 1.Nucleus – positively charged central part of an atom Contains two types of particles – proton – positive charge – neutron – no charge; neutral 2.Electrons – negatively charged particles Outside of the nucleus electron (-) proton (+) neutron (0)

The electrons’ charge + Nucleus’ charge = neutral charge (Zero) ParticleSymbolLocationChargeMass Electrone-e- Space surrounding the nucleus amu ProtonP+P+ Nucleus+1amu NeutronN0N0 Nucleus Neutral 1amu From this table you can deduce that protons and neutrons make up the mass of an atom and the electrons are too small to count!!

Looking at a periodic table: **Be careful, not all periodic tables have the numbers in the same places!!

Atomic Number The number of protons in an atom of an element. Because all atoms are neutral, the number of electrons is equal to the number of protons.

Atomic Mass This is an average of all the isotopes of an element. For example: Chlorine exists naturally 75.77% of the time as chlorine-35 (mass = amu). It exists as chlorine-37 (mass = amu), about 24.23% of the time. Chlorine-35 = amu X = Chlorine-37 = amu X =  35.45amu Thus, taking a weighted average gives us amu (atomic mass units) for chlorine.

Mass Number this is the total number of protons and neutrons in the nucleus of an atom. (This is the atomic mass rounded to a whole number.) Example: Fluorine’s Atomic Mass = Fluorine’s Mass number = 19

To calculate the number of neutrons in an atom:

Nuclear Symbol (Symbolic Notation) this is a way to notate the element’s symbol, mass number & atomic number. Example: Cl Hyphen Notation the name of the element, then hyphen, then the mass number written after the element. Example: Chlorine-35

Isotopes atoms of the same element which have different numbers of neutrons. Example: A stable Hydrogen atom has 0 neutrons & is called Hydrogen -1. Some have 1 or 2 neutrons, such as Hydrogen-2 & Hydrogen-3. These are called isotopes of hydrogen.

Isotopes Hyphen Notation ProtonsElectronsNeutrons Nuclear Symbol Hydrogen -1 Hydrogen -2 Hydrogen -3 Isotopes of Hydrogen