The Structure of the Atom Or Modern Atomic Theory

Dalton’s Atomic Theory revised Democritus’s ideas based on his research matter is made of tiny particles called atoms atoms of the same element are identical (size, mass, chemical properties) English school teacher (1766-1844)

Dalton’s Atomic Theory (continued) atoms of one element differ from atoms of another element atoms cannot be created, destroyed or divided atoms combine to form compounds atoms can combine, separate or rearrange in chemical reactions

Structure of the Atom atom – the smallest particle of an element that can exist that retains the properties of the element.

The size of an atom a single copper penny contains: 29,000,000,000,000,000,000 atoms in scientific notation that number is 2.9 x1019 atoms of copper atoms can be seen with a special kind of microscope

Questions remained what is the shape of an atom? are atoms made of smaller particles? is an atom homogenous or heterogenous?

Subatomic particles and the atomic nucleus

Static Electricity have you ever been shocked when you touched a doorknob static electricity made scientists in the 1800s look for a relationship between matter and electric charge

Experiments with Electricity how would electricity behave in the absence of matter (in a vacuum) they connected electrodes to the ends of vacuum tubes allowing electricity to travel through the tube

Cathode Rays streams of negatively charged particles could travel through the tubes similar cathode rays were found in all types of matter

JJ Thompson researched the ratio of a cathode ray particle’s mass and charge placed electrical and magnetic fields around cathode ray tubes and measured changes in the particle’s path 1856-1940

JJ Thompson discovered the mass of a cathode ray particle was smaller than the smallest known atom (hydrogen) this was the electron based on the mass, atoms must contain other particles This was called the Cathode Ray Tube Experiment

Lingering Questions How can atoms be neutral, if they contain negative particles? Thompson proposed his plum pudding model cookie dough dough is positively charged sphere chips are negatively charged particles in it

Ernest Rutherford studied positively charged alpha particles shot alpha particles at gold foil (a few atoms thick) to see if the path would deflect An alpha particle is a helium nucleus Gold Foil Experiment 1871-1937

Rutherford’s Results he thought the alpha particles would go straight through or deviate a little some particles bounced almost straight back they must have hit something BIG

Rutherford’s Experiments Showed an atom is mostly empty space there is a tiny dense region (nucleus) in the center of an atom the nucleus contains most of the atom’s mass positive charges are found in the nucleus (Be sure to write these down. The results/analysis/conclusions are the most important parts of an experiment.)

More Rutherford Discoveries later he concluded the atomic nucleus contains protons the charge of a proton is equal (but opposite) to that of an electron

Nuclear Atomic Model

Atomic Particles Particle Charge Mass (kg) Location Electron -1 9.109 x 10-31 Electron cloud Proton +1 1.673 x 10-27 Nucleus Neutron 1.675 x 10-27

Subatomic Particles Neutrons Protons Electrons Nucleus Orbitals

Modern View The atom is mostly empty space Two regions Nucleus- protons and neutrons Electron cloud- region where you might find an electron

How atoms differ Chapter 4.3

How atoms differ There are over 110 different kinds of atoms. What makes atoms of one element different than another?

Henry Moseley (1887-1915) discovered atoms of each element have a unique positive charge the number of protons determines the element (e.g. every atom with 6 protons is carbon) corresponds to the number of protons corresponds to the atomic number

Atomic Number elements are listed in order of increasing atomic number in periodic table the number of protons is the atomic number (e.g. every atom with 6 protons is carbon) since an atom has no net charge: Atomic # = # of protons = # of electrons

Isotopes atoms of the same element always have the same number of protons but they can have different numbers of neutrons most elements are a mixture of isotopes isotopes are atoms of the same element with a different number of neutrons and a different mass

How atoms differ Chapter 4.3B

atom the smallest piece of an element that has all the properties of that element

electron small negatively charged particle found in all atoms in the electron cloud. It has a charge of -1

proton subatomic particle found in an atom’s nucleus. It has a charge of +1

neutron subatomic particle found in an atom’s nucleus it has no charge (it is neutral)

Atomic Particles Location Mass (g) Charge Particle Nucleus 1.67 x 10-24 Neutron +1 Proton Electron cloud 9.11 x 10-28 -1 Electron Location Mass (g) Charge Particle

atomic number the number of protons in an atom. It identifies the atom Remember: APE Atomic number = # Protons = # Electrons (since atoms are neutral)

mass number number written after an elements name, representing the sum of its protons and neutrons

isotope atoms of the same element with the same # of protons, but different # of neutrons

Mass Number the mass of an atom is called the mass number to calculate the number of neutrons in an isotope: mass number - atomic number = number of neutrons

Isotope Notation To distinguish between two isotopes the mass number is given along with the elements name Potassium-39 Potassium-40 Potassium-41

X Symbols of isotopes Mass number Atomic number Contain the symbol of the element, the mass number and the atomic number Mass number X Atomic number

Messy Math the mass of subatomic particles are so small, they are difficult to work with scientists adopted an atomic standard the mass of a carbon-12 atom is said to be 12 atomic mass units (amu) protons and neutrons have an approximate mass of 1 amu

Why aren’t atomic masses whole numbers? atomic mass reported on the periodic table are a weighted average of the isotopes of that element Chlorine is a mixture of 2 isotopes 75.770% chlorine-35 (atomic mass = 34.969) 24.230% chlorine-37 (atomic mass = 36.766) (0.75770)(34.969) + (0.24230)(36.966) = 26.496 +8.9569 = 35.4529  35.453 amu

atomic mass unit 1/12th the mass of a carbon-12 atom

atomic mass weighted average mass of the isotopes of that element

Measuring Matter Chapter 11.1

Common Counting Units eggs paper shoes pencils dozen (12) ream (500 sheets) shoes pair (2) pencils gross (144)

Chemist’s Counting Units need a counting unit for atoms and molecules since they are extremely small mole = counting unit to measure the amount of a substance 1 mole = the number of particles in exactly 12 grams of pure carbon-12

Avogadro’s Number 1 mole = 6.0221367 x 1023 particles rounded to 6.02 x 1023 particles particles can be atoms (elements) molecules formula units

How much is a mole?

Converting from moles to particles use conversion factors how many roses are in 3.5 dozen? 3.5 dozen  12 roses = 42 roses dozen

mole conversions how many particles are in 3.50 moles of sucrose? 3.50 moles  6.02 x 1023 particles mole 21.1 x 1023  2.11 x 1024 particles

particle to mole conversions how many moles are in 4.50 x 1024 atoms of Zn? 4.50 x 1024 atoms  1 mole 6.02 x 1023 atoms 7.48 moles Zn

mole SI base unit to measure the amount of a substance (element or compound)

molar mass mass in grams of any pure substance; for elements, the molar mass is the atomic mass

Avogadro’s number the number 6.0221367 x 1023, represents particles in a mole