Ch 9 Understanding the Atom

Discovering Parts of the Atom Democritus & Aristotle Can you cut a piece of paper in half and cut those halves in half and continue until you are left with 1 particle?   If you could do this you would end up with what Democritus called an atom= not able to be divided Aristotle disagreed with democritus’s idea. He believed that You would never end up with a particle that could not be cut. Who was correct? Aristotle or Democritus?? Size of an atom

Discovering Parts of the Atom Democritus was correct! Matter is made of particles which we call atoms. Atom - the smallest unit of an element that maintains the properties of that element

Discovering Parts of the Atom Dalton – 1803 Dalton’s atomic theory was based on Democritus's ideas! All substances are made of atoms Atoms are small particles that cannot be created, divided, or destroyed Atoms of the same element are exactly alike and atoms of different elements are different. Atoms join with other atoms to make new substances.

Discovering Parts of the Atom New information didn’t quite fit with Dalton’s ideas. His atomic theory had to be changed. Thomson- 1897 Discovered the negatively charged particles There are small particles inside the atom. Atoms can be broken down further. (Dalton thought atoms were solid spheres – so he’s wrong)

CATHODE-RAY TUBE A positively charged plate was attracted to a source of electric current. Therefore, the beam must be made up of negative charges. Thomson concluded these negative charges are present in every kind of atom.

Discovering Parts of the Atom Electron- A subatomic particle that has a negative charge. Thomson described his model of the atom like plum pudding. Today you might call it the chocolate chip cookie model. (with electrons represented by chocolate chips)

Discovering Parts of the Atom Rutherford- 1909 Rutherford was a student of Thomson’s. He tried to help prove Thomson theory was correct and ended up proving him wrong! He aimed a beam of small, positively charged particles (α particles) at a thin piece of gold foil. Photo paper behind the gold recorded where the charged particles hit. Surprising Results He thought the particles would pass right through the gold in a straight line. Some particles were deflected. Some shot right straight back

Rutherford’s Gold-Foil Experiment Rutherford determined an atom must be mostly empty space with a tiny part made of highly dense material.

Rutherford’s Model Nucleus - The centrally located, tiny, extremely dense positively charged part of an atom.

Discovering Parts of the Atom Bohr – 1913 Niels Bohr proposed that electrons move around the nucleus in certain paths or energy levels Electrons can jump from path to path but cannot exist in between paths. Think of rungs on a ladder. You can stand on the rungs, but not between the rungs. Or think of planets in our solar system – they do not stray off the set path or go in-between paths.

The Modern Atomic Theory Schrӧdinger and Heisenberg Electrons do not travel in definite paths as Bohr suggested. The exact path of the electron cannot be predicted. There are regions where it is likely to be found called electron clouds

In Your Journal (#10 )Why was Thomson’s model of the atom incorrect? Which scientists changed the model and describe how they changed it. Write neatly Your journal should take up at least 8 lines

9.2 Protons, Neutrons, and Electrons – How Atoms Differ How small is an atom? A penny contains 2 x 1022 atoms. (20,000,000,000,000,000,000,000 atoms) An atom is made up of electrons, protons, & neutrons The Nucleus Protons – a particle with a positive charge & found in the nucleus of the atom The mass of protons are measured in atomic mass units (amu). Each proton has a mass of 1 amu. Neutrons- Particles in the nucleus that have no electrical charge. Neutrons have a mass of 1 amu. (Same as protons)

The Atom Outside the nucleus Electrons are found outside the nucleus. They have a negative charge. They have a mass of almost 0. If the number of protons equals the number of electrons, the atom is neutral

HOW DO ATOMS OF DIFFERENT ELEMENTS DIFFER? Atoms of different elements have different numbers of protons. No two elements have the same number of protons (atomic #). Atomic number is the number of protons in an atom. All atoms of the same element have the same atomic number.

HOW DO ATOMS OF DIFFERENT ELEMENTS DIFFER? So how do we figure out the number of neutrons for an atom? The number of electrons = the number of protons Mass number – the number of protons & neutrons in the nucleus The mass number does not include the mass of electrons because they are so small and have very little effect.

HOW DO ATOMS OF DIFFERENT ELEMENTS DIFFER? Where do you find these numbers?  On the periodic table ! C C C     # of neutrons = mass # - atomic # # of neutrons in C = 12 - 6 C P+ = 6 No= 6 E- = 6 Mass # # n0+ # p+ 12 Atomic # # p+ 6

HOW DO ATOMS OF DIFFERENT ELEMENTS DIFFER? Mass # Atomic # Protons Neutrons Electrons Li Be B C N O F Na k

DRAWING BOHR MODELS OF ATOMS Draw the nucleus and label with #p & #n Draw electron orbitals 1st orbital can have 2 electrons only 2nd and 3rd ring can each have 8 electrons Fill – the lowest energy level first Inner rings must be filled first before any electron enters a higher ring!!!!! Examples:

Changing Atoms If you change a persons hair color does it change who they are? Weight? What subatomic particle determines an atoms identity? protons Therefore, the number of electrons and neutrons can change in the atom and it is still the same atom. If the number of protons equals the number of electrons, the atom is neutral If the number of protons does not equal the number of electrons, the atom becomes charged and we call it an ion  Positively charged ions – cation – lost electrons Negatively charged ions – anion – gained electrons

ISOTOPES Isotopes- Atoms that have the same number of protons, but have different numbers of neutrons. Examples: Hydrogen -1 and Hydrogen -2 Naming Isotopes Write the name of the element, followed by a hyphened and the mass number of the isotope. Ex. A hydrogen atom with 1 proton and 0 neutrons A hydrogen atom with 1 proton and 1 neutron

ISOTOPES Mass Number – The sum of the no and p+ in an atom. (electrons are not included.) Mass # = protons + neutrons Atomic Mass – The average of the masses of all the naturally occurring isotopes of that element.

Solving the Atomic Mass Ex: Skittles consist of Orange – 1 which is 25% of the Skittles and Red – 2 which is 75% of skittles. Step1: change percents to decimals. Step 2: multiply the decimal by the mass Step 3: add the two answers together

Solving for Atomic Mass Ex: Calculate the average atomic mass for Chlorine which is made of Chlorine-35 which is 76% of the Chlorine atoms and Chlorine- 37 which is 24% of Chlorine atoms. Step1: change percents to decimals. Step 2: multiply the decimal by the mass Step 3: add the two answers together

Solving for atomic mass Calculate the atomic mass of silicon, which occurs naturally as 92% silicon-28, 5% silicon-29, and the remaining ___ is silicon-30