Chapter 10- Introduction to Atom

Element substance that cannot be broken down into simpler substances Element substance that cannot be broken down into simpler substances. It is a matter made of atoms of only one kind 118 Elements on Periodic Table Example—Iron is only made of iron atoms

Atom smallest part of an element Consist of a nucleus surrounded by one or more electrons

Scientists who contributed to the knowledge of the Atom  The Theory of the Atom has changed over time

Timeline BCE means Before Common Era AD means Anno Domini (Year of Our Lord)

Democritus (460 BC – 370 BC)

Democritus A Greek Philosopher, who theorized that if you cut an object in half repeatedly, you would end up with a particle that could not be cut. (Atom) Atom (Greek word: Atomas) meaning “not able to be divided”

Aristotle (384 BC – 322 BC)

Aristotle Greek Philosopher, who believed that you would never end up with a particle that could not be cut More Influential He was WRONG

John Dalton (AD 1766 – AD 1844)

John Dalton British School Teacher / Chemist Combined his idea of Element with Greek Theory of the Atom He proposed the following ideas about Matter:

Matter is made up of Atoms. Atoms cannot be divided into smaller pieces. All the Atoms of an Element are exactly alike. Different Elements are made of different kinds of Atoms.

William Crookes(AD 1832 –AD 1919)

William Crookes Start of Scientific Evidence (No Lab Experiments before this time) Glass tube that had almost all the air removed from it. The glass tube had two pieces of metal called Electrodes.

Electrode is a piece of metal that can conduct electricity. Anode an electrode that has a positive end. Cathode an electrode that has a negative end.

Crooke hypothesized that the green glow in the tube was caused by rays, or streams of particles. Rays were called Cathode Rays Used in TV and Computer Screens

J.J. Thomson(AD 1856 – AD 1940)

J. J. Thomson Tested Crooke’s Experiment Was the Greenish Glow Light, or was it a stream of charged particles? Placed a Magnet beside the tube from Crooke’s experiment.

The beam is bent in the direction of the magnet. Light cannot be bent by a Magnet, as the beam couldn’t be light. Conclusion—must be a charged particle that came from the cathode.

He showed that there was a mistake with Dalton’s Theory He showed that there was a mistake with Dalton’s Theory. The Atom could be divided into smaller pieces. Electrons  negatively charged particle that exists in an Electron Clouds formation around an Atom’s nucleus

Ernest Rutherford (AD1871-AD1937)

Ernest Rutherford Former student of Thomson’s Designed an experiment to study the parts of the Atom He aimed a beam of small, positively particles at a thin sheet of Gold Foil He placed a coated screen around the foil The coating glowed when hit by the positively charged particles

Hypothesized that almost all the mass of the atom and all of its positive charge are crammed in the center of the atom called the Nucleus. Proton positive charge particle present in the nucleus of all atoms.

 An Atom’s Electron have almost no mass. Neutron neutral particle that has the same mass as a proton and is found in an atom’s nucleus

Niels Bohr (AD 1885 – AD 1962)

Niels Bohr Danish scientist who worked with Rutherford Studied the way Atoms react with light Electrons travel in a region surrounding the nucleus called the Electron Cloud.

Parts of the Atom Dot on your paper – 2 Million Atoms Size of an Atom: Dot on your paper – 2 Million Atoms Penny – 20,000,000,000,000,000,000,000 Atoms or 2 X 10²² Atoms of Copper and Zinc

Nucleus (Center Atom) Proton  positively charged particle --All Protons are identical Neutron  no electrical charged particle --All Neutrons are identical Neutrons are slightly larger than Protons Majority of Mass in Atom(Protons/Neutrons)

Outside the Nucleus Electron  negatively charged particle Has hardly any Mass Very Small (1,800 Electrons equal Mass of 1 Proton)  Electrons are found around the Nucleus in what is called Electron Clouds

Mass Proton = Mass Neutron Mass Proton > Mass Electron Mass Neutron > Mass Electron

How do Atoms in an Element Differ? Elements differ by their number of Protons, Neutrons, and Electrons in each Atom.  The different numbers of each will give the Atom special properties

Neutral Charge Elements can differ in charge  The Element will either have a neutral charge, positive charge, or negative charge

Atomic Structure of an Atom All Atoms are made up of 3 Subatomic Particles Proton Neutron Electron *** Most of an Atom is Empty Space

You must know 3 things about each Subatomic Particle What is the Charge? Where can it be Found? What is its JOB?

Proton Positive Charge Found in Nucleus of an Atom Identification of Element

Neutron Neutral Charge Found in Nucleus of Atom Radioactive (Yes or No) Yes if Neutrons > Protons No if Protons > Neutrons

Electron Negative Charge Found Outside the Nucleus in a region called the Electron Cloud Binding or Glue to form Compounds

Atomic Number  Is the number of protons in the nucleus of an atom of that element. The Atoms of different elements contain different numbers of protons.

Examples Hydrogen has 1 proton, Carbon has 6 protons, Oxygen has 8 protons, Iodine has 53 protons Atoms of an element are identified by the number of protons because this number never changes without changing the identity of the element.

Mass Number  the sum of Neutrons and Protons in the nucleus of an Atom. To calculate the number of Neutrons in an isotope you must subtract the Atomic Number from the Mass Number of the isotope.

Neutrons The atomic number is the number of protons, but what about the number of Neutrons in an Atom’s nucleus. A particular type of Atom can have a varying number of neutrons in its nucleus

Isotopes  are Atoms of the same element that have different numbers of Neutrons. Carbon-12 (Normal) Carbon-13 (Isotope) Carbon-14 (Isotope)

Helium Isotopes

Radioactive Decay Many atomic nuclei are stable when they have about the same number of Protons and Neutrons. Some become unstable when too many or too few Neutrons.

Examples Uranium and Plutonium Repulsion builds up because the nucleus must release a particle to become stable When particles are released, energy is given off

Radioactive Decay release of nuclear particles and energy from unstable nuclei Transmutation the changing of one element into another through radioactive decay Example – Smoke Alarm

Half-Life Half-Life time needed for one-half of the mass of a sample of a radioactive isotope to decay.

Half –Life Problem: Tritium has a half-life of 12.5 years. If you start with 20 grams of Tritium, how much is left after 50 years?

Carbon Dating Radioactive decay is useful in determining the age of artifacts and fossils. Carbon-14 Half-Life of 5,730 years. In a living organism, the amount of Carbon-14 remains in constant balance with the levels of the isotope in the atmosphere or ocean.

The balance occurs because living organisms take in and release Carbon. While life processes go on, any Carbon-14 nucleus that decays is replaced by another from the environment. When the plant and animal dies, the decaying nuclei no longer can be replaced.

Calculating the Mass of an Element Atomic Mass  of an Element is the weighted average of the Masses of all the naturally occurring Isotopes of that Element Atomic Mass Unit = AMU

Atomic Mass Problem  Chlorine-35 makes up 76% of all the Chlorine in nature, and Chlorine-37 makes up the other 24%. What is the Atomic Mass of Chlorine?

Forces of an Atom Gravitational Force – depends on mass and distance apart -- since atom very small this force is small Electromagnetic Force – objects that have the same charge repel each other, while objects with opposite charge attract each other -- strong force

Strong Force -- Protons push away from each other -- strongest force 4. Weak Force – Occurs in Radioactive Atoms when Neutrons change to Protons -- weak force