Atoms and ElementsChemical Foundations

Atoms and ElementsElement - Substances that can not be broken down further by ordinary chemical meansAtoms - The smallest particle of an element that retains the properties of the element

PostulatesElements consist of tiny particles called atomsAtoms retain their identity in reactionsThey do not change into other types of atomsCarbon stays carbon, Oxygen stays oxygenIn a compound, atoms of two or more elements are combined

Atoms have partsAtoms are composed of protons, neutrons and electronsAll of the parts of the atom stay together as that atom in chemical reactions

ElementsElements represent the different types of atoms that are knownCome in solid, liquid and gas forms at earths normal temperatures

Top Ten

Most Abundant Elements on Earth

Most Common Elements

A collection of argon atoms.

Diatomic Elements

Nitrogen gas contains N2 molecules.

Oxygen gas contains O2 molecules.

Representation of NO, NO2, and N2O.

History of the discovery of the atom300 BC in GreeceSchool of thought that matter is made up of tiny indivisible, invisible, indestructable, fundamental units of matter called atmosDemocritus of Abdera most well known atmostist. Did no experimentsNo proofphilosopher

1770sAntoine Lavosier made observations and did experiments resulting in the Law of Conservation of MassLaw of Conservation of MassMatter can not be created or destroyedMass of materials before the chemical reaction is the same mass after the reaction

1799 - ProustJoseph Proust showed that the proportion by mass of the elements in a compound is always the sameNow called the Law of Definite ProportionsPart of our definition of a chemical - any substance with a definite composition

John DaltonEnglish School TeacherAlchemists were trying to change different metals into goldWanted to learn in what ratios that different elements combinePerformed experimentsFormulated theories to explain his observationsProposed his theories in 1808Based on the Greek idea of the atom

Key idea - Daltons Atomic TheoryAll matter is made of indivisible, invisible, and indestructible particles called atomsAtoms of the same element are identicalAtoms of different elements differ in their physical and chemical properties

Daltons Atomic Theory ContAtoms of different elements combine in simple whole number ratios to form compoundsChemical Reactions occur when atoms are separated, joined or rearranged. Atoms of one element are NEVER changed into atoms of another element as a result of a reaction

1870sWilliam Crooks developed the Crooks TubeA tube with sealed gases and metal plates at the endRun a current through the metal, and a glowing beam forms

Schematic of a cathode ray tube.

1897 - Sir J.J.ThompsonThompson proposed that the cathode ray formed in the tube is a stream of negatively charged particlesKey idea - ELECTRONSShowed that all cathode rays are electrons regardless of the type of gas in the tubeAtoms have parts that are negatively chargedSince atoms are neutral, they must also have positively charged areasGave rise to the plum pudding model of the atom

Plum Pudding model of an atom.

1886 - GoldsteinE. Goldstein discovered a positively charged particle.Called them PROTONS Work was largely ignored

1909 - Ernst RutherfordRutherford physicist who experimented with the positively charged particleHe figuredAtoms have no chargeElectrical charges are properties of matterElectrical charges exist in single whole number units + or - (no fractions)Electrical Charges cancel each other, therefore a + and a - together = neutral atom

Rutherfords experiment.

Results of foil experiment if Plum Pudding model had been correct.

Actual Results.

Rutherfords modelKey Idea -NUCLEAR ATOMAll of the mass of the positive particles (protons) is at the center region of the atomCenter region called the NUCLEUSElectrons surround the nucleus in a cloud

1913 - Niels BohrDanish PhysicistRutherfords model did not work because the electrons are so small compared to the positive mass in the center of the atom

Bohrs AtomThere are certain energy levels in the space of the electron cloud that mathematically allow the electron to stay in the cloud (not crash into the nucleus)Key idea - There are certain ORBITS in which an electron can travel around the nucleus without losing energy

Bohrs atom contSuggested circular orbitsThe farther away from the nucleus, the higher the energy levelUnder normal conditions, electrons are arranged in the GROUND STATE. In the ground state, the electrons occupy the lowest energy level closest to the nucleus

All models have problemsAll models are not exactly correct, but they lead to further science and discoveryAll have important key ideas

We now knowAtoms are made up of subparticlesAtoms are divisible, but not by ordinary chemical meansAtoms of elements can vary (isotopes)Electrons reside in ORBITALS, not orbits