Atoms, Molecules, and Electronic Configuration

Outlines

• Focus is placed on defining atoms, molecules, ions, and chemical reactions.
• Discussion on the modern electronic theory of atoms.
• Writing electronic configurations of elements on the periodic table.

Definition of Atoms and Molecules

• An atom represents the basic unit of a chemical element.
• Atoms are the fundamental building blocks of all matter.
• Subatomic particles comprise atoms.
• Protons, neutrons, and electrons are the three types of extremely small particles.

Element

• An element is a substance with only one type of atom.
• Hydrogen consists of an atom containing one proton and one electron.

Molecule

• A molecule is formed when two or more atoms combine.
• Single atoms of elements are not molecules.
• "O" as a single atom, does not qualify as a molecule.
• Oxygen can bond with itself (e.g., O2 or O3) or with another element, like carbon (CO2) or sodium (Na2O), to create a molecule.
• Molecules can be simple or complex.
• Common example molecules:
  • Water (H₂O)
  • Nitrogen (N₂)
  • Ozone (O3)
  • Ammonia (NH3)
  • Glucose (C6H12O6)

Compounds

• A compound contains at least two or more different elements.
• Examples of compounds include water (H₂O), carbon dioxide (CO₂), and sulfuric acid (H₂SO₄).
• All compounds are molecules, but not all molecules are compounds.
• Molecules that only contain one element are not compounds
• Molecular oxygen (O2)
• Molecular hydrogen (H2)
• Molecular chlorine (Cl2)
• Carbon monoxide, ethane, and ammonia are compounds since each is made from more than one element.

Ions

• An ion is a substance possessing a positive or negative charge.
• An ion is an atom, or group of atoms, with a positive or negative charge.
• A particle with a neutral charge has the same number of protons and electrons.
• Ions do not have the same number of electrons and protons.
• Examples of ions:
  • He+ is a helium atom missing one electron and carries a positive charge due to having one more proton than electrons.
  • CO32- is the formula for Carbonate, which has two more electrons than protons.

Class Work Question Examples

• Predict whether the following compounds are ionic or molecular:
  • KI, the compound used as a source of iodine in table salt
  • H2O2, the bleach and disinfectant hydrogen peroxide
  • CHCl3, the anesthetic chloroform
  • Li₂CO₃, a source of lithium in antidepressants

Development of Atomic Theory and Structure

• Atomic theory developed from many models/theories
• Hypothesis and experiments
• Ideas and theories in science change as new information is gathered.
• Models and theories are based off each other and further developed.
• Understanding of atom changed over time as new studies are done.

Early Theories

• Democritus, a Greek philosopher (460 - 370 B.C.), held that all matter is made of various basic elements.
• Democritus proposed that if one continues to cut matter, there would be a point where the resulting particle could no longer be divided.
• Democritus called this uncuttable particle "atom," derived from "atomos," a Greek word meaning indivisible.
• Democritus concluded about cutting matter in half that:
  • There was a limit to how far you could divide matter.
  • You would eventually end up with a piece of matter that could not be cut.
  • There are various basic elements from which all matter is made.
• Democritus proposed about the atom:
• Atoms are small hard particles.
• Atoms are made of a single material that's formed into different shapes and sizes.
• Atoms are always moving
• Atoms form different materials by joining together.
• Aristotle (384 – 322 B. C.), a Greek Philosopher, disagreed with Democritus.
• Aristotle believed matter could be divided forever.
• His idea became more popular at the time than the idea of Democritus.
• Democritus' ideas were rejected by leading philosophers/scientists for thousands of years.

John Dalton

• John Dalton (1776-1844) was a British chemist and teacher.
• Roughly two millennia later, Dalton revived Democritus's atomic concept.
• In the late 1700s, it was discovered that elements combine in specific mass ratios to create compounds.
• Dalton used experiments to investigate how elements combine to form substances and introduced his idea in 1803.
• John Dalton's Atomic Theory:
  • All substances are made up of atoms which are small particles that cannot be created, divided, or destroyed.
  • All atoms of the same element are exactly alike. Atoms of different elements are different.
  • Atoms of different elements combine in simple whole-number ratios to form chemical compounds. This is the explanation of the law of definite proportion.
  • In chemical reactions, atoms are combined, separated, or rearranged but never changed into atoms of another element. This explains the law of conservation of mass.
  • Chemical equations can describe chemical reactions.
• Dalton's atomic theory was widely accepted because it explained:
  • Conservation of mass
  • Definite proportions
  • Multiple proportions
  • Other observations
• Dalton was incorrect in his assumption that all elements of the same type are identical.
• Atoms of the same element can have different numbers of neutrons; these variations are called isotopes.
• Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912, nearly 30 years after Dalton's original idea.
• Isotopes are atoms of the same element with different masses, owing to varying neutron counts.
• Soddy won the Nobel Prize in Chemistry (1921) for isotope-related and radioactive substance studies.

Subatomic Particles and the Changing Atomic Model

• About fifty years after his proposal, evidence indicated that atoms may not be indivisible spheres
• Evidence of electrically charged particles and radioactive materials were identified
• The discovery of electrically charged particles and radioactive materials led to Dalton's solid, indestructible atom model being abandoned.
• Michael Faraday, a British physicist, discovered evidence that atoms had an electrical component in the 1830's.
• Faraday found that elements of a dissolved compound would accumulate on one of two electrodes in a solution.
• Sir William Crookes studied the effects of sending an electric current through a gas in a sealed tube in 1879.
• The tube had electrodes at either end and particles moved from one electrode.
• This electrode was called the cathode, and the particles were known as cathode rays.
• The particles were first believed to be negatively charged atoms or molecules
• J.J. Thomson (1856-1940) discovered that atoms were made of smaller negatively charged particles called electrons.

Thomson's Plum Pudding Model

• J.J. Thomson deduced the presence of negatively charged particles (electrons) using a cathode ray tube in 1897.
• Because atoms have a neutral charge, there must also be a positive charge.
• Thomson proposed that electrons existed within a soup of positive charges. In his model, electrons were mixed throughout the atom with no knowledge of how they arranged.
• Thomson's model was known as the "Plum Pudding Model" where the atom sphere of positive charge with negatively charged electrons spread throughout, like plums in a pudding or chocolate chips in ice cream.

Rutherford's Gold Foil Experiment

• Ernest Rutherford (1871 - 1937) disagreed with Thomson and devised an experiment to investigate the structure of positive and negative charges in the atom.
• Rutherford received the Nobel Prize in Chemistry for discovering alpha particles, positively charged particles released by radioactive elements.
• Rutherford's gold foil experiment involved shooting particles at gold foil and measuring their deflection.
• Rutherford found that:
• Most particles traveled straight through the gold foil.
• The observed behavior of a few particles deflecting off the gold foil was unexpected.
• Rutherford's Revised Atomic Theory (1911):
  • Most positively charged particles passed straight through the gold foil.
  • The atom's mass is mostly in the nucleus.
  • Some positively charged particles deflected, or even bounced back.
  • The diameter of the nucleus is 100,000 times smaller than the diameter of the entire gold atom.
  • Because like charges repel, the nucleus must have a positive charge.
  • Electrons must surround the nucleus at a distance.
  • Atoms are mostly empty space with a tiny, massive nucleus at the center.

Bohr Model

• Niels Bohr (1885 – 1962) proposed a planetary model for the structure of the atom in 1913.
• Nucleus is surrounded by orbiting electrons at different energy levels (different distances from nucleus).
• Electrons have definite orbits with no paths in between.
• Electrons can "jump" from level to level.
• Atoms interact and are reactive, because electrons can "jump" from level to level.
• Bohr proposed that electrons move in paths at certain distances around the nucleus and the electrons can jump from a path on one level to a path on another level.
• Bohr further suggested that electrons can only revolve in certain orbits, or at certain energy levels (ie, the energy levels are quantized). These levels follow defined "steps".