CHAPTER 2 Atoms, Molecules & Ions

Early History of Chemistry Greeks were the first to try to explain why chemical changes occur (around 400 B.C.) Before 16th Century Alchemy – Attempts (scientific or otherwise) to change cheap metals into gold. Some alchemists were mystics and fakes; others were serious scientists. Alchemists discovered several elements.

17th Century – Robert Boyle First chemist to perform truly quantitative experiments – Robert Boyle (1627-1691). Measured the relationship between pressure and volume of air Boyles Law : P1V1 = P2V2 Other major contributions consisted of his ideas about the chemical elements (believed a substance was an element unless it could be broken down into simpler substances)

PHLOGISTON THEORY 17TH & 18TH Century Hypothesis regarding combustion that was popularized by G. E. Stahl. Postulated that in all flammable materials there is present phlogiston, a substance without color, odor, taste, or weight that is given off in burning. “Phlogisticated” substances are those that contain phlogiston and, on being burned, are “dephlogisticated.” The ash of the burned material was believed to be the true material.

The theory received strong and wide support throughout a large part of the 18th century until it was refuted by the work of A. L. Lavoisier, who revealed the true nature of combustion. Joseph Priestley defended the theory throughout his lifetime. Priestly discovered oxygen gas (O2) in 1774 when he isolated oxygen by heating mercuric oxide (HgO). Oxygen was called “dephlogisticated air.” He also discovered the solubility of carbon dioxide (CO2) in water to produce seltzer.

Antoine Lavoisier (1743-1794) Lavoisier was a French chemist who finally explained combustion. Boyle & Lavoisier regarded measurement as the essential operation of chemistry. Performed experiments where he massed the reactants and products of various chemical reactions Proposed Law of Conservation of Mass

Law of Conservation of Mass Discovered by Antoine Lavoisier. Mass is neither created nor destroyed. His experiments showed that combustion involved oxygen not phlogiston. Lavoisier published the first chemistry textbook. Executed by guillotine during French Revolution for collecting taxes for the government.

Law of Definite Proportions Proposed by Joseph Proust (1754-1826) Law states: A given compound always contains exactly the same proportion of elements by mass. Example:Carbon tetrachloride (CCl4) is always 1 carbon atom per 4 chlorine atoms

John Dalton (1766-1784) best known for his atomic theory prepared a table of relative atomic weights proposed Dalton’s Law of Partial Pressures of Gases: a gas will exert the same pressure in a system as it would by itself and the pressure exerted by a combination of gases is the sum of the pressures exerted by each gas. Ptotal = Pgas a + Pgas b + Pgas c ...

Law of Multiple Proportions Dalton performed careful weighing experiments and proposed the Law of Multiple Proportions. Law of Multiple Proportions states: When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers.

Examples of Law of Multiple Proportions The ratio of the masses of oxygen in H2O and H202 (hydrogen peroxide) will be a small whole number (“2”). H202 mass of O = 32 = 2 H2O mass of O = 16 1

Dalton’s Atomic Theory (1808) 1. Each element is made up of tiny particles called atoms. 2. The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways. 3. Atoms were indivisible and indestructible.

Dalton’s Atomic Theory (continued) 4. Chemical compounds are formed when different elements combine with each other. A given compound always has the same relative numbers and types of atoms. 5. Chemical reactions involve reorganization of the atoms--changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

Problems with Dalton’s Atomic Theory All atoms of the same element are not “exactly” identical; Dalton did not know about isotopes. Atoms are destructible in nuclear reactions. They are also made up of subatomic particles (protons, neutrons, and electrons).

Joseph Gay-Lusaac Known primarily for his studies on the combining of volumes of gases Gay-Lusaac’s Law: For a given sample of a gas, pressure with vary directly with temperature. Gay-Lusaac’s motivation arose from his passion for ballooning. P = k P1 = P2 T T1 T2

Gay-Lusaac (continued) Performed experiments in which he measured (under the same conditions of temperature and pressure) the volumes of gases that react with each other.

Amadeo Avogadro (1776-1856) Expanded on Dalton’s and Gay-Lusaac’s work to propose that at the same temperature and pressure, equal volumes of different gases contain the same number of particles. Known as Avogadro’s hypothesis. Example: 5 liters of O2 (gas) 5 liters of N2 (gas) Same number of particles!

A representation of combining gases at the molecular level A representation of combining gases at the molecular level. The spheres represent atoms in molecules.

Early Experiments to Characterize the Atom What is an atom made of, and how do atoms of the various elements differ?

J.J. Thomson and the Electron Postulated the existence of electrons using cathode ray tubes. The fast-moving electrons excite the gas in the tube, causing a glow between the electrodes. The green color is due to the response of the ZnS coating of the screen to the electron beam.

J.J. Thomson (continued) Thomson found that when high voltage was applied to the tube a “ray” was produced. He called it a cathode ray. The ray was produced at the negative electrode and was repelled by the negative pole of an applied electric field. He postulated the ray was a stream of negatively charged particles, now called electrons. These experiments were conducted from 1898 to 1903.

Deflection of cathode rays by an applied electric field.

Thomson’s Plum Pudding Model (or Chocolate Chip Cookie) Thomson concluded that since electrons could be produced from electrodes made of various types of metals, all atoms must contain electrons. Atoms were known to electrically neutral. Thomson assumed atoms also must contain positive charge.

Thomson’s Plum Pudding Model Postulated that an atom consisted of a diffuse cloud of positive charge with the negative electrons embedded randomly in it.

Robert Millikan (1868-1953) Performed experiments involving charged oil drops. Determined the magnitude of the charge on an electron. Mass of electron = 9.11 X 10-31 kg

Millikan’s Oil Drop Experiment

Henri Becquerel Becquerel accidentally found that piece of a mineral containing uranium could produce an image on a photographic plate in the absence of light. Postulated this to a spontaneous emission of uranium he called radioactivity.

Three types of radioactive emission Alpha Particle (α) contains 2 p+ and 2 no and has a 2+ charge (the same configuration as a He2+ ion). Mass of α particle is 7300 times that of an electron. Beta Particle (β ) is a high speed electron. Gamma Ray (λ) is high-energy “light”

Ernest Rutherford’s Gold Foil Experiment In 1911 Rutherford carried out experiments to test Thomson’s Plum Pudding Model. Shot α-particles at a thin sheet of gold foil.

Rutherford expected the α-particles to travel through the foil with minor deflections. Although most of the α-particles passed straight through, many were defected at large angles. Rutherford knew the plum pudding could not be correct.

Rutherford’s Conclusion The deflected alpha particles had a “close encounter” with the massive positive center of the atom. (Remember alpha particles have a 2+ charge.) An atom has a dense center of positive charge (the nucleus). The atom is mostly empty space.

Modern Atomic Structure Three subatomic particles Protons (p+) located in the nucleus. The charge on a proton is equal in magnitude to the electron’s negative charge. The mass of a proton is 1 atomic mass unit (amu). Neutrons (no) also located in the nucleus. The neutron has the same mass as a proton. Protons and neutrons are referred to as “nucleons.” Electrons (e-) orbit the nucleus.

If all atoms are composed of these same components, who do different chemicals have different chemical properties?

Isotopes Atoms of the same elements with different numbers of neutrons, thus different mass. All isotopes of the same element will always have the same number of protons. Mass number (A#) → 23 Na Atomic number (Z#) → 11 This isotope of sodium is known as sodium-23.

Molecules and Ions Molecules and ions are created when atoms form a bond. A chemical bond involves a either: Sharing electrons Transfer of electrons Covalent bonds involve a sharing of electrons Ionic bonds involve a transfer of electrons

The chemical formula of a compound is written using the element symbols followed by the subscript for the number of atoms of each element. Ex: NH3  1 Nitrogen bonded to 3 Hydrogen The structural formula shows the shape of the molecule using the elemental symbols connected by lines. Ex: H – O – H or O H H

Ions Ions- an atom or a group of atoms that has a net charge Cation- an ion with a net positive charge Anion- an ion with a net negative charge Ex: Na  Na1+ + e- (cation) Ex: O + 2 e-  O2- (anion) Ionic bonding is the force created by these oppositely charge ions

Remember Ionic solids, such as NaCl, are also called salts. We tend to see these in neutralization reactions. Ions that contain more than one atom are called polyatomic ions. Please be familiar with all of the polyatomic ions studied last year, as well as the list given to you this year.

The Periodic Table of Elements The PTOE is arranged in order of increasing atomic number. Most of the elements are metals, they are located on the left side of the stair-step. Metals are: good conductors of heat and electricity, malleable, ductile and lustrous. Metals always lose electrons and therefore will form cations. Metals tend to be solids at room temperature.

There are much fewer nonmetals There are much fewer nonmetals. These are located to the right side of the stair-step. Hydrogen is a nonmetal however it is located in the top left hand corner of the periodic table. Nonmetals are: brittle as solids, found in all three states of matter at room temperature, poor conductors of heat and electricity. Nonmetals tend to gain electrons and therefore will form anions.

Groups and Families The columns of the periodic table are referred to as groups or families. This is due to the fact that within each group on the periodic table, all of the elements tend to have the same number of valence electrons. Groups or Families also tend to have similar properties.

Families Alkali Metals- Group 1 or 1A, very active metals that form a 1+ ion. Alkaline Earth Metals- Group 2 or 2A, metals that form 2+ ions. Halogens- Group 17 or 7A, nonmetals that form a 1- charged ion and all diatomic molecules. The only group with all three phases of matter. Nobel Gases- Group 18 or 8A, all nonmetal gases that are stable.

Periods Periods are the mane for the horizontal rows of the PTOE. Periods represent the Principle Energy Levels in which an electron can be found. All elements in the same period will have their valence electron in the same shell or step.

Naming Compounds Please see attached file in the Chapter 2 folder titled Naming Rules Packet.