CHEM-1411: Chapter 2 - Atoms, Molecules, and Ions

Law of Conservation of Mass

The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place.

Law of Multiple Proportions

When two elements, A and B, form more than one compound, the masses of B that combine with a given mass of A are in the ratio of small whole numbers

Postulates of Dalton’s Atomic Theory

All matter is composed of indivisible, indestructible atoms, with each element featuring unique mass and properties

Cathode Rays

• Streams of negatively charged particles were found to emanate from cathode tubes, causing fluorescence'

• J.J. Thomson discovered in 1897

The Electron

• Thomson’s measurements allowed for calculation of the charge/mass ratio of the electron.

• 1.76×10⁸ coulombs/grams

Millikan Oil-Drop Experiment

• Once the charge/mass ratio of the electron is known, the determination of either the charge or the mass of an electron yields the other

• Robert Millikan determined the charge on the electron in 1909 was 1.602×10^-19C. Electron mass is now known

Radioactivity

the spontaneous emission of high-energy radiation by an atom

Three types of radiation:

• Alpha: α particles - positively charged

• Beta: β particles - negatively charged, like electrons

• Gamma: γ rays - uncharged

Discovery of the Nucleus

Ernest Rutherford shot α particles at a thin sheet of gold foil and observed the pattern of scatter of the particles

Subatomic Particles

• Proton: Positive (1+) - 1.0073 (Mass (amu)

• Neutron: None (neutral) - 1.0087 (Mass (amu)

• Electron: Negative (1-) - 5.486×10^-4 (Mass (amu)

Atomic Number

The humber of protons in the nucleus of an atom
Since atoms have no overall charge, the number of protons equals the number electrons in an atom

Atoms of an Element

• Atomic number = number of protons (above the element’s symbol)

• Mass number = protons and neutrons (below the element’s symbol)

Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons (different masses).

Example: ¹³C - 6 protons, 6 electrons, 7 neutrons

Atomic Mass Unit (amu)

1 amu = 1.66054×10^-24g

Atomic Weight

An average mass is found using all isotopes of an element, weighted by their relative abundances

• Atomic Weight=Σ[(isotope mass)x(fractional natural abundance)] for ALL isotopes

• ^-12C - 6 protons and 6 neutrons, has a mass of 12

Reading the Periodic Table

Organization of the Periodic Table

• The rows on the periodic table are called periods

• Columns are called groups

• Elements in the same group have similar chemical properties

Periodicity-Rows

When one looks at the chemical properties of elements in a row, one notices a repeating pattern of properties and reactivity

Group 1A - Alkaline Metals

Li, Na, K, Rb, Cs, Fr

Group 2A - Alkaline Earth Metals

Be, Mg, Ca, Sr, Ba, Ra,

Group 6A - Chalcogens

O, S, Se, Te, Po

Group 7A - Halogens

F, Cl, Br, I, At

Group 8A - Noble gases

He, Ne, Ar, Kr, Xe, Rn

Metals

are on the left side of the periodic table
Some properties of metals include:

• Shiny luster

• Conducting heat and electricity

• Solids (except mercury)

Nonmentals

• Are on the right side of the periodic table (they include H)

• They can be solid (like carbon), liquid (like bromine), or gas (like neon) at room temperature

Metalloids

• Steplike purple line on the periodic table (Except Al, Po, and At)

• Their properties are sometimes like metals and sometimes like nonmetals