STEM Chemistry: Chapter 3: The Atom

Avogadro's Number

number of representative particles in a mole, 6.022 X 10^23. Also the number of atoms there are in exactly 12 grams of carbon-12.

Mole

The amount of substance that contains the Avogadro's number of chemical units

Molar mass

mass in grams of one mole of an element. Determined for an element by using average atomic mass. Units of grams per mole. Molar masses of elements contain equal numbers of atoms.

Atomic mass

The mass of an atom expressed in atomic mass units. (amu).

Average atomic mass

Sum of the atomic masses of each isotope multiplied by the decimal of its percent abundance. The weighted average of the atomic mass of every naturally occurring isotope of an element.

Atomic number

Refers to the number of protons found in the nucleus of an element

Mass number

the total number of protons and neutrons in the nucleus of an atom

Isotopes

Atoms of one element that vary only in the number of neutrons in the nucleus, and thus have different atomic masses.

Hyphen notation

Element-mass number

Nuclear symbol

the superscript indicates the mass number and the subscript indicates the atomic number.

Atom

the smallest complete part of an element that can exist alone or in combination with other atoms

Nucleus

the positively charged dense center of an atom

Electron cloud

a region with a negative charge around the nucleus of an atom where electrons are likely to be found

Cathode Ray Tube Experiment

J.J. Thomson carried out an experiment where he measured the ratio of the charge to mass of the cathode ray, and the ratio was always the same. He concluded that all cathode rays are made of identical negatively charged particles named electrons.

Oil Drop Experiment

A drop of oil was placed in a cylinder with negative and positive plates, the drops were charged using an x-ray. Different waves of xrays let the drops fall at different rates. Determined that the mass of the electron is 9.10 x 10^-28 by Robert Milikan.

Conclusions of electron experiments

Because atoms are electrically neutral, they must contain a positive charge to balance the negative electrons. Because electrons have much less mass than atoms, atoms must contain other particles that account for most of their mass.

Plum pudding model

J.J Thomsons model of an atom, in which he thought electrons were randomly distributed within a positively charged cloud.

Gold Foil Experiment

Conducted by Ernest Rutherford in which alpha particles that were shot at gold foil were deflected when they hit the positive center of gold atoms. Conclusions: Alpha particles must have experience a powerful force. Source of the force must take up a very small space. Source must be a densely packed bundle of matter with a positive electric charge, called the nucleus.

Protons

Positive charge equal in magnitude to that of an electron. Relatively heavy particles. Equals the number of electrons in a neutral atom. Found in the nucleus. Identifies an element.

Neutrons

No electrical charge, same weight as a proton, found in the nucleus.

Electrons

Negative charge. Very light: 1/1837th the mass of a proton. Exist in regions called shells or energy levels, which they can go between.

Elements

pure substances that cannot be broken down chemically into simpler kinds of matter

Vertical columns on the periodic table

groups or families (each group contains elements with similar chemical properties)

Horizontal rows on the periodic table

periods, physical and chemical properties change somewhat regularly across a period.

Elements placed below

Lanthanide series and the actinide series: metallic elements that are placed below to keep the table from being too wide.

Metals

Good electrical conductor and good heat conductor. Most are solid at room temp. Malleable. Ductil, can be drawn into a wire. High tensile strength: won't always break when pulled.

Nonmetals

Poor conductor of heat and electricity. Many are gases at room temperature. Brittle.

Metalloid

Between metals and nonmetals. All are solid at room temperature. Less malleable than metals but not as brittle as nonmetals. Semiconductors of electricity.

Noble gases

Unreactive, very unique, gases at room temperature, used in lighting.

Early Ideas of the Atom

The particle theory of matter, supported by Democritus. Aristotle thought all matter was continuous. Neither view is supported by experimental evidence.

Foundations of Atomic Theory

Late 1700s: Virtually all chemists accept the modern definition of an element. Elements can combine to form compounds that have different physical/chemical properties than the elements that form them. 1790s: New emphasis on analytical analysis of chemical reactions, improved balances, chemists began to accurately measure the masses of elements and compounds.

Law of conservation of matter

Matter is not created nor destroyed in a chemical reaction.

law of definite proportions

A compound contains the same elements in the same proportions by mass

Law of Multiple Proportions

Two elements can come together in more than one way, but those mass rations will equal a small whole number.

Dalton's Atomic Theory

1) elements are composed of atoms. 2) atoms of same element are identical, but differ from other elements. 3) atoms cannot be subdivided, created, or destroyed. 4) atoms of different elements combine in simple whole number ratios to form chemical compounds. 5) in chemical reactions, atoms are combined, separated, or rearranged, but not destroyed.

Incorrect parts of Dalton's theory

2, a given element CAN have atoms with different masses, and 3, atoms ARE DIVISIBLE into even smaller pieces.

Reactants

A starting material in a chemical reaction

Products

The elements or compounds produced by a chemical reaction.

Significant figures

All the digits that can be known precisely in a measurement, plus a last estimated digit

Matter

Anything that has mass and takes up space, measured by the mole

Modern atomic theory

1. all matter is composed of atoms 2. atoms of any one element differ in properties from atoms of another element remain unchanged

Accuracy

A description of how close a measurement is to the true value of the quantity measured.

Precision

a measure of how close a series of measurements are to one another

Scientific notation

A method of writing or displaying numbers in terms of a decimal number between 1 and 10 multiplied by a power of 10.

Metric prefixes

King Henry Died Unexpectedly Drinking Chocolate Milk

Compound

A substance made up of atoms of two or more different elements joined by chemical bonds

Heterogeneous mixture

A mixture that is not uniform in composition; components are not evenly distributed throughout the mixture

Homogeneous mixture

A mixture in which substances are evenly distributed throughout the mixture and it is uniform throughout

Density

mass/volume

Mass

the amount of matter in an object