Atoms and Periodic Table

Antoine Lavoisier

Law of Conservation of Matter: matter can not be destroyed nor created.

Dalton’s Atomic Theory

1. All elements are composed of atoms.2. Atoms of the same element are identical.

Atoms of any one element are different than atoms of another element.

3. Atoms of different elements can physically mix together or can chemically combine in whole number ratios to form compounds.

4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element.

Law of multiple proportions: different compounds made of the same elements, have mass ratios related by small whole numbers.

Ernest Rutherford

Rutherford’s gold-foil experiment: the atom is mainly empty space with all the positive charge and almost all the mass centrally located in the nucleus.

Atoms

Particle Charge Mass Location

Proton (p+) + 1 1 amu nucleus

Neutron (n0) 0 1 amu nucleus

Electron (e-) - 1 0 amu electron cloud

Atoms are made of protons, neutrons, and electrons.

Atomic number

The atomic number (often given as Z) of an element is the number of protons in the nucleus of an atom of that element.

Since atoms are electrically neutral, the number of electrons (-) must equal the number of protons (+).

Atomic symbol

Periodic Table

Isotopes

Isotopes: same # protons, different # neutrons

Symbol:

Atomic mass

Atomic mass is a weighted average mass of the atoms in a naturally occurring sample of the element.

Calculate: multiply the atomic mass of an isotope by its percentage/100.

Do this for every isotope.Add the atomic masses found.

Isotopes

Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the atomic mass of gallium?

Ga-69

68.9 amu x 60.2 = 41.5 amu for 69Ga

100

Ga-71

70.9 amu x 39.8 = 28.2 amu for 71Ga 100

Atomic mass Ga = 69.7 amu

Abundance

Percent abundance: % of totalRelative abundance: Most abundant isotope

set at 100 %, other relative to most abundant one.

Mass spectroscopy

Mass spectroscopy is an analytical technique that helps identify the type and amounts of molecules and atoms (including isotopes) present in a sample.

It measures the mass-to-charge ratio (m/z) of ionized samples.

Mass spectroscopy provides an accurate way of measuring the mass of atoms and

molecules.

Mass Spectroscopy

Periodic Table

• Horizontal rows: periods• Vertical columns: groups or families• Groups 1 and 2 (1A and 2A) and groups 13-18

(3A – 8A) are called representative elements• Groups 3-12 are the transition metals• Lanthanides and Actinides

Periodic Table

Three main classes of elements: 1. metals 2. nonmetals 3. metalloids

Metals

Most elements are metals.Properties:1. Good conductors of heat and electricity2. Solid at room temperature (except

Mercury)3. Reflect light (shiny)4. Lose electrons in reactions

Non-metals

Located in the upper right corner of PTGreater variation among these than metals.Most are gases at room temperature.A few are solids (C, S, P) and one is a liquid

(Br)Tend to have properties opposite of metals.Gain electrons in reactions.

Metalloids

Generally have properties similar to metals and nonmetals.

An element in this group may behave like a metal under certain conditions, and then behave like a nonmetal under different conditions.

For example, the metalloid Silicon is a poor conductor of electricity, but it becomes a good conductor when it is mixed with another metalloid, Boron.

Main groups

Group IA alkali metalsGroup IIA alkaline earth metalsGroup VIIIA noble gasesGroup VIIA halogens – “salt formers”Group VIA chalcogensGroup VA Nitrogen groupGroup IVA IVA groupGroup IIIA IIIA group