Introduction to Chemistry

Chemistry Is the Science that deals with the composition, structure, and properties of matter and the transformations which matter undergoes.

The Atom The atom is the building block of all matter. The nucleus of the atom contains the protons and neutrons (nucleons) and almost all of the atom’s mass. The shells (layers of electrons) make up the atom’s volume.

Protons  subatomic particles located in the nucleus that have a positive charge. Neutrons  subatomic particles located in the nucleus that have no charge. Electrons  subatomic particles that orbit around the nucleus. They have a negative charge and have relatively no mass as compared to nucleons.

Atoms can not be further divided into other elements. Atom : The smallest particle of an element having the same properties of that element. Element: A pure substance that cannot be broken down into simpler substances by ordinary means such as chemical reactions.

An atom’s nucleus accounts for 99.97% of the total mass of the atom. The nucleus occupies only about 1/ of the diameter. The atom is mainly empty space.

Hans Geiger and Ernest Marsden irradiated a thin foil of gold with alpha- particles.

Molecule : The smallest particle of a compound that has the same properties of that compound. Compound: A pure substance consisting of two or more elements. A compound can have very different characteristics than the elements that make it up. Ex: H 2 + O 2 and H 2 O.

The Periodic Table Many elements have similar regularities through physical and chemical properties. These observations led to the modern periodic table. The elements of the periodic table can be grouped into several different categories.

Metals Good Conductors of heat Good conductors of electricity Solids at room temperature (exception- Mercury- Hg) Shiny lustre Malleable (sheet) and ductile (wire) Form positive ions (cations)

Non-metals Poor conductors of heat and electricity Can be solid, liquid, or gas at room temperature Not very shiny Brittle, not ductile Form negative ions (anions)

Metalloids Solids at room temperature May conduct electricity but not heat Can be shiny or dull Brittle not ductile May or may not form ion

Layout of an Element on the Table Elements are presented in the periodic table like this:

From this information, we can determine several different things about an atom of scandium. 21  This is the atomic number of scandium. This number indicates the number of protons in the nucleus of a scandium atom. All atoms of scandium have the same atomic number.

44.96  This is the atomic mass number of scandium. This number, once rounded, indicates the number of protons and neutrons combined in the nucleus of a scandium atom. Atoms of the same element may have different numbers of neutrons giving them a different atomic mass number. Atoms that have the same atomic number but different atomic mass numbers are called isotopes. Hydrogen: HDeuterium: HTritium: H

Calculating Protons, Neutrons, and Electrons In any atom, we will assume protons and electrons cancel each other out unless a charge is indicated on the element (this will come later). Atomic # = number of protons Mass # = number of protons + neutrons

Another way to write this is: Neutrons = mass # - atomic # Protons = atomic # Electrons = atomic #

Example problems: How many protons are in an atom of Chlorine? How many electrons are in an atom of Potassium? How many neutrons are in an atom of Boron?

Groups and periods yield a distinct pattern. Electrons move around an atom’s nucleus is regions called shells. Each shell can only hold a certain number of electrons.

Patterns on the Periodic Table Elements are arranged into periods and groups according to their atomic numbers. Periods  The horizontal rows Groups  The vertical columns

For example, Carbon is in what group and period?

Ultimately: Look at figure 5.4 on page 141. Period number tells you the number of electron shells. Group number tells you the number of electrons in the outer most shell.

Alkali metals Group One on the periodic table Highly reactive, seldom found as elements in nature. Hydrogen is top of group one, but it is not a true alkali metal. All alkali’s have one electron in their outer shell.

Alkaline Earth Metals Group two React strongly with halogens (group 17), but not as strongly as alkali metals. Have two electrons in their outer energy shell.

Halogens Group 17 Halogens are diatomic when in natural form (F 2, Cl 2,…) Very reactive with metals Missing one electron from outer shell

Noble Gases Group 18 Their outer shell has the maximum number of electrons possible. Therefore, they are not reactive and very stable.