Introduction to Chemistry Earth Science Introduction to Chemistry

Nature of Atoms Atoms are made up of protons, neutrons, and electrons Protons have a mass of 1.67x10-24g and an electric charge of +1. Neutrons have a mass of 1.67x10-24g and no electric charge. Electrons have a mass of 9.11 x10-28g and an electric charge of -1. The mass of the electron is negligible to the protons and neutrons.

Atoms and Elements Protons and neutrons reside in the nucleus. Each element has a different atomic number; this is the number of protons in each nucleus. This defines that element…all atoms with a given number of protons are atoms of the same element. In an electrically neutral atom, the number of electrons equals the number of protons.

Atoms and Elements, cont. Electrons are found outside the nucleus. Electrons reside in orbitals which we define mathematically; one way we define them is as energy levels. The electrons in the outermost occupied energy level are called valence electrons. These valence electrons determine the physical properties and bonding behavior of a given element; those elements with the same number of valence electrons behave similarly (very important!).

Chemistry Atoms with 1, 2, 3 valence electrons usually lose them and form cations (positive ions). Atoms with 5, 6, 7 valence electrons usually gain electrons from other atoms and form anions (negative ions). Atoms with 4 valence electrons usually share them (covalent bonding). Atoms with 8 valence electrons usually are relatively inactive (“happy”).

Periodic table Useful collection of information about the elements; Arranged by increasing atomic number Broken down by number of electrons in various orbitals Each column/group exhibits similar behavior. Cations have fewer electrons than protons, hence the positive charge; anions have more electrons than protons, hence negative charge.

Periodic Table

Atoms and Ions Cations are positive ions: they can only lose as many electrons as they have in their outermost energy level (their valence electrons). So, an element in the first column (from the left) only has one valence electron to lose, so it will have a +1 charge when it has lost this electron. An element in the second column, two electrons, and so for another column or two.

Atoms and Ions, cont. Elements in the columns with 5, 6, or 7 valence electrons, columns Va, VIa, or VIIa, find it easier to gain 3, 2, or 1 electrons, so these get -3, -2, or -1 charges, as appropriate. Atoms in VIIIa are generally nonreactive, so they rarely gain or lose electrons. Elements in IVa generally share their electrons, although they can also form +4 ions.

Isotopes Isotopes are atoms of a given element which have different numbers of neutrons…this in no way affects their bonding or physical characteristics, but it DOES affect their nuclear stability. These atoms have different mass numbers, which is how we tell them apart.

Atomic and Mass Number Atomic number is the number of protons in a given atom’s nucleus. Mass number is the sum of the number of protons and the number of neutrons in a given atom’s nucleus. To calculate the number of neutrons, you take the mass number and subtract the atomic number: M# (p+n) – A# (p) = number of neutrons

Atomic Weight A given element may have several naturally-occurring stable isotopes. The weighted average of these isotopes is the atomic weight, given on the periodic table…these are not whole numbers, such as 1.00797 for Hydrogen…this means that the atomic weight of each times its relative abundance gives a weighted average for any atom.

Nomenclature Ions: atoms that have gained or lost electrons Denoted with superscripts: H+1, O-2, Sn+4 Isotopes: different mass numbers Denoted with preceding superscripts: 2H, 3H, 14C Molecules and compounds are denoted with subscripts indicating the number of each element’s atoms contained: H2, H2O, CaCO3.