Basic concepts for general chemistry Lec.2

Important definitions Element  Any substance that contains only one kind of an atom.  Each element is represented by a unique symbol.  Each element differs from any other in weigh, size, and chemical properties.  The elements can be divided into three categories :metals, nonmetals, and semimetals. The Atom  Elements are made up of atoms, the smallest particle that has any of the properties of the element.  An atom is the defining structure of an element, which cannot be broken by any chemical means

Structure of the Atom A typical atom consists of a nucleus of protons and neutrons with electrons circling this nucleus. Modern Theory Of The Atom by John Dalton, Based On The Following Assumptions. 1. All atoms of an element are identical. 2. Atoms of different elements have different weights and different chemical properties. 3. Atoms of different elements combine in simple whole numbers to form compounds 4. Atoms cannot be created or destroyed. When a compound decomposes, the atoms are recovered unchanged.

Isotopes  Atoms with the same number of protons, but differing numbers of neutrons.  Isotopes are different forms of a single element, e.g. Carbon 12 and Carbon 14 are both isotopes of carbon, one with 6 neutrons and one with 8 neutrons (both with 6 protons). The number of protons, neutrons, and electrons in an atom can be determined from a set of simple rules: 1. The number of protons in the nucleus of the atom is equal to the atomic number (Z). 2. The number of electrons in a neutral atom is equal to the number of protons. 3. The mass number of the atom (M) is equal to the sum of the number of protons and neutrons in the nucleus. 4. The number of neutrons is equal to the difference between the mass number of the atom (M) and the atomic number (Z).

Examples: Determine the number of protons, neutrons, and electrons in the following isotopes. 12 C 13 C 14 C The different isotopes of an element are identified by writing the mass number of the atom in the upper left corner of the symbol for the element. 12 C 13 C 14 C are isotopes of carbon (Z = 6) and therefore contain six protons. If the atoms are neutral, they also must contain six electrons. The only difference between these isotopes is the number of neutrons in the nucleus. 12C: 6 electrons, 6 protons, and 6 neutrons 13C: 6 electrons, 6 protons, and 7 neutrons 14C: 6 electrons, 6 protons, and 8 neutrons

Isotopes of hydrogen Atomic Weight  Atomic mass or atomic weight is the average mass of atoms of an element.  Examples: C=12, O=16, H=1 Molecular Weight  The molecular weight of a compound is the sum of the atomic weights of the atoms in the molecules that form these compounds.  Example: H 2 O=1*2+16=18 CO 2 =12+16*2=44

The Mole  The term mole means a small mass.  a mole of any substance contains the same number of elementary particles.  A mole of any atoms has a mass in grams equal to the atomic weight of the element.  Number of moles =mass /mass per mole(M.wt).  1 Mole is equivalent to x (Avogadro's number) units of any substance.

Molar mass  A sample of any element with a mass equal to that element's atomic weight (in grams) will contain precisely one mole of atoms (6.023x atoms).  For example, helium has an atomic weight of Therefore, 4.00 grams of helium will contain one mole of helium atoms (6.023 x helium atoms)

 When atoms form molecules, the atoms bond together, and the molecule's weight is the combined weight of all of its parts.  For example, every water molecule (H 2 O) has two atoms of hydrogen and one atom of oxygen. One mole of water molecules will contain two moles of hydrogen and one mole of oxygen