Chapter 2 Notes - Beckford

Chapter 2 Notes
Dr. Floyd Beckford

CHAPTER 2
ATOMS AND MOLECULES Dalton’s Atomic Theory
The essence of Dalton’s atomic theory may be summed up as follows:
1. All matter is made of atoms, which are minute, indivisible and indestructible particles. 2. All atoms of a given element are identical, both in mass and in properties. Atoms of different elements have different masses and properties.
3. Compounds are formed by the combination of two or more different kinds of atoms. The atoms combine in ratios of simple whole numbers. e.g. one atom of A combines with one atom of B or two atom of A combine with one atom of B.
4. Atoms cannot be created or destroyed or transformed into another atom (except in a nuclear reaction).

An atom is the smallest particle of an element that maintains its chemical identity through all physical and chemical changes. While atoms are indivisible, they contain what are called sub-atomic particles. There are three (3) fundamental particles: protons, neutrons and electrons.

Protons and electrons have equal but opposite charges and neutrons have no charge. Since atoms are electrically neutral atoms have equal numbers of electrons and protons.
The three particles are located in and around the nucleus. Protons and neutrons are located in the nucleus itself and the electrons in diffuse layers surrounding the nucleus. The nucleus is therefore positive.
The number of protons in the nucleus is called the atomic number of the element. It is given the symbol Z.

Two or more atoms can combine together in small, discrete, electrically neutral units called molecules. A molecule is the smallest unit of an element or compound that can have a stable independent existence. A molecule that has two atoms is said to be diatomic and those with two or more atoms are said to be polyatomic.

Chemical Formulas
The chemical formula for a substance shows its chemical composition. The formula of a compound tells us which element it is composed of and how many atoms of each element are present.

For a single element, its chemical formula is a symbol governed by the following rules:
1. Symbols are composed of one or two letters
2. If one letter is used, it is capitalized
3. If two letters are used, the first is capitalized and the second is a lower case letter.
4. A small subscript following the letter indicates the number of that particular element. e.g. P4 – a phosphorus molecule containing 4 P atoms.

A chemical formula gives only the number of atoms of each type in the compound. It does not indicate how the atoms are arranged. The structural formula shows this information. e.g. H2O (chemical formula); H-O-H (structural formula).

Ions and ionic compounds
Some compounds (e.g. KCl) are made up of ions. An ion is a positive or negative electrically charged atom or group of atoms. Ions that are positively charged are called cations and those that are negatively charged are called anions. An ion is formed when neutral atoms lose (cations) or gains (anions) one or more electrons. Ions can be monoatomic e.g. K⁺, Cl^- or polyatomic e.g. SO₄^2-, NH_4⁺

THE MOLE CONCEPT
Atoms and molecules are extremely small and almost impossible to work individually. This has led to the establishment of a measurement scale based on the atomic mass unit (amu). This unit is defined as exactly 1/12 of the mass of an atom of the carbon-12 isotope. The use of the amu involves very large numbers. The unit use to handle large numbers is the mole (mol). A mole is the amount of substance that can contain as many particles as there are atoms in exactly 12 g of the isotope carbon-12.

1 mole = 6.022 x 10²³ particles This number is known as Avogadro’s number.

Understanding the mole concept depends on one mole contains the same number of particles, whether the particles are ions, atoms, molecules or eggs. One mole of copper atoms has the same number of atoms as one mole of hydrogen atoms. Likewise a mole of molecules has the same number of molecules as a mole of atoms has atoms.

1 mol of Cu = 6.022 x 10²³ atoms
1 mol of H₂ = 6.022 x 10²³ molecules

Taking the definitions of the mole and the atomic mass unit together, we find that the mass of one mole of atoms of an element (6.022 x 10²³ atoms) is the molar mass of the element in units of g/mol and is numerically equivalent to the atomic mass in atomic mass units.

Molar masses of the elements are used as conversion factors between units of moles and grams for quantities of elements.

The formula weight (FW) of a substance is the sum of the atomic weights of the elements in the formula, each taken the number of times the element occurs.

Formula weight = molar mass In most cases the two terms can be used interchangeably.

Percent is parts per 100 so that 20% means 20 out if every 100.

EMPIRICAL AND MOLECULAR FORMULAS

The empirical (or simplest), formula of a compound gives the smallest whole-number ratio of the atoms that are present in a compound.
The molecular formula is the true formula, representing the total number of each element present in one molecule or one formula unit of a compound.

Its important to note the difference as two or more substances may have the same empirical formula. e.g. C₆H₆, benzene and C₂H₂, ethylene both have empirical formula CH.

Guidelines for calculating empirical formulas.
1. Divide the weight (grams) of each element by their molar masses to convert gram to moles. This conversion gives the number of moles of atoms of each element.
2. Divide each of the values obtained in step 1 by the smallest of these values.
3. If the numbers from step 2 are not whole numbers, multiply them by the smallest number that will convert them to whole numbers. *

For many compounds the molecular formula can be calculated if the molecular weight, in addition to data for calculating the empirical formula, is known.