# Chapter 2: Atoms, Molecules, and Ions

### The Atom

• Atoms are mostly empty space.

#### Subatomic Particles

• Protons have a positive charge.

• Neutrons have a neutral charge.

• Electrons have a negative charge.

• Protons and neutrons are found in the nucleus and essentially have the same mass.

• Electrons travel around the nucleus.

#### Atomic Number

• Atomic number - the number of protons in the nucleus of an atom; it also provides the number of electrons an atom has.

• The number of protons equals the number of electrons in an atom.

#### Atoms of an Element

• Elements are represented by a one or two letter symbol.

• All atoms of the same element have the same number of protons, which is the atomic number. It is the subscript.

• The mass number is the total number of protons and neutrons in the nucleus of an atom. It is the superscript.

### Isotopes

• Isotopes are atoms of the same element with different masses.

• Isotopes have different numbers of neutrons, but the same number of protons.

• Example: Isotopes of Carbon

Symbol

Number of Protons

Number of Electrons

Number of Neutrons

11C

6

6

5

12C

6

6

6

13C

6

6

7

14C

6

6

8

### Atomic Mass Unit (amu)

• Atoms have extremely small masses.

• A mass scale on the atomic level is used, where an atomic mass unit (amu) is the base unit.

• 1 amu = 1.66054 × 1024- g

### Atomic Weight

• Atomic weight - an average mass is found using all isotopes of an element weighted by their relative abundances.

• Atomic Weight Formula:

• $\sum$ [(isotope mass)x(fractional natural abundance)] for ALL isotopes.

### The Periodic Table

• The periodic table is a systematic organization of the elements.

• Elements are arranged in order of atomic number.

#### Reading the Periodic Table

• The atomic number is above the symbol.

• The atomic weight is below the symbol.

#### Organization of the Periodic Table

• The rows on the periodic table are called periods.

• Columns are called groups.

• Elements in the same groups have similar chemical properties.

#### Group Names

• 1A are Alkali metals

• 2A are Alkaline earth metals

• 3-12 are Transition metals

• 6A are Chalcogens

• 7A are Halogens

• 8A are Noble gases

#### Metals, Nonmetals, and Metalloids

• Metals are on the left side of the periodic table.

• Properties include shiny luster, conducting heat and electricity, and solids (except Hg).

• Nonmetals are on the right side of the periodic table (they include Hydrogen, H).

• They are mostly gases, but can be solid (Carbon), liquid (Bromine), or gas (Neon) at room temperature.

• Metalloids occur at a “stair step”.

• They include Boron, Silicon, Germanium, Arsenic, Antimony, and Tellurium.

• Their properties are sometimes like metals and sometimes like nonmetals.

### Chemical Formulas

• The subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound.

• Molecular compounds are composed of molecules and almost always contain only nonmetals.

### Diatomic Molecules

• These seven elements occur naturally as molecules containing two atoms:

• Hydrogen (H2)

• Nitrogen (N2)

• Oxygen (O2)

• Fluorine (F2)

• Chlorine (Cl2)

• Bromine (B2)

• Iodine (I2)

• They can never exist alone, they must be paired with something else.

### Types of Formulas

• Empirical formulas give the lowest whole-number ratio of atoms of each element in a compound; simplest formula.

• Molecular formulas give the exact number of atoms of each element in a compound; actual formula.

• If we know the molecular formula of a compound, we can create its empirical formula.

### Ions

• When an atom of a group of atoms loses or gains electrons, it becomes an ion.

• Ion - a charged atom.

• Cations are formed when at least one electron is lost.

• Monoatomic cations are formed by metals.

• Anions are formed when at least one electron is gained.

• Monoatomic anions are formed by nonmetals, except the noble gases.

### Polyatomic Ions

• Sometimes a group of atoms will gain or lose electrons.

• Polyatomic ions - many atoms grouped together.

• Polyatomic cation example: Ammonium is $NH_{4}^{+}$

• Polyatomic anion example: Sulfate is $SO_{4}^{2-}$

### Ionic Compounds

• Ionic compounds (such as NaCl) are generally formed between metals and nonmetals.

• Metal + Nonmetal

• Electrons are transferred from the metal to the nonmetal. The oppositely charged ions attract each other. Only empirical formulas are written.

### Writing Formulas

• When writing formulas, the charge of the cation becomes the subscript for the anion.

• The charge for the anion becomes the subscript for the cation.

• If they are not the lowest whole-number ratio, divide them by the greatest common factor.

### Chemical Nomenclature

• The system of naming compounds is called chemical nomenclature.

• Inorganic nomenclature

• Write the name of the cation. If the cation can have more than one possible charge, write the charge as a Roman numeral in parentheses. If it is a polyatomic cation, it will end in -ium.

• If the anion is an element, change its ending to -ide; if the anion is a polyatomic ion, simply write the name of the polyatomic ion.

• Nomenclature of Binary Molecular Compounds

• The name of the element farther to the left in the periodic table or lower in the same group is usually written first.

• A prefix is used to denote the number of atoms of each element in the compound.

• The prefix mono is not used on the first element listed.

• Prefixes:

Prefix

Meaning

mono-

1

di-

2

tri-

3

tetra-

4

penta-

5

hexa-

6

hepta-

7

octa-

8

nona-

9

deca-

10

• The ending of the second element is changed to -ide.

• CO2: Carbon dioxide

• CCl4: Carbon tetrachloride

### Acid Nomenclature

#### Three Rules

• If the anion in the acid ends in -ide, change the ending to -ic acid and add the prefix hydro-.

• HCl: hydrochloric acid

• HBr: hydrobromic acid

• HI: hydroiodic acid

• If the anion ends in -ite, change the ending to -ous acid.

• HClO: hypochlorous acid

• HClO2: chlorous acid

• If the anion ends in -ate, change the ending to -ic acid.

• HClO3: chloric acid

• HClO4: perchloric acid

• Helpful mnemonic device for naming acids:

• My ride has hydrolics (-ide changes to hydro____ic acid)

• I ate something icky (-ate changes to -ic acid)

• Sprite is delicious (-ite changes to -ous acid)