4 - Atoms and Elements

Electron-Dot Symbol

An electron-dot symbol, also known as a Lewis structure, represents the valence electrons as dots that are placed on the sides, top, or bottom of the symbol for the element. One to four valence electrons are arranged as single dots. When an atom has five to eight valence electrons, one or more electrons are paired.

Isotopes

Isotopes are atoms of the same element that have the same atomic number but different numbers of neutrons.

Atomic Mass

The weighted average mass of all the naturally occurring isotopes of an element.

Chemical Symbols

are one- or two-letter abbreviations for the names of the elements. Only the first letter of an element's symbol is capitalized. If the symbol has a second letter, it is lowercase so that we know when a different element is indicated. If two letters are capitalized, they represent the symbols of two different elements.

Electrons

A negatively charged subatomic particle having a very small mass that is usually ignored in calculations; its symbol is e−.

Neutrons

A neutral subatomic particle having a mass of about 1 amu and found in the nucleus of an atom; its symbol is n or n0.

Protons

A positively charged subatomic particle having a mass of about 1 amu and found in the nucleus of an atom; its symbol is p or p+.

Atomic Mass Unit (amu)

A small mass unit used to describe the mass of very small particles such as atoms and subatomic particles; 1 amu is equal to one-twelfth the mass of a atom.

Alkali Metals

Elements of Group 1A (1) except hydrogen; these are soft, shiny metals with one valence electron. Group 1A (1) elements—lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr)—are a family of elements known as the alkali metals.

Metalloids

Except for aluminum, the elements located along the heavy line are metalloids: B, Si, Ge, As, Sb, Te, Po, and At. Metalloids are elements that exhibit some properties that are typical of the metals and other properties that are characteristic of the nonmetals. For example, they are better conductors of heat and electricity than the nonmetals, but not as good as the metals. The metalloids are semiconductors because they can be modified to function as conductors or insulators. Table 4.3 compares some characteristics of silver, a metal, with those of antimony, a metalloid, and sulfur, a nonmetal.

Periods

Each horizontal row in the periodic table is a period (see Figure 4.2). The periods are counted from the top of the table as Periods 1 to 7.

Group

Each vertical column on the periodic table contains a group (or family) of elements that have similar properties.

Representative Elements

Elements found in Groups 1A (1) through 8A (18) excluding B groups (3-12) of the periodic table.

Transition Elements

Elements located between Groups 2A (2) and 3A (13) on the periodic table.

Dalton's Atomic Theory

1. All matter is made up of tiny particles called atoms. 2. All atoms of a given element are similar to one another and different from atoms of other elements. 3. Atoms of two or more different elements combine to form compounds. A particular compound is always made up of the same kinds of atoms and always has the same number of each kind of atom. 4. A chemical reaction involves the rearrangement, separation, or combination of atoms. Atoms are never created or destroyed during a chemical reaction.

Periodic Table

An arrangement of elements by increasing atomic number such that elements having similar chemical behavior are grouped in vertical columns.

Atom

An atom is the smallest particle of an element that retains the characteristics of that element.

Metallic Character

An element that has metallic character is an element that loses valence electrons easily. Metallic character is more prevalent in the elements (metals) on the left side of the periodic table and decreases going from left to right across a period. The elements (nonmetals) on the right side of the periodic table do not easily lose electrons, which means they are the least metallic. Most of the metalloids between the metals and nonmetals tend to lose electrons, but not as easily as the metals.

Energy Level

In an atom, each electron has a specific energy known as its energy level, which is assigned values called principal quantum numbers (n), (n = 1, n = 2, ...). Generally, electrons in the lower energy levels are closer to the nucleus, while electrons in the higher energy levels are farther away. The energy of an electron is quantized, which means that the energy of an electron can only have specific energy values, but cannot have values between them.

Ionization Energy

In an atom, negatively charged electrons are attracted to the positive charge of the protons in the nucleus. Thus, a quantity of energy known as the ionization energy is required to remove one of the outermost electrons. When an electron is removed from a neutral atom, a positive particle called a cation with a 1+ charge is formed. Na(g) + energy (ionization) → Na+(g) + e−

Metals

In general, most metals are shiny solids, such as copper (Cu), gold (Au), and silver (Ag). Metals can be shaped into wires (ductile) or hammered into a flat sheet (malleable). Metals are good conductors of heat and electricity. They usually melt at higher temperatures than nonmetals. All the metals are solids at room temperature, except for mercury (Hg), which is a liquid.

Nonmetals

Nonmetals are not especially shiny, ductile, or malleable, and they are often poor conductors of heat and electricity. They typically have low melting points and low densities. Some examples of nonmetals are hydrogen (H), carbon (C), nitrogen (N), oxygen (O), chlorine (Cl), and sulfur (S).

Alkaline Earth Metals

The alkaline earth metals are found in Group 2A (2). They include the elements beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). The alkaline earth metals are shiny metals like those in Group 1A (1), but they are not as reactive.

Atomic Number

The atomic number of an element is equal to the number of protons in every atom of that element. The atomic number is the whole number that appears above the symbol of each element on the periodic table. Atomic number = number of protons in an atom

Valence Electrons

The chemical properties of representative elements in Groups 1A(1) to 8A(18) are mostly due to the valence electrons, which are the electrons in the outermost energy level. The group number gives the number of valence electrons for each group of representative elements. For example, all the elements in Group 1A(1) have one valence electron. All the elements in Group 2A(2) have two valence electrons. The halogens in Group 7A (17) all have seven valence electrons.

Nucleus

The compact, very dense center of an atom, containing the protons and neutrons of the atom.

Halogens

The halogens are found on the right side of the periodic table in Group 7A (17). They include the elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). The halogens, especially fluorine and chlorine, are highly reactive and form compounds with most of the elements.

Noble Gases

The noble gases are found in Group 8A (18). They include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). They are quite unreactive and are seldom found in combination with other elements.

Mass Number

Thus, for a single atom, we assign a mass number, which is the total number of protons and neutrons in its nucleus. However, the mass number does not appear on the periodic table because it applies to single atoms only. Mass number = number of protons + number of neutrons

Atomic Symbol

To distinguish between the different isotopes of an element, we write an atomic symbol for a particular isotope that indicates the mass number in the upper left corner and the atomic number in the lower left corner.

Elements

are pure substances from which all other things are built. Elements cannot be broken down into simpler substances.

Group Number

is written at the top of each vertical column (group) in the periodic table.