Atoms, Molecules, and Ions

The Atomic Theory

The Greek Philosopher Democritus expressed the belief that all matter consists of tiny,

indivisible particles. He named these particles atomos, which means indivisible. In 1808, John

Dalton formulated a definition of the atom and thus Dalton’s Atomic theory says this:

1)-Elements are made up of extremely small particles called atoms. All atoms of a given element

are identical having the same mass, size, and chemical properties; however, the atoms of

differing elements are different.

2)-Compounds are composed of atoms of more than one element and in any compound the ratio

of the number of atoms of any two elements present is an integer or simple fraction.

3)-A chemical reaction involves the separation, combination, or rearrangement of atoms, however

atoms are not created or destroyed in any chemical reaction.

Dalton’s first hypothesis says that atoms of one element are different than atoms of other

elements. His second hypothesis says that in order to form compounds, we need specific atoms

as well as specific numbers of those atoms. According to Proust, a French chemist, and his law

of definite proportions, different samples of the same compound always contain its constituent

elements in the same proportion by mass. So in analyzing samples of say carbon dioxide gas,

the ratio of the mass of carbon to oxygen is always the same or the ratio of the number of atoms

of carbon to oxygen is the same in each carbon dioxide molecule. Dalton’s second hypothesis

also supports the law of multiple proportions, which states that if two elements can combine to

form more than one compound, the masses of one element that combine with a fixed mass of the

other element are in ratios of small numbers. This means that different compounds made up of

the same element differ in the number of atoms of each kind that combine. So carbon dioxide

and carbon monoxide are both made up of carbon and oxygen, but carbon dioxide contains two

oxygen atoms while carbon monoxide contains only one oxygen atom. Dalton’s third hypothesis

supports the law of conservation of mass, which states that matter can be neither created nor

destroyed.

The Structure of the Atom

According to Dalton’s atomic theory, an atom is the basic unit of an element that can

enter into a chemical reaction. Dalton believed that the atom was extremely small and indivisible,

however it was later found that the atom actually posses internal structure and is made up of

even smaller particles called subatomic particles, mainly electrons protons and neutrons.

Figure 4. -Structure of an atom

Radiation is the study of the emission and transmission of energy through space in the

form of waves. The cathode ray tube was used to study this phenomenon. As seen in figure 1, It

is a glass tube from which most of the air has been evacuated. Two metal plates are connected

to a high voltage source and the negatively charged plate called the cathode emits an invisible

ray. This cathode ray is drawn to the positively charged plate called the anode, where it passes

through a hole where it strikes a special surface and produces a bright light. When a magnet and

two electrically charged plates are added to the outside of the cathode ray tube, the direction of

the ray can be controlled. According to electromagnetic theory, a moving charged body behaves

like a magnet and can interact with electric and magnetic fields. So the ray, as a moving charged

body, is affected by the electric filed of the two oppositely charged plates and the magnetic filed

of the magnet. The cathode ray is attracted to the positive plate and repelled by the negative

plate, so it must consist of negatively charged particles called electrons. JJ Thomson, an English

physicist, used a cathode ray tube and his knowledge of electromagnetic theory to come up with

the ratio of electric charge to its mass as –1.76*102C/g. C stands for Coulomb, the unit of electric

charge. R.A. Millikan succeeded in measuring the charge of the electron by examining the

motion of single tiny drops of oil that picked up static charge from ions in the air. He came up

with a charge of –1.66022*10-19. He calculated the mass of the electron as 9.10*10-28 g.

Figure 1.

Becquerel, a French physicist discovered that when he exposed thickly wrapped

photographic plates to a certain uranium compound, it caused them to darken. In fact he had

found that the radioactivity, or spontaneous emission of particles or radiation cause the plated to

darken. There are three types of rays that are produced by the decay of radioactive substances

as uranium. Alpha rays are positively charges particles called alpha() particles. Beta rays are

electrons called beta() particles. Gamma rays are high-energy rays otherwise known as

gamma() rays.

In the 1900s Thomson proposed that an atom is essentially a charged positive sphere

with electrons embedded in it. In 1910, Rutherford carried out experiments in which he targeted

alpha particles at thin foils of gold as seen in Figure 2. He noticed that most of the particles

penetrated the foil with no or small deflection. However, he also noticed that occasionally an

alpha particle bounced back. Based on these findings, Rutherford developed a new model of the

atom in which most of the atom is empty space. The atom’s positive charges, called protons are

concentrated in the nucleus, the dense central core of the atom. The charge of the proton is

equal to the charge of the electron except it is positive. The mass of the proton is 1.67262*10-24

g., 1840 times bigger than the mass of the electron. Also, while typical atomic radius is 1 pm, the

atomic nucleus is only 5*10-3 pm.

Figure 2.

Rutherford did not however account for the neutrons. It was Chadwick, an English

physicist who discovered the existence of the neutron when he bombarded a thin sheet of

beryllium with alpha particles and a high-energy radiation similar to gamma rays was emitted by

the metal. This radiation turned out to be neutrons, electrically neutral particles having a mass

only slightly greater than the proton.

Example 1:Name the types of radiation known to be emitted by radioactive elements.

Answer:-Alpha particles-Beta particles-Gamma Rays

Atomic Number, Mass Number, and Isotopes

Atoms can be identified by the number of protons and neutrons they contain. The atomic

number, Z, is the number of protons in the nucleus of each atom of an element. Atoms with no

charge have the same number of electrons as protons. From the atomic number of an element,

one can identify that element. So an atom with an atomic number of seven has seven protons as

well as seven electrons and is identified as the element nitrogen.

The mass number, A, is the total number of neutrons and protons present in the nucleus

of an atom of an element. The mass number is just equal to the number of protons plus the

number of neutrons. So the number of neutrons in an atom is equal to the difference between the

mass of the atom, A and the atomic number, Z. So if fluorine has a mass of 19 and an atomic

number of 9, the number of neutrons in fluorine is 19-9-10.

Atoms of an element often have isotopes-atoms that have the same atomic number but

different mass numbers. For example, there are three isotopes of hydrogen. The hydrogen

isotope contains one proton and no neutrons. The deuterium isotope contains one proton and

one neutron. The tritium isotope has one proton and two neutrons. The mass number, A, of an

element, X, is written in the upper left hand corner of the element as such: AX. The atomic

number of an element, Z, is written in the lower left hand corner of the element, X, as such: ZX.

Since the chemical properties of an element are determined mostly by the protons and electrons

in its atoms, isotopes of the same element have similar chemistries.

Example 2:Calculate the number of neutrons of 239Pu.

Answer:145

The Periodic Table

The periodic table is a chart in which elements having similar chemical and physical

properties are grouped together. It was made from the need to organize large amounts of

information about the structure and properties of elemental substances. Thus, elements are

grouped according to similar physical and chemical behavior. Figure 3 shows the periodic table

of the elements. The elements are arranged by atomic number in horizontal rows called periods

and in vertical columns known as groups or families, according to their similarities in chemical

properties.

Figure 3.

The elements are divided into metals, nonmetals, and metalloids. Metals are good

conductors of heat and electricity. Nonmetals are poor conductors of heat and electricity.

Metalloids have properties that are intermediate between those of metals and nonmetals. The

elements change from metallic to nonmetallic as you go across the table left to right. Elements

can be referred to collectively by their group number such as 2A or special names that the groups

have been give. Group 1A elements are called alkali metals, group 2A elements are called

alkaline earth metals, group 7A elements are known as halogens, and group 8A elements are

called noble gasses. We can make easy use of the periodic table to predict chemical behavior

between elements.

Example 3:

Define, with two examples, the following terms:

1) Alkali metals2) Alkaline earth metals3) Halogens4) Noble gasses

Answer:

1) Li, Na2) Be, Mg3) F, Cl4) Ne, He

Molecules and Ions

A molecule is an aggregate of at least two atoms in a definite arrangement held together

by chemical bonds. The molecule may contain atoms of the same element or atoms of different

elements. Molecules are not always compounds, which are made up of two or more elements.

Water is a molecular compound because it is made up of the hydrogen element and the oxygen

element. Hydrogen gas is a pure element but also diatomic molecule because it is made up of

two atoms. Molecules that contain more than two atoms are called polyatomic molecules.

Ammonia, NH3 is a polyatomic molecule as well as is ozone, O3.

An ion is an atom or a group of atoms hat has a net positive or negative charge. The

number of protons in the nucleus of an atom always remains the same during ordinary chemical

reactions, however electrons may be gained or lost by certain elements. A cation is an ion with a

positive charge resulting from the loss of electrons. Sodium, Na, can lose an electron to become

sodium cation, Na+. An anion is an ion with a negative charge resulting from the gain of

electrons. Chlorine, Cl, can gain an electron to become a chlorine anion, Cl-. Together sodium

and chlorine can combine to form sodium chloride, table salt. This type of reaction is called an

ionic compound because it is formed from two ions. Monatomic ions are ions that form from only

one element such as Na+. Polyatomic ions are ions that form from more than one element such

as OH-.

Chemical Formulas

Chemists make use of chemical formulas to express the composition of molecules ad

ionic compounds in terms of chemical symbols. A molecular formula shows the exact number of

atoms of each element in the smallest unit of a substance. So H2 is the molecular formula for

hydrogen and H2O is the molecular formula for water. An allotrope is one of two or more distinct

forms of an element. O2 and O3 are allotropes of oxygen since they are two different forms of the

same element, oxygen.

Since molecules are too small for us to observe directly, we can use molecular models to

observe them physically. In ball-and-stick models the atoms are represented by plastic or

wooden balls with holes in them. Sticks or springs are used to represent chemical bonds. The

angles of these bonds approximate the bond angles in actual molecules. Except for the hydrogen

atom, the balls are the same size and each atom is represented with a different color. In space-

filling models, atoms are represented by truncated balls held together by snap fasteners. The

bonds are not visible. Figure 4 shows a ball-and-stick model of a molecule on top and a space-

filling model of the same molecule on the bottom. The structural formula shows how atoms are

bonded to one another in a molecule. In a water molecule, two hydrogen atoms are bonded to

oxygen so its structural formula would be written as H-O-H. The lines connecting the two atomic

symbols represent chemical bonds.

Figure 4.

The empirical formula tells which elements are present and the simplest whole number

ration of their atoms. The molecular formula for hydrogen peroxide is H2O2. The ratio of H to O is

2:2 or in simplest whole number terms 1:1. Thus, the empirical formula for H2O2 is HO. The

empirical formula for N2H4 is NH2. Empirical formulas are the simplest chemical formulas while

molecular formulas are the true formulas of molecules. Determining a compounds empirical

formula is the first step in determining its molecular formula through experimentation. For water,

H2O, its ratio of elements cannot be reduced further, so it is already an empirical formula as well

as well as a molecular formula.

The formulas of ionic compounds are usually the same as their empirical formulas. To

explain this further, consider the ionic compound NaCl. NaCl contains an equal number of Na

and Cl- ions because each is charged positive and negative one. However, Na+ is not associated

with just one Cl-, it is surrounded by six Cl- ions and vice versa. So the ratio of the number of Na

to Cl- ions will always be written in lowest terms. In order to form ionic compounds, they must be

electrically neutral. This means that the sum of the charges of the cation and anion must be zero.

To do this we make the number of cations present numerically equal to the charge of the cation

and vice versa. So if a zinc cation, Z2+ has a charge of two and an iodine anion, I- has a charge of

1, there must be two iodines and one zinc in the ionic formula, ZnI2.

Example 4:What are the empirical formulas of the following compounds?1) C2N2

2) C6H6

3) C9H20

4) P4O10

5) B2H6

Answer:

1) CN2) CH3) C9H20

4) P2O5

5) BH3

Naming Compounds

There are over thirteen million known chemicals today. Many more are being

synthesized everyday. Chemists have devised a system for naming these chemicals and other

chemicals that may yet be discovered. Moreover, these rules are accepted worldwide so as to

facilitate communication between scientists around the world.

Organic compounds are compounds containing the element carbon, usually in

combination with elements such as hydrogen, oxygen, nitrogen, and sulfur. All the other

compounds are considered inorganic compounds. However, some carbon containing

compounds such as carbon dioxide, carbon monoxide, carbon disulfide, and carbonate are

considered inorganic compounds.

Except for the ammonium ion, most cations are derived from metal ions. Metal cations

are named simply by naming the metal and adding the word ion after it. So sodium in its ionic

state would be named sodium ion. Aluminum in its ionic state would be named aluminum ion.

For ionic compounds formed from just two elements, these are referred to as binary compounds.

In binary compounds the first element named is the metal cation, followed by the nonmetallic

anion. The anion is named by taking the first part of the element name and adding “ide” to the

end. So chlorine would be called chloride and bromine would be called bromide. So the binary

compound NaCl would be called sodium chloride. Certain anions such as hydroxide, OH- and

cyanide, CN- also use the “ide” ending. So the compound LiOH is called lithium hydroxide. This

compound consists of three elements and so is called a ternary compound.

Some metals, mostly transition metals, can form more than one type of cation. Iron can

form two cations, Fe2+ and Fe3+. An old system of naming compounds uses the ending “ous” for

the cation with fewer positive charges and the ending “ic” for the cation with more positive

charges. So Fe2+ would be called ferrous ion and Fe3+ would be called ferric ion. So the binary

compound FeCl2 would be called ferrous chloride and FeCl3 would be called ferric chloride.

However, some elements may form more than two cations. Under the newer Stock system, the

Roman numeral indicates the charge on a cation. So Mn2+ would be named manganese(II) oxide

and Mn3+ would be named manganese(III) oxide. These compounds would be pronounced

“manganese-two oxide” for the former and “manganese-three oxide” for the latter.

Molecular compounds are usually made up of nonmetallic elements. Many molecular

compounds are binary and naming these compounds is similar to naming binary compounds.

The first element is named and then the second element is named by adding “ide” to the end of

the name. So HCl would be named hydrogen chloride and HBr would be named hydrogen

bromide. When one pair of elements forms more than one compound, Greek prefixes are used to

denote the number of each element present. The Greek prefixes can be seen in Table 1.

Prefix Meaning

Mono- 1

Di- 2

Tri- 3

Tetra- 4

Penta- 5

Hexa- 6

Hepta- 7

Octa- 8

Nona- 9

Deca- 10

Table 1.

The molecules CO and CO2 would be named carbon monoxide and carbon dioxide

respectively. The “mono” in front of the carbon can be emitted for the first element, as is the case

here. The molecules SO2 and SO3 would be named sulfur dioxide and sulfur trioxide. The

molecule N2O4 would be name dinitrogen tetroxide. Instead of tetraoxide, the element is named

tetroxide as can usually be done for oxides. Molecular compounds containing hydrogen however

do not follow the rule. B2H6 is named diborane and CH4 is named methane. H2O as well that

most of us are familiar with is called water.

An acid is a substance that yields hydrogen ions, H+, when dissolved in water. Acids

contain hydrogen atoms and an anionic group. If the anionic group ends in “ide”, then acids are

formed with a “hydro” prefix and an “ic” ending. HCl is called hydrogen chloride in the gaseous or

liquid state. However, when HCl is dissolved in water, H+ ions are released and the molecule

becomes hydrochloric acid.

Oxoacids are acids that contain hydrogen, oxygen, and another element. The formulas

for oxoacids are written with the H first, the other element in the middle, and the O at the end.

HNO3-nitric acid, H2CO3-carbonic acid, and H2SO4-sulfuric acid are oxoacids. When two or more

oxoacids have the same central atom but a different number of O atoms the following rules apply

in naming the acid:

-The addition of an O atom to the “ic” acid adds a “per” prefix to the acid. Adding an O to HClO3

changes chloric acid to perchloric acid, HClO4.

-The removal of an O atom from the “ic” acid changes the suffix to “ous”. So nitric acid, HNO3

becomes nitrous acid, HNO2.

-The removal of two O atoms from the “ic” acid adds a “hypo” prefix and changes the suffix to

“ous”. So bromic acid, HBrO3 becomes HBrO, hypobromous acid.

Oxoanions, the anions of oxoacids also have certain rules:

-When all the H ions are removed from the “ic” acid, the anions name ends with “ate”. Carbonic

acid becomes carbonate.

-When all the H ions are removed from the “ous” acid, the anions name ends with “ite”. Chlorous

acid becomes chlorite.

-The names of anions in which one or more but not all hydrogen ions have been removed must

indicate the number of H ions present.

A base is a substance that yields hydroxide ions, OH-, when dissolved in water. NaOH,

KOH, and Ba(OH)2 are all bases. Ammonia, NH3 does not even contain an OH however, when it

dissolves in water, it reacts with water to yield among other products OH- ions. So it is indeed a

base.

Hydrates are compounds that have a specific number of water molecules attached to

them. Copper(II) sulfate has five water molecules attached to it, so the systematic name for the

compound is copper(II) sulfate pentahydrate. Its formula is CuSO45H2O. The water molecules

can be evaporated by heating and the resulting substance would be called anhydrous copper(II)

sulfate. Its formula is CuSO4. BaCl22H2O is another hydrate named barium chloride dihydrate.

One more hydrate is LiClH2O named lithium chloride monohydrate. The amount of water

molecules present is named using the Greek prefixes seen in Table 1.

Example 5:Define the following terms1) Acids2) Bases3) Oxoacids4) Hydrates

Answer:

1) An acid is a substance that yields hydrogen ions, H+, when dissolved in water.2) A base is a substance that yields hydroxide ions, OH-, when dissolved in water.3) Oxoacids are acids that contain hydrogen, oxygen, and another element.4) Hydrates are compounds that have a specific number of water molecules attached to them.

Example 6:

Name the following compounds1) KClO2) Ag2CO33) HNO24) KmnO45) CsClO36) KNH4SO47) FeO8) Fe2O3

Answer:

1) Potassium hypochlorite2) Silver carbonate3) Nitrous acid4) Potassium permanganate5) Cesium chlorate6) Potassium ammonium sulfate 7) Iron(II) oxide 8) Iron(III) oxide