Atom is a basic unit of matter, composed of atomic nuclei and negatively charged electron cloud surrounding it. The nucleus consists of positively charged protons and electrically neutral neutrons (except hydrogen-1 nucleus, which has no neutrons). The electrons in an atom bound to the nucleus by the electromagnetic force. As well as a collection of atoms can bind to each other, and form a molecule. Atoms containing the number of protons and electrons of the same neutral, while containing the number of protons and electrons of different positive or negative and is called ions. Atoms are grouped based on the number of protons and neutrons contained in the nucleus. The number of protons in an atom determines the chemical element the atom, and the number of neutrons determine the isotope of the element.

The term comes from the Greek atom (ἄτομος / Atomos, α-τεμνω), which means it can not be cut or something that can not be divided again. The concept of the atom as a component that can not be divided again was first proposed by the philosophers of India and Greece. In the 17th century and into the 18th, the chemists laid the foundations of this idea by showing that certain substances can not be broken down further using chemical methods. During the late nineteenth century and early twentieth century, physicists <ol>

have managed to find the structure and subatomic components inside the atom, to prove that the ‘atom’ is not never be divided again. The principles of quantum mechanics used by physicists then successfully model the atom.

In the day-to-day observations, the relative atomic considered a very small objects with proportionately tiny masses anyway. Atoms can only be monitored using special equipment such as atomic force microscopy. More than 99.9% of the mass of the atom is concentrated in the nucleus, [note 1] with protons and neutrons are nearly the same mass. Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in transmutation, which changes the number of protons and neutrons in the nucleus. [2] Electrons are bound to the atom contains a number of energy cedar, or orbital, which is stable and can undergo transitions between these cedar to absorb or emit photons corresponding to the difference between the energy levels. The electrons in the atom determines the chemical properties of an element, and affects the magnetic properties of the atom.

The Structure of Atoms

An atom (ours, not Carroll’s) consists of a positively charged nucleus, surrounded by one or more negatively charged particles called electrons. The positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. Most of an atom’s mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. Although the nucleus is heavy, it is quite small compared with the overall size of an atom. The radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6 nm. * If an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.<code>

The nucleus of an atom contains protons and neutrons. Protons and neutrons have nearly equal masses, but they differ in charge. A neutron has no charge, whereas a proton has a positive charge that exactly balances the negative charge on an electron. Table 1-1 lists the charges of these three fundamental particles, and gives their masses expressed in atomic mass units. The atomic mass unit (amu) is defined as exactly one-twelfth the mass of the nucleus of a carbon atom consisting of six protons and six neutrons. With this scale, protons and neutrons have masses that are close to, but not precisely, 1 amu each. [As a matter of information at this point, there are approximately 6.022 X 1023 amu in 1 gram (g). This number is known as Avogadro's number, N, and later in the chapter we will show one of the ways this number can be calculated.]

The number of protons in the nucleus of an atom is known as the atomic number, Z. It is the same as the number of electrons around the nucleus, because an atom is electrically neutral. The mass number of an atom is equal to the total number of heavy particles: protons and neutrons. When two atoms are close enough to combine chemically-to form chemical bonds with one another-each atom “sees” mainly the outermost electrons of the other atom. Hence these outer electrons are the most important factors in the chemical behavior of atoms. Neutrons in the nucleus have little effect on chemical behavior, and the protons are significant only because they determine how many electrons surround the nucleus in a neutral atom. All atoms with the same atomic number behave in much the same way chemically and are classified as the same chemical element. Each element has its own name and a one- or two-letter symbol (usually derived from the element’s English or Latin name). For example, the symbol for carbon is C, and the symbol for calcium is Ca. The symbol for sodium is Na-the first two letters of its Latin (and German) name, natrium- to distinguish it from nitrogen, N, and sulfur, S.

*One nanometre equals 10-9 meters (m), or 10-7 centimeters (cm). If you are not familiar with metric units, see Appendix 1 for more information on the Système International (SI), a simplified version of the metric system adopted by scientists throughout the world. We shall generally use SI units in this book. If you are not familiar with the use of exponential numbers (scientific notation), read Appendix 4.

Isotopes

Although all atoms of an element have the same number of protons, the atoms may differ in the number of neutrons they have (Table 1-2). These differing atoms of the same element are called isotopes. Four isotopes of helium (He) are shown in Figure 1-1. All atoms of chlorine (Cl) have 17 protons, but there are chlorine isotopes having 15 to 23 neutrons. Only two chlorine isotopes exist in significant amounts in nature, those with 18 neutrons (75.53% of all chlorine atoms found in nature), and those with 20 neutrons (24.47%). To write the symbol for an isotope, place the atomic number as a subscript and the mass number (protons plus neutrons) as a superscript to the left of the atomic symbol. The symbols for the two naturally occurring isotopes of chlorine then would be Cl and Cl. Strictly speaking, the subscript is unnecessary, since all atoms of chlorine have 17 protons. Hence the isotope symbols are usually written without the subscript: 35Cl and 37Cl. In discussing these isotopes, we use the. terms chlorine-35 and chlorine-37. For a nucleus to be stable, the number of neutrons should (for the first few elements) equal or slightly exceed the number of protons. The more protons, the greater the ratio of neutrons to protons to ensure stability. Nuclei that have too many of either kind of fundamental particle are unstable, and break down radioactively in ways that are discussed in Chapter 23.

Molecules

The formation of atoms from fundamental particles, interesting as this might be to the physicist, is far from being the ultimate stage in the organization of matter. As we mentioned earlier, when atoms are close enough to one another that the outer electrons of one atom can interact with the other atoms, then attractions can be set up between atoms, strong enough to hold them together in what is termed a chemical bond. In the simplest cases the bond arises from the sharing of two electrons between a pair of atoms, with one electron provided by each of the bonded atoms. Bonds based on electron sharing are known as covalent bonds, and two or more atoms held together as a unit by covalent bonds are known as a molecule. One of the principal triumphs of the theory of quantum mechanics in chemistry (see Chapter  has been its ability to predict the kinds of atoms that will bond together, and the three-dimensional structures and reactivities of the molecules that result. (A major section of this book, Chapters 8-14, is devoted to chemical bonding theories.)

Figure 1-2 Shapes and relative sizes of some simple molecules. Two bonded atoms appear to interpenetrate because their electron clouds overlap. By convention. a tapered bond in a drawing represents a bond pointing out toward the observer, with the wide end of the taper closest. and a dashed line is used for a bond that points back behind the plane of the page.

In molecular diagrams, a covalent, electron-sharing bond is represented by a straight line connecting the bonded atoms. In the water molecule, one atom of oxygen (0) is bonded to two hydrogen (H) atoms. The diagram for the molecule can be drawn two ways:

The second version acknowledges the fact that a water molecule is not linear; the two H -0 bonds make an angle of 105° with one another. Molecules of hydrogen gas, hydrogen sulfide, ammonia, methane, and methyl alcohol (methanol) have the following bond structures:

These diagrams show only the connections between atoms in the molecules. They do not show the three-dimensional geometries (or shapes) of the molecules. Figure 1-2 shows the shapes and the relative bulk of several molecules. Note that the bond angle in molecules having more than two atoms can vary. The angle in the water molecule is 105°, and the angle in hydrogen sulfide is 92°; the four atoms connected to the central carbon in methane and methyl alcohol are directed to the four corners of a tetrahedron. The bond structure in straight-chain octane, one of the components of gasoline, is

Each of the molecular diagrams shown can be condensed to a molecular formula, which tells how many atoms of each element are in the molecule, but provides little or no information as to how the atoms are connected. The molecular formula for hydrogen is H2; water, H20; hydrogen sulfide, H2S; ammonia, NH3; methane, CH4 ; methyl alcohol, CH30H or CH4O; and octane, C8H18. The formula for octane can also be written

The sum of the atomic weights of all the atoms in a molecule is its molecular weight. Using the atomic weights on the inside back cover, we can calculate molecular weights. The molecular weight of hydrogen, H2, is

2 X 1.0080 amu = 2.0160 amu

A water molecule, H2O , has two atoms of hydrogen and one atom of oxygen, so:

(2 X 1.0080 amu) + (15.9994 amu) = 18.0154 amu

Ions

Figure 1-5 Common table salt (sodium chloride. NaCl) is built from closely packed sodium ions, Na+ (small spheres). and chloride ions. CI- (large. colored spheres). Each ion of one charge is surrounded by six ions of the opposite charge at the four compass points and above and below. This is a particularly stable arrangement of charges. and it occurs in many salts. From Dickerson and Geis. Chemistry, Matter. and the Universe The Benjamin / Cummings Publishing Co .. Menlo Park. Ca .. © 1976 .

The idea of a covalent bond suggests equal sharing of the electron pair by the bonded atoms, but the brief discussion of polarity in Section 1-4 indicated that the sharing is not always equal. The relative electronegativity or electron-attracting power of atoms is of great importance in explaining chemical behavior, and is treated in detail in Chapters 9 and 10. Sodium atoms (and all metals in general) have a weak hold on electrons, whereas chlorine atoms are very electronegative. Hence in common table salt (sodium chloride, NaCl), each sodium atom, Na, loses one electron (e-) to form a sodium ion, Na+. Each chlorine atom picks up one electron to become a chloride ion, Cl-:

Na → Na+ + e-wordandword Cl2 + e- → Cl-

We write  Cl2 because free chlorine gas exists as diatomic (two-atom) molecules, not as free chlorine atoms. Solid sodium chloride (Figure 1-5) has sodium and chloride ions packed into a three-dimensional lattice in such a way that each positive Na+ ion is surrounded on four sides and top and bottom by negative Cl- ions, and each Cl- is similarly surrounded by six nearest neighbor Na+ ions. This is a particularly stable arrangement of positive and negative charges.

Metals in general lose one to three electrons easily to become positively charged ions, or cations:

Li         →        Li+       +          e-         ttt        lithium ion

Na        →        Na+      +          e-         ttt        sodium ion

K          →        K+        +          e-         ttt        potassium ion

Mg       →        Mg2+   +          2e-       ttt        magnesium ion

Ca        →        Ca2+    +          2e-       ttt        calcium ion

Al         →        Al3+     +          3e-       ttt        aluminum ion

Some nonmetals, in contrast, pick up electrons to become negatively charged ions, or anions:

F2       +          e-         →        F-         ttt        fluoride ion

Cl2      +          e-         →        Cl-        ttt        chloride ion

O2      +          2e-       →        O2-      ttt        oxide ion

S2       +          2e-       →        S2-       ttt        sulfide ion

Table 1-4 Some Simple Ions of Elements

Other simple ions made from single atoms are shown in Table 1-4. The charge on a simple, single-atom ion such as AP+ or S2- is its oxidation state or oxidation number. It is the number of electrons that must be added to reduce (or removed to oxidize) the ion to the neutral species:

Reduction: AI3+ + 3e- → Al

Oxidation: S2- → S + 2e-

Pulling electrons away from an atom or removing them altogether is oxidation. Adding electrons to an atom or merely shifting them toward it is reduction.