THE STRUCTURE OF AN ATOM

An atom is built up of protons, electrons and neutrons according to the six rules mentioned below  

An atom is not charged, in other words it is electrically neutraF . This means that the number of protons p+ is equal to the number of electrons e.  

It is not possible to establish the number of neutrons in an atom with the help of simple rules. Their number is always bigger than or equal to the number of protons, with the exception of the hydrogen atom which has one proton and most of its atoms no neutrons. 

The electrons circulate around the nucleus in specific orbits. These orbits are also called shells andean be compared to the orbits in which satellites travel around the Earth. When more electron orbits are present in one atom, these differ in diameter. 

The maximum number of electrons in a shell depends on the diameter. The bigger the diameter, the larger the maximum number of electrons will be. The shells are designated with the letter K, L, M, etc. The K-shell has the smallest diameter. In that order the shells are also denoted with n = 1, n = 2, etc. The maximum number of electrons per shell can then be calculated using the formula 2 x n2. 

Electrons move continuously. Like anything that moves, electrons have kinetic energy, and this energy is what counteracts the attractive force of the positively charged protons that would otherwise pull the negatively charged electrons into the nucleus. 

Protons and neutrons have approximately the same mass and are about 1800 times more massive than an electron. Most of the mass of an atom, therefore, is in its nucleus. Most of the volume of an atom, however, is occupied by its electrons, and this is where our focus will be because it is the electrons that form chemical bonds. 

The atomic number of an atom is the number of protons in its nucleus. The atomic number is imique to a particular element. For example, the atomic number of carbon is 6, which means that all uncharged carbon atoms have six protons and six electrons. Atoms can gain electrons and thereby become negatively charged, or they can lose electrons and become positively charged, but the number of protons in an atom of a particular element never changes. 

Although all carbon atoms have the same atomic number, they do not all have the same mass number because they do not all have the same number of neutrons. The mass number of an atom is the sum of its protons and neutrons. For example, 98.89% of all carbon atoms have six neutrons— giving them a mass number of 12—and 1.11% have seven neutrons—giving them a mass number of 13. These two different kinds of carbon atoms and C) are called isotopes. 

Carbon also contains a trace amount of which has six protons and eight neutrons. This isotope of carbon is radioactive, decaying with a half-life of 5730 years. (The half-life is the time it takes for one-half of the nuclei to decay.) As long as a plant or animal is alive, it takes in as much as it excretes or exhales. When it dies, however, it no longer takes in so the in the organism slowly decreases. Therefore, the age of a substance derived from a living organism can be determined by its content. 

The isotope is usually referred to as deuterium (D), or heavy hydrogen, but most isotopes of other elements are identified simply by their mass number. The atomic mass listed for each element in the periodic table is a weighted average, the fractional abundance of each isotope times its exact mass, summed over all naturally occurring isotopes. 

The mass number A is 3 and therefore, from Eq. (2.1), N = 2. Thus, the nucleus eonsists of one proton and two neutrons.  

Before we delve further into the properties of the nucleus, let us momentarily shift our attention back to one of the electrons zooming around the nucleus. Just like photons, electrons exhibit both wave and particle properties. Each electron wave in an atom is characterized by four quantum numbers. The first three of these numbers can be taken as the electron s address and describe the energy, shape, and orientation of the volume the electron occupies in the atom. This volume is called an orbital. The fourth quantum number is the electron spin quantum number s, which can assume only two values, + -ox-. (Why J was selected rather than, say, 1 will be described a little later.) The Pauli exclusion principle tells us that no two electrons in an atom can have exactly the same set of four quantum numbers. Therefore, if two electrons occupy the same orbital (and thus possess the same first three quantum numbers), they must have different spin quantum numbers. Therefore, no orbital can possess more than two electrons, and then only if their spins are paired (opposite). 

Perhaps surprisingly, neutrons also exhibit a magnetic moment and a nuclear spin of / = J, even though they are uncharged. Therefore, they too can adopt two different orientations in a magnetic field. But because the sign of p for a neutron is negative (Table 2.1), the more stable orientation corresponds to /n = - J. 

we have established that H nuclei (i.e., protons) exhibit two possible magnetic spin orientations. What about other isotopes From Chapter 1 you might remember that 

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Two of Rutherford s students conducted a classic experiment to determine the structure of an atom. They beamed alpha particles through a sheet of gold foil that was 1/50,000 of an inch thick. Since the thickness of this thin foil was only about two thousand atoms of gold, it... 

Clearly distinguish the following aspects of the structure of an atom and sketch the appropriate function for Is, 2j, 2p, 3s, and 3p orbitals. 

The electrons in the outermost shell of the structure of an atom. Since these electrons are commonly the means by which the atom enters into chemical combinations—either by giving them up. or by adding others to their shell, or by sharing electrons in this shell—these outermost electrons arc called valence electrons. 

Why did it take scientists so long to discover the structure of an atom ... 

The structure of an atom showing all the particles on the nucleus and also the electrons orbiting around it. 

However, if a hypothesis or set of hypotheses, based on an observation of a natural phenomenon, cannot be tested or experiments fail to show a direct conclusion, it is identified as a theory. A theory explains why something happens. For example, the atomic theory (see Lessons 9 and 10) of small electron particles orbiting around a dense nucleus containing protons and neutrons is based on indirect observations of the atom. This is only a possible explanation for the structure of an atom. A model is the description of the theory, such as the structure of an atom. 

For hundreds of years, alchemists searched for ways to turn various metals into gold. How would the structure of an atom of soHg (mercury) have to be changed for the atom to become an atom of toAu (gold) ... 

Solids aren t really solid They are mostly empty space When we discuss the structure of an atom in Chapter 3 you will learn that the size of the nucleus is actually very tiny compared to the size of the atom. The space that an atom effectively occupies really has to do with the fact that the electrons are moving very quickly and that the repulsive forces between electrons of different atoms keep them a certain distance apart. If you could strip the electrons off an atom and neutralize the charge of the nucleus, the atom would occupy only a fraction of its original size. To picture the size change, if the original size of the atom was about the size of a baseball stadium, the altered atom would be about the size of a marble. Just think The solid floor that you stand on is almost completely empty space ... 

Bohr returned to Denmark and became a professor. He wrote papers in which he described the structure of an atom. Bohr showed that electrons have stable orbits around the nucleus, which allows them to keep spinning. Electrons give off energy only when they jump to a different orbit. In 1922, Bohr won the Nobel Prize for his studies of atoms. 

Chemistry makes connections among the composition, structure, and behavior of matter. As you study chemistry, you will learn how the chemical and physical properties of matter are important clues to its submicro-scopic structure and behavior. You U see how knowing the structure of an atom of an element can enable you to predict the chemical behavior of that element. You U learn how the state of a substance at room temperature provides clues to the way its atoms are arranged. You ll find out why some things dissolve in water but others do not, how metals corrode, how batteries work, why compounds containing carbon are important to life, and how a nuclear reactor works. 

Inferring Why did Rutherford conclude that an atom s nucleus has a positive charge instead of a negative charge Summarize the conclusions that Rutherford s team made about the structure of an atom. 

Until now we have dealt with rather simple models of the atom. For example, Dalton s model did not include electrons at all. In order to answer our questions about the colors of fireworks, we need to develop a more complicated model of the atom, and especially of the electrons in the atom. We have not yet looked deeply at the structure of an atom, but that is what this chapter is all about. 

Once chemists came to "helieve" in atoms, logical questions followed What are atoms like What is the structure of an atom In Chapter 3 we learned to picture the atom with a positively charged nucleus composed of protons and neutrons at its center and electrons moving around the nucleus in a space very large compared to the size of the nucleus. 

Since we are not able to study the internal constitution of atoms by the kind of methods we might use to study the internal constitution of a tape worm by dissection and abstraction from what we observe, models of atomic innards caimot be analytical in the above sense. Nor can they be iconic, since to project only one of the properties of product electrons back in to the structure of an atom is to commit the second mereological fallacy. Clearly, the planetary electron hypothesis, if taken seriously as a contribution to our knowledge of the internal constitution of atoms, is an example of that fallacy. The only remaining possibility is Mulliken s inspired proposal that we clearly disassociate ourselves from electrons and their orbits by proposing a pregnant new concept - that of the orbital . It reminds us of the history of quantum chemistry from its beginnings in Bohrian orbits , but offers a purely formal model of the structure of chemical entities. 

Quantum mechanics describes and defines the behavior of electrons in atoms. One of the rules of quantum behavior is that electrons are constrained to specific locales known as orbitals within the structure of an atom and are not allowed to exist at the boundaries of those locales. An observable property called quantum mechanical tunneling occurs, however, which permits electrons to move from one locale to another across the orbital boundaries. In scanning probe microscopy the miniscule electrical current due to quantum mechanical tunneling between the atoms at a surface and the atoms at the tip of an atomic-scale probe is measured as a function of their relative positions. This provides a corresponding atomic scale map of the surface structure. 

Each element, in fact, has a characteristic line spectrum because of the emission of light from atoms in the hot gas. The spectra can be used to identify elements. How is it that each atom emits particular colors of light What does a line spectrum tell us about the structure of an atom If you know something about the structures of atoms, can you explain the formation of ions and molecules We will answer these questions in this and the next few chapters. 

If you know or assume the structure of an atomic crystal, you can calculate the length of the unit-cell edge from the density of the substance. This is illustrated in the next example. Agreement of this value with that obtained from x-ray diffraction confirms that your view of the structure of the crystal is correct. 

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