Structure of the Atom

Stacy Kopso, M.E.d (RT)(R)(M)

History of Atomic Theory

• Leucippus is most often associated with the earliest atomic theory.

• Leucippus’ student and follower, Democritus is credited with truly formalizing and elaborating on the earliest atomic theory. FIGURE 2-1 Early Greek Theory of the Atom.

Atom

• Atom– Comes from the Greek word atomos meaning

indivisible (impossible to divide or separate)– Democritus hypothesized that all things were

made of tiny indivisible structures called atoms

John Dalton

• English chemist John Dalton in the early 1800s gave us a sound atomic theory based on scientific evidence.– He recognized that elements combined to form compounds(combination

of elements) and developed an atomic theory to explain why this happened.

– His theory is now more than 200 years old, but remains fundamentally valid.

FIGURE 2-2 Dalton’s Atom Model. Dalton’s wooden models of the atom. Reprinted with permission Science Museum (London).

JJ Thomson

• Studied the glowing stream that is visible when an electric current is passed through the cathode ray tube.

• Discovered that the glowing stream was attracted to a positively charged electrode

• This glowing stream was actually negatively charged pieces of atoms (electrons)

• Concluded that the atom is a positively charged sphere with negatively charged electrons embedded in it

Ernest Rutherford

• Based on his experiment he concluded that a model of an atom consists of a positively charged and very dense nucleus with tiny electrons orbiting it in defined paths.

Niels Bohr

• Modern Theory• The atom is considered the basic building

block of matter• The atom has three fundamental components– Electrons– Neutrons– Protons

Bohr Atom

• The atom has a nucleus made up of protons and neutrons

• Orbiting that nucleus are electrons

Parts of the Atom

• Proton– Located in nucleus of the atom– One unit of Positive electrical charge– Mass of 1.673 x 10-27 kg

• Neutron– Located in nucleus of the atom– No electric charge– Mass of 1.675 x 10-27 kg

Parts of the Atom

• Electron– One unit of Negative electrical charge– Orbit the nucleus in defined energy bands– Mass of 9.109 x 10-31 kg

Atomic Charge

• Electric charge is a characteristic of matter• If an atom has an equal number of protons (+) and

electrons(-), it is neutral– They cancel each other out– Protons are very strongly bound in the nucleus, the cause of

the electrical change usually involves the gain or loss of electrons

• If an atom loses and electron (-) the atoms charge becomes positive (positive ion)

• If an atom gains an electron (-) the atoms charge becomes negative (negative ion)

Nucleus

• The nucleus is held together by a strong nuclear force, creating a binding energy

• This holds the protons and neutrons together• Also a measure of the amount of energy

necessary to split an atom• If a particle strikes the nucleus with energy

equal to the binding energy, the atom could break apart

Binding Energy

• The force of the negative charge electrons and positive charged protons keep the electrons in the orbit

• The binding energy depends on – How close the electron is to the nucleus– How many protons are in the atom

• An atom has defined energy levels each at a different distance from the nucleus– Electrons orbit three-dimensionally around the nucleus.– They are not simply orbiting the nucleus in a single plane– They are illustrated this way for simplicity

• Class work Chapter 2 part 1

Electron Shells

Electron Shells

• Each electron shell is lettered beginning with K which is nearest to the nucleus

• Move outward with L,M,N,O, P and so on• Formula for the maximum number of

electrons that may occupy each shell2n2 (n represents the shell number, beginning with

the K shell as 1)

Electron Shell

• The K shell has the greatest binding energy• Binding energy decreases with each shell• Even though a shell can hold a certain number

of electrons, they are not necessarily full• A maximum of eight electrons can exist in the

outermost shell of any atom• Valence shell- Outermost shell

– The M shell can contain 18 electrons if there are more electrons present, they will be in an N shell. If the M is the outermost shell than it can only hold a maximum of 8 electrons

Work it Out

• K shell– 2n2

2N=1 (1 squared =1) 2x1=2

Work it Out

• L shell– 2n2

8 N=2 (2 squared is 4) 4x2=8

Work it Out

• M shell– 2n2

18M=3, 3 squared =9, 9x2=18

Classwork

• Draw an atom– Label • Protons• Neutrons• Electrons• Label shells and # of electrons in each

– An atom with 28 electrons

Elements

Classification

• Atomic number– The number of protons it contains in its nucleus– Also the number of electrons if it is a stable atom

• Atomic mass– Number of protons and neutrons an atom has in

its nucleus

Classification

• Elements• Simplest forms of substances that compose matter• Each element is made up of only one unique type

of atom with an unchanging number of protons– 92 different elements exist in the natural world

and more than a dozen others have been created artificially

Bonding

• Pure elements– Naturally occurring elements– Not combined with other elements• Iron, zinc, nickel,oxygen,carbon, hydrogen

• Chemical compounds– Two or more atoms bonded together form a

molecule• Water

– 2 atoms of hydrogen & 1 atom of oxygen

Chemical shorthand

Classifying Elements

Connection to Radiology

• Elements– Barium– Molybdenum– Tungsten– Lead1. Research their purpose in radiology2. Atomic number and mass3. Symbol

Classification

• Isotope– Elements whose atoms have the same number of protons but

different number of neutrons• Isotone

– Elements whose atoms have the same number of neutrons but a different number of protons

• Isobar– Elements whose atoms have a different number of protons but the

same total number of protons and neutrons (atomic mass number)• Isomer

– Elements whose atoms have the same number of protons and neutrons but with different amounts of energy within their nuclei

Examples of isos• Isotopes(same atomic number)

11H & 2

1H

• Isotones (same number of neutrons)131

53I & 13254XE subtract the atomic # from atomic

mass (78)• Isobars (same atomic mass number)

73Li & 7

4Be

• Isomer(different amounts of energy)99mTc (m stands for metastable/chemically unstable)

Bonding

• It is the chemical bonds between atoms that allow complex matter(living tissue) to exist

• A molecule is formed when two or more atoms join together chemically

• The bonding of various atoms to form molecules permits the highly complex matter about us to exist.

• There are 2 types of ways atoms bond to form molecules– Ionic bond– Covalent bond

Bonding

• Ionic bonding– Based on the attraction of opposing charges– Atoms are electrically neutral (same # of protons

and electrons)– When in the presence of other atoms, some atoms

have the tendency to give up or gain electrons– In an ionic bond, one of the atoms gives up an

electron and the other takes the extra electron• Difference in their electrical charge attracts and bonds

the two together

Covalent Bonding

• Based on two atoms sharing electrons that then orbit both nuclei

• As the electron shells of atoms fill, they do so from the one nearest the nucleus

• The outermost shells are not always full• An outermost electron from one atom begins

to orbit the nucleus of another adjacent atom

X-ray Connection

• The interactions in the x-ray tube that produce x-rays occur at the atomic level

• The nature of the x-ray photon produced depends on how an electron interacts with an atom

X-ray Connection

• The interactions between the x-ray photons and the human body also occur at the atomic level– The type of interactions determine both the

radiation dose delivered and how the body part will be imaged

X-ray Connection

• The interactions between the x-ray photons exiting the patient to produce the image interact at the atomic level of the image receptor to generate the final image.

• Class work chapter 2 part 2