3.3 Parts of an Atom Handout and highlight. Parts of an Atom.
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Transcript of Jan 14_Fundamentals of an Atom
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Fundamentals of anAtom
Chemistry 1
By: Ms. Mialo C. Lacaden
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Fundamentals of the Atom
An atom consists of a nucleus of protons and neutrons surrounded byelectrons
Each of the elements in the periodictable is classified according to itsatomic number
Protons have a charge of +1, electronshave a charge of -1, and neutronshave no charge
Electrically, neutral atoms have the
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Within a given element, atoms with differentnumbers of neutrons are isotopes of that
element and exhibit the same properties
Electrons have such little mass that theyexhibit properties of both particles and waves
Heisenberg's Uncertainty Principle states that itis impossible to know the precise location of an electron
Despite this limitation, there are regionsaround the atom where the electron has a
high probability of being found. Such regionsare referred to as orbitals.
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Atomic Orbitals
For isolated atoms (meaning non-bonded),electrons reside in the atomic orbitals of those atoms
Atomic orbitals are classified according to aset of four quantum numbers whichdescribe the energy, shape, and
orientation of the orbital.
1. Principle Quantum Number (n):Indicates how far the orbital is from thenucle.
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2. Angular Momentum Quantum Number(l): Describes the shape of the orbital.
The angular momentum number (orsubshell) can be represented either bynumber (any integer from 0 up n -1) or by aletter ( s , p , d , f , g , and then up thealphabet), with 0 = s, 1 = p, 2 = d , and so
on. For example: when n = 1, l can only equal 0; meaningthat shell n = 1 has only an s orbital (l = 0).
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when n = 3, l can equal 0, 1, or 2;meaning that shell n = 3 has s , p ,
and d orbitals. s orbitals are spherical, whereas p orbitals are dumbbell-shaped. d
orbitals and beyond are muchharder to visually represent.
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3. Magnetic Quantum Number (m): Gives theorientation of the orbital in space; in other words,the value of m describes whether an orbital lies
along the x -, y -, or z -axis on a three-dimensionalgraph, with the nucleus of the atom at the origin.
m can take on any value from -l to l. For ourpurposes, it is only important that this quantum
number tells us that for each value of n there maybe up to
one s –orbital three p –orbitals
five d - orbitals, and so on:
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The s orbital (l = 0) has one orbital,since m can only equal 0. That
orbital is spherically symmetricalabout the nucleus.
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The p orbital (l = 1) has three orbitals,since m = -1, 0, and 1. These threeorbitals lie along the x -, y -, and z -axes.
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The d orbital (l = 2)has five orbitals, since
m = -2, -1, 0, 1, and 2.It is far more difficult todescribe the orientationof d orbitals.
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4. Spin Quantum Number (s): Tellswhether a given electron is spin up
(+1/2) or spin down (-1/2).
Pauli Exclusion Principle tells usthat no two electrons of an atom can
have the same set of quantumnumbers, each orbital is limited toholding two electrons at most.
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Orbital Energy Diagrams
Note that the relative spacing betweenorbitals becomes smaller for larger n .In fact, as n gets large the spacing
becomes infinitesimally small.
Figure %: Energy diagram of theunoccupied atomic orbitals of hydrogen.
Potential energy is on the y -axis.
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Multi-electron atoms
Breaking of degenerancies is due to the factthat the attractive nuclear force any electronfeels is shielded by the other electrons. s -orbitals tend to be closer to the nucleus than
p -orbitals and don't get as much shielding,and hence become lower in energy. Thisprocess of breaking degeneracies within ashell is known as splitting. In general s orbitals become lowest in energy, followed by
p orbitals, d orbitals, and so forth.
Figure %: Splitting of orbital -
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Electron Configuration andValence Electrons
Electron Configuration The electrons in an atom fill up its atomic orbitals
according to the Aufbau Principle; "Aufbau," in
German, means "building up.” Electrons always fill orbitals of lower energy
first. 1s is filled before 2s , and 2s before2 p .
If two electrons occupy the same orbital,they must have opposite spin, as requiredby the Pauli Exclusion Principle.
When electrons have to choose between twoor more orbitals of the same energy,
electrons prefer to go into different orbitals.
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electrons tend to half-fill orbitals of the same energy before pairing
with existing electrons to fillorbitals. This is known as Hund'sRule
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Valency and Valence Electrons The outermost shell of an atom is its
valence shell, and the electrons in the
valence shell are valence electrons.
Valence electrons are the highestenergy electrons in an atom and aretherefore the most reactive.
Inner electrons (those not in thevalence shell) typically don'tparticipate in chemical bonding and
reactions,
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Valence electrons can be gained, lost,or shared to form chemical bonds.
For this reason, elements with thesame number of valence electronstend to have similar chemicalproperties, since they tend to gain,lose, or share valence electrons in thesame way.
The Periodic Table was designed withthis feature in mind. Each element
has a number of valence electrons
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The Octet Rule
The octet rule states that atoms becomeespecially stable when their valence shellsgain a full complement of valence electrons.For example, Helium (He) and Neon (Ne)
have outer valence shells that are completelyfilled, so neither has a tendency to gain orlose electrons. Therefore, Helium and Neon,two of the so-called Noble gases, exist in freeatomic form and do not usually form
chemical bonds with other atoms.
Most elements, however, do not have a fullouter shell and are too unstable to exist asfree atoms. Instead they seek to fill theirouter electron shells by forming chemicalbonds with other atoms and thereby attain
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An element will tend to take the shortestpath to achieving Noble Gasconfiguration, whether that means
gaining or losing one electron.
For example, sodium, which has a singleelectron in its outer 3s orbital, can losethat electron to attain the electronconfiguration of neon. Chlorine, withseven valence electrons, can gain oneelectron to attain the configuration of argon.