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Structure and Bonding - Lecture notes 1-2

Introduction to Structure and Bonding in Organic Molecules
Course

Organic Chemistry (Chem 201)

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Academic year: 2021/2022
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1 Structure and Bonding

Organic chemistry refers to the studies of the structure, properties and reactions of organic

compounds which contain carbon in covalent bonding. Organic compounds relate to living

things and all contain carbon.

1 Atomic structure

 All matter is composed of atoms.

 An atom consists of a dense, positively charged nucleus surrounded at a relatively large

distance by negatively charged electrons.

 The nucleus consists of subatomic particles called neutrons, which are electrically neutral,

and protons, which are positively charged.

 An atom is neutral overall, the number of positive protons in the nucleus and the number of

negative electrons surrounding the nucleus are the same.

Fig. 1 The simple schematic diagram to the left shows this simple model of a helium atom.

The nucleus contains two protons and two neutrons, and then there are two electrons.

 According to the quantum mechanical model of atomic structure, the behavior of a specifi c

electron in an atom can be described by a mathematical expression called a wave equation— the same sort of expression used to describe the motion of waves in a fluid. The solution to a wave equation is a wave function, or orbital, denoted by the Greek letter psi, ψ.

 An orbital can be thought of as defining a region of space around the nucleus where the

electron can most likely be found.

 There are four different kinds of orbitals, denoted s, p, d , and f , each with a different shape.

 In organic chemistry one needs to be most familiar with the s- and p-type orbitals, these are

the most common in organic and biological chemistry.

Fig 1 The orbitals for the electrons in carbon and their quantum numbers.

 Draw the shapes of the 2px and 2py orbitals

1 Electronic configurations

 The electron configuration of an atom describes how the electrons of an atom are

arranged.

 Different layers, or electron shells , contain different numbers and kinds of orbitals, and

each orbital can be occupied by 2 electrons.

 The first shell contains only a single s orbital, denoted 1s, and thus holds only 2 electrons.

 The second shell contains an s orbital (designated 2s) and three mutually perpendicular p

orbitals (each designated 2p) and thus holds a total of 8 electrons. The third shell contains an s orbital (3s), three p orbitals (3p), and five d orbitals (3d), for a total capacity of 18 electrons.

 The lowest-energy arrangement, or ground-state electron confi guration, of an atom is a

listing of the orbitals that the atom’s electrons occupy.

 We can predict this arrangement by following three rules.

RULE 1 The orbitals of lowest energy are filled first, according to the order 1s → 2s → 2p → 3s → 3p → 4s → 3d.

RULE 2 Only two electrons can occupy an orbital, and they must be of opposite spin. (Electrons act in some ways as if they were spinning on an axis, somewhat as the earth spins. This spin can have two orientations, denoted as up ↑ and down ↓.)

If two or more empty orbitals of equal energy are available, one electron occupies each with the spins parallel until all orbitals are half-full.

 The number of covalent bonds an atom forms depends on how many additional valence

electrons it needs to reach a noble-gas configuration. Hydrogen has one valence electron (1s) and needs one more to reach the helium confi guration (1s 2 ), so it forms one bond.

 Carbon has four valence electrons (2s 2 2p 2 ) and needs four more to reach the neon confi

guration (2s2 2p6), so it forms four bonds.

 Nitrogen has five valence electrons (2s 2 2p 3 ), needs three more, and forms three bonds;

oxygen has six valence electrons (2s2 2p4), needs two more, and forms two bonds; and the halogens have seven valence electrons, need one more, and form one bond.

 Valence electrons that are not used for bonding are called lone-pair electrons, or

nonbonding electrons. The nitrogen atom in ammonia (NH 3 ), for instance, shares six valence electrons in three covalent bonds and has its remaining two valence electrons in a nonbonding lone pair.

Predicting the Number of Bonds Formed by an Atom

1. How many hydrogen atoms does phosphorus bond to in forming phosphine, PH??

 Identify the periodic group of phosphorus, and tell from that how many

electrons (bonds) are needed to make an octet.

 Phosphorus is in group 5A of the periodic table and has fi ve valence electrons. It

thus needs to share three more electrons to make an octet and therefore bonds to three hydrogen atoms, giving PH 3. Drawing Electron-Dot and Line-Bond Structures

1. Draw both electron-dot and line-bond structures for chloromethane, CH 3 Cl.

 Hydrogen has one valence electron, carbon has four valence electrons, and

chlorine has seven valence electrons. Thus, chloromethane is represented as

Problem Write both electron-dot and line-bond structures for the following molecules,

showing all non-bonded electrons:

(a) CHCl 3 , chloroform (b) H 2 S, hydrogen sulphide (c) CH 3 NH 2 , methylamine

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Structure and Bonding - Lecture notes 1-2

Course: Organic Chemistry (Chem 201)

4 Documents
Students shared 4 documents in this course

University: Solusi University

Was this document helpful?
1.0 Structure and Bonding
Organic chemistry refers to the studies of the structure, properties and reactions of organic
compounds which contain carbon in covalent bonding. Organic compounds relate to living
things and all contain carbon.
1.1 Atomic structure
All matter is composed of atoms.
An atom consists of a dense, positively charged nucleus surrounded at a relatively large
distance by negatively charged electrons.
The nucleus consists of subatomic particles called neutrons, which are electrically neutral,
and protons, which are positively charged.
An atom is neutral overall, the number of positive protons in the nucleus and the number of
negative electrons surrounding the nucleus are the same.
Fig. 1.1 The simple schematic diagram to the left shows this simple model of a helium atom.
The nucleus contains two protons and two neutrons, and then there are two electrons.
According to the quantum mechanical model of atomic structure, the behavior of a specifi c
electron in an atom can be described by a mathematical expression called a wave equation—
the same sort of expression used to describe the motion of waves in a fluid. The solution to a
wave equation is a wave function, or orbital, denoted by the Greek letter psi, ψ.
An orbital can be thought of as defining a region of space around the nucleus where the
electron can most likely be found.
There are four different kinds of orbitals, denoted s, p, d, and f, each with a different shape.
In organic chemistry one needs to be most familiar with the s- and p-type orbitals, these are
the most common in organic and biological chemistry.

Students also viewed

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