(Concepis in a Box

transfer of forms ionic bond

electrons may polar or
be
nonpolar
covalent
combine sharing of forms
bond single, double
Atoms through electrons Can
be or triple bond
metallic
sharing of sea forms -
of electrons bond

Lesson 5.1 Electronsin Atoms

their location and energy.
Main ldea: Electrons can be described in terms of
atom?
EQ: How do atomic models describe the electrons in an

Electrons play a key role in chemical bonding. Their number and

bond.
arrangement in an atom explain vhy and how atoms interact and
What does an atom look like and where are the electrons located inside
it?

Since the time that ancient Greek philosophers Leucippus and

Democritus conceived the idea of an atom, different models have been
proposed to describe atomic structure. The simplest model, propOsed
in 1803 by English chemist John Dalton (1766-1844), depicted the
atom as an indivis1ble sphere. This idea of the indivisibility of an atom
was later corrected as scientists discovered subatomic particles. When
British plhysicist JJ. Thomson (1856-1940) discovered the electron, he
proposed in 1904 the plum pudding model to account for his observations.
This model described the atom as alarge, positively charged sphere
in which 'negatively charged electrons are embedded. British physicist
Ernest Rucherford (l871-1937) and his students tested the viability of
chis model chrough cheir gold foil experiment, but found instead that
the atom consists mostly of empty space and has a massive positively
charged center. Rutherford later discovered that this positively charged
center contains positively charged protons. The gold foil experiment did
not reveal much information about electrons, only that they can be found
around and at great distances from the positive center. These findings
led to the development of the nuclear mnodel of the atom. The discovery
of uncharged neutrons in 1932 by British physicist James Chadwick
(1891-1974) completed the model, with a nucleus composed of protons
g 5-1 Models of the alo
and neutrons at the center and electrons surrounding it. Dalton's to Bohr's

96 Exploring Life Through Science | Grade 9

 Ivae properties of matter wer explored, studies on atomic struCture focused
yhintrtizing the electrons in an atoM In 1913, Dan1sh physicist Niels Bohr (1885-1962) cheorized
clectron rravels around the nucleus in a specific orbit having a discrete amount of energy
enetgy) In this View, the energv of the electrons in an orbit increases with
increas1ng
nucleus. Bohr also postulated that electrons can jump from one orbit to another when
n r eleas1ng quantized energy With these findings, Bohr modified Rutherford's model by
Circles around the nucleus to represent the orbits of the
electrons.
Honever. while Bohr's model worked for hydrogen, it failed to
explain the atomic
her elements. S Cientists-then came up with a new model called the quantum mechantcal properties of
model, which
n egionsnot detined points or orbits-around the nucleus as
probable locations of electrons.
e00s are called atonuc orbitals. The farther the orbital is from che
electrons in It.
nucleus, the higher is the
NTEY Ot the
The quantum mechanical model uses quantum numbers to
describe an electron and its orbital.
The przncifil quantum number (n) specifies the main energy level, or
Integral values of 1, 2. 3, and so on. The value increases as the shell, of the orbital. It can have
farther from the nucleus. orbital (and thus an electron) gets
The aigalar momentum quantun nimber (l) describes the
shape of the orbital. It is also referred to
as subshelI. It has a value of0 to n-1. The
values 0, 1, 2, and 3 correspond to s, þ, d, and f orbitals,
respectrvel.
The agnetic quantum number (m ) describes the orbital's
to .
orientation in space. It has a value of -{
The sprn quantunn number (m) indicates the
clockvise or counterclockwise spin of an electron in a
magnetic field. ItS possible values are +1/2 and -1/2.
Every electron in an atom is assigned a unique combination of four
quzntumn numbers. No two electrons can have the same set of quantum
tumbers. Scan this QR code to
review the details of
the atomic models.

Valence Elecirons
Electrons in the outermost (i.e., highest) energy level of an atom are known as valence electrons.
itctrons are involved in chemical bonding. Valence comes from the Latin These
word valentia, which means
-<pacity. For atoms, valence pertains to the capacity of atoms to form bonds.
The distribution of electrons in an atom can be represented with electron
configuration. Electron
Configurations help determnine che number of valence electrons of an atom. For example, the electron
COntiguration of hvdrogen is s'. The superscript I is equal to the number of valence clectrons in the
itorn. For oxvgen. which has the elecCtron configuration If2r2p', the valence electrons are found in the
and ýorb1tals at =2: that is, oxygen has Gvalence electrons.
nunber of electrons
in the Subshell

Scan this QR <ode

principolavoitum to learn Ore
fumoer ot shel about electron
3ubsie| conhqiE Iion

Chapter 5 Ties That Bind 97

 Inthe periodic table, the number of valenceelectrons determines which group the element belongs
to. For example, hydrogen has one valence electron; hence, it is in Gtoup 1A (alkali metal family). All
orher elements in this family (Li, Na, K, Rb, Cs, and Fr) also contain only one valence electron. In
another example, fluorine has seven valence electrons as shown by its configuration ls»22p. This places
fluorine in Group 7A (halogen family). Table 5-1 shows the number of valence electrons and electron
configuration for each element in Periods 1 and 2. Note that He is used as a core symbol to shorten
the configuration.
TABLE 5-VALENCE ELECTRONS OF ELEMENTS IN PERIODS 1AND 2
Element Group No. Electron Configuration No. of Valence Electrons
I

1s'
He 2

Li (Hel2s!
Be 2 IHel2s2 2

B
IHel2s?2p! 3

4
IHel2s2p 4

5 IHel2s°2p 5

IHel2s2p* 6

F 7 IHel2s?2p$ 7

Ne IHel2s?2p*

Pause How do different models describe the atom?

hink 2. How do the Bohr's model and the quantum mechanical model differ in describing
Recall the location and energy of the electrons in an atom?
3 What are valence electrons?

Lesson 5.2 Lewis Electron-DotStructures of Elements

electron-dot structures.
Main ldea: Valence electrons are represented using
elements can be observed?
EQ: What patterns in electron-dot structures of

after
representative elements can be represented by Lewis electron-dot structures, named
Atoms of in chemical
Lewis (1875-1946), who proposed that only the valence electrons are involved
Gilbert N. it. The element
of an element symbol and dots around
bonding. An electron-dot structure consists atom. The dots represent the valence electrons.
nucleus and inner electrons of the
symbol represents the

Grade 9
98 Explóring Life Through Science
 LoVE OF LAB

Using clay and beads, create the

elements. Based on the strUctures, which two electron-dot structure of the Period 4
atoms can you pair up to representative
eight electrons? Find out the significance
of this method inthe next lesson. obtain a total of
.s) shows the electrON-dot
similar structures,
structures of some elements, Notice that elements in a
group have
TARLE 6-2 LeWIS ELECTRONDor STRUCTURES OF SOME
IA 2A 3A
4A
RerRESENTATIVE ELEMENTS
5A 6A
H"
7A 8A
He:
Li" Be" "B.
·O: Ne:
Ng Mg. " Al
·P:
:Cl: :Ar :
In drawing an
electron-dot
svmbol until a maximum structure, one dot is placed on each of the four
the remaining valence
of four valence electrons are represented. The dots are sides of che element
eight valence electrons,
electrons of an atom. Beryllium, carbon, and neon, then paired to represent
respectively, are represented with two, four, and eight which
dots as
have two, four, and
The maximum numþer of dots that
can be drawn around the shown in the table.
accordance with the octet rule, which will be element symbol is eight. This is in
discussed further in the next lesson.
ause 1, What are the
parts of an electron-dot structure of an
Dhink signify? element? What does each part
Recall 2. How is an electron-dot structure of an element
drawn?

Lesson 5.3 Bond Formation

Main ldea: Atoms form chemical bonds to
achieve anoble gas configuration.
What are the ways by which atoms attain an octet or a duet?

For most elements, the most stable

configuration has
Outermost energy level. This means that it has eight valencecompletely filled
s and p orbitals in the
electrons, or an octet. Atoms of noble
gases have this stable configuration. Helium, however, h¡s a stable
configuration with only two valenc
electrons, or a duet, because its valence shell contains only one s orbital. This is the reason noble gases
are generally unreactivethat is, chey do not form bonds with other atoms.

Chapter 5 Ties That Bind 99