<< Chapter < Page Chapter >> Page >

The second electron shell may contain eight electrons. This shell contains another spherical s orbital and three “dumbbell” shaped p orbitals, each of which can hold two electrons, as shown in [link] . After the 1 s orbital is filled, the second electron shell is filled, first filling its 2 s orbital and then its three p orbitals. When filling the p orbitals, each takes a single electron; once each p orbital has an electron, a second may be added. Lithium (Li) contains three electrons that occupy the first and second shells. Two electrons fill the 1 s orbital, and the third electron then fills the 2 s orbital. Its electron configuration    is 1 s 2 2 s 1 . Neon (Ne), on the other hand, has a total of ten electrons: two are in its innermost 1 s orbital and eight fill its second shell (two each in the 2 s and three p orbitals); thus, it is an inert gas and energetically stable as a single atom that will rarely form a chemical bond with other atoms. Larger elements have additional orbitals, making up the third electron shell. While the concepts of electron shells and orbitals are closely related, orbitals provide a more accurate depiction of the electron configuration of an atom because the orbital model specifies the different shapes and special orientations of all the places that electrons may occupy.

Watch this visual animation to see the spatial arrangement of the p and s orbitals.

Chemical reactions and molecules

All elements are most stable when their outermost shell is filled with electrons according to the octet rule. This is because it is energetically favorable for atoms to be in that configuration and it makes them stable. However, since not all elements have enough electrons to fill their outermost shells, atoms form chemical bonds with other atoms thereby obtaining the electrons they need to attain a stable electron configuration. When two or more atoms chemically bond with each other, the resultant chemical structure is a molecule. The familiar water molecule, H 2 O, consists of two hydrogen atoms and one oxygen atom; these bond together to form water, as illustrated in [link] . Atoms can form molecules by donating, accepting, or sharing electrons to fill their outer shells.

In the first image, an oxygen atom is shown with six valence electrons. Four of these valence electrons form pairs at the top and right sides of the valence shell. The other two electrons are alone on the bottom and left sides. A hydrogen atom sits next to each the lone electron of the oxygen. Each hydrogen has only one valence electron. An arrow indicates that a reaction takes place. After the reaction, in the second image, each unpaired electron in the oxygen joins an electron from one of the hydrogen atoms so that the valence rings are now connected together. The bond that forms between oxygen and hydrogen can also be represented by a dash.
Two or more atoms may bond with each other to form a molecule. When two hydrogens and an oxygen share electrons via covalent bonds, a water molecule is formed.

Chemical reactions occur when two or more atoms bond together to form molecules or when bonded atoms are broken apart. The substances used in the beginning of a chemical reaction are called the reactants (usually found on the left side of a chemical equation), and the substances found at the end of the reaction are known as the products (usually found on the right side of a chemical equation). An arrow is typically drawn between the reactants and products to indicate the direction of the chemical reaction; this direction is not always a “one-way street.” For the creation of the water molecule shown above, the chemical equation would be:

2 H + O   H 2 O

An example of a simple chemical reaction is the breaking down of hydrogen peroxide molecules, each of which consists of two hydrogen atoms bonded to two oxygen atoms (H 2 O 2 ). The reactant hydrogen peroxide is broken down into water, containing one oxygen atom bound to two hydrogen atoms (H 2 O), and oxygen, which consists of two bonded oxygen atoms (O 2 ). In the equation below, the reaction includes two hydrogen peroxide molecules and two water molecules. This is an example of a balanced chemical equation    , wherein the number of atoms of each element is the same on each side of the equation. According to the law of conservation of matter, the number of atoms before and after a chemical reaction should be equal, such that no atoms are, under normal circumstances, created or destroyed.

Questions & Answers

What is the Antibiotic
Tamara Reply
antibiotics is any medication that stop the growth of bacteria
onuoha
what is biomolecules
Lawerence Reply
This is a group of molecules produced by a living organism
Odion
thank alot,I had a hard time getting the answer
Lawerence
wat is the meaning of Mr.niger. d
It Reply
Now Mr. Niger Dac M;movement R;respiration N;nutrition G;growth E;excretion R;reproduction D;death A;adaptation C;competition
Odion
Sorry.... I;irritability
Odion
GOOD MORNING MY NAME IS MESUMBE PRECIOUS WHAT IS YOUR NAME
Mesumbe Reply
ccc
Peace
hi
Emmanuel
Emmanuel Daniel
Emmanuel
what is the meaning of Mr Niger?
Emmanuel
state all the elements and their symbols
Taiwo Reply
what types of muscles are found in the heart
Diamond Reply
Cardiac muscle
Divya
cardiac or myocardia muscle
onuoha
cardiac muscles
Elvis
cardiac muscle
Jemima
Cardiac muscle
Dr
Thank you dear!
yimam
please can you tell me the meaning of Mr Niger?
Emmanuel
It is an initial to represent life processes of organisms. M means movement, R for reproduction, N stands for nutrition, I for irritability or sensitivity,G means growth , E for excretion and R stands for respiration.
Quartey
cardiac muscle
Stanisla
what is digestion
Dolla Reply
it is the chemical break down of insoluble food substances such as fatty acid to soluble substances which are then used for body processes
Lawerence
does human being sperm performed in the ovary of goat
sadeeq Reply
levels of ecological study
sadeeq
no
Dolla
human sperms contains acid
Dolla
what is an artery
Dolla
It is one of the organs of the circulatory system that carries oxygenated blood from the heart to other parts of the body.
Quartey
artery is type of blood vessel which carries blood away from heart...
shams
how liver destroy red blood cells?
shams
big bang theory was discovered by
Sweety Reply
Georges Lemaître
Mr
the meaning of nucleus
Spring Reply
nucleus Is also known as the cell's headquarters
Feranmi
what is pathology?
Azi Reply
is the study of diseases, it's cause and development of a disease
Boadi
State the law of segregation
Theolla Reply
what is a cell
Bunmi Reply
A cell is a basic structural unit of an organism
Addo
what are molecules?
Veronica
molecule:a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction.
Black
cell is basic structural and functional unit of life which can perform all the metabolic activities
Prakash
which organ function as modulator & effector in homeostasis glucose level ( Liver & islet of Langerhans ) & How ?
Dr
a cell is the basic unit of life
Taiwo
A cell is the basic unit of life
Odion
basic structural and functional unit of any living organism
Lawerence
comapare and contrast food chain
Chengo Reply
what are the classes of biology
Oluebube Reply
Microbiology Biochemistry Botany Zoology Horticulture
Odion

Get the best Biology course in your pocket!





Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask