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Lithium (Li), whose atomic number is 3, has three electrons. Two of these fill the first electron shell, and the third spills over into a second shell. The second electron shell can accommodate as many as eight electrons. Carbon, with its six electrons, entirely fills its first shell, and half-fills its second. With ten electrons, neon (Ne) entirely fills its two electron shells. Again, a look at the periodic table reveals that all of the elements in the second row, from lithium to neon, have just two electron shells. Atoms with more than ten electrons require more than two shells. These elements occupy the third and subsequent rows of the periodic table.

Electron shells

This four panel figure shows four different atoms with the electrons in orbit around the nucleus.
Electrons orbit the atomic nucleus at distinct levels of energy called electron shells. (a) With one electron, hydrogen only half-fills its electron shell. Helium also has a single shell, but its two electrons completely fill it. (b) The electrons of carbon completely fill its first electron shell, but only half-fills its second. (c) Neon, an element that does not occur in the body, has 10 electrons, filling both of its electron shells.

The factor that most strongly governs the tendency of an atom to participate in chemical reactions is the number of electrons in its valence shell. A valence shell    is an atom’s outermost electron shell. If the valence shell is full, the atom is stable; meaning its electrons are unlikely to be pulled away from the nucleus by the electrical charge of other atoms. If the valence shell is not full, the atom is reactive; meaning it will tend to react with other atoms in ways that make the valence shell full. Consider hydrogen, with its one electron only half-filling its valence shell. This single electron is likely to be drawn into relationships with the atoms of other elements, so that hydrogen’s single valence shell can be stabilized.

All atoms (except hydrogen and helium with their single electron shells) are most stable when there are exactly eight electrons in their valence shell. This principle is referred to as the octet rule, and it states that an atom will give up, gain, or share electrons with another atom so that it ends up with eight electrons in its own valence shell. For example, oxygen, with six electrons in its valence shell, is likely to react with other atoms in a way that results in the addition of two electrons to oxygen’s valence shell, bringing the number to eight. When two hydrogen atoms each share their single electron with oxygen, covalent bonds are formed, resulting in a molecule of water, H 2 O.

In nature, atoms of one element tend to join with atoms of other elements in characteristic ways. For example, carbon commonly fills its valence shell by linking up with four atoms of hydrogen. In so doing, the two elements form the simplest of organic molecules, methane, which also is one of the most abundant and stable carbon-containing compounds on Earth. As stated above, another example is water; oxygen needs two electrons to fill its valence shell. It commonly interacts with two atoms of hydrogen, forming H 2 O. Incidentally, the name “hydrogen” reflects its contribution to water (hydro- = “water”; -gen = “maker”). Thus, hydrogen is the “water maker.”

Chapter review

The human body is composed of elements, the most abundant of which are oxygen (O), carbon (C), hydrogen (H) and nitrogen (N). You obtain these elements from the foods you eat and the air you breathe. The smallest unit of an element that retains all of the properties of that element is an atom. But, atoms themselves contain many subatomic particles, the three most important of which are protons, neutrons, and electrons. These particles do not vary in quality from one element to another; rather, what gives an element its distinctive identification is the quantity of its protons, called its atomic number. Protons and neutrons contribute nearly all of an atom’s mass; the number of protons and neutrons is an element’s mass number. Heavier and lighter versions of the same element can occur in nature because these versions have different numbers of neutrons. Different versions of an element are called isotopes.

The tendency of an atom to be stable or to react readily with other atoms is largely due to the behavior of the electrons within the atom’s outermost electron shell, called its valence shell. Helium, as well as larger atoms with eight electrons in their valence shell, is unlikely to participate in chemical reactions because they are stable. All other atoms tend to accept, donate, or share electrons in a process that brings the electrons in their valence shell to eight (or in the case of hydrogen, to two).

Visit this website to view the periodic table. In the periodic table of the elements, elements in a single column have the same number of electrons that can participate in a chemical reaction. These electrons are known as “valence electrons.” For example, the elements in the first column all have a single valence electron—an electron that can be “donated” in a chemical reaction with another atom. What is the meaning of a mass number shown in parentheses?

The mass number is the total number of protons and neutrons in the nucleus of an atom.

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Questions & Answers

what is Endocrine system?
Islam Reply
which secrete hormones and other products direct into the blood
Cell is basic, structural and functional unit of life
Kabuja Reply
The cell is the structural and functional unit of all living organisms, and is sometimes called the "building block of life." Some organisms, such as bacteria, are unicellular, consisting of a single cell.
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what is the basic function of the lymphatic system
The other main function is that of defense in the immune system. Lymph is very similar to blood plasma: it contains lymphocytes. It also contains waste products and cellular debris together with bacteria and proteins. Associated organs composed of lymphoid tissue are the sites of lymphocyte producti
the function of lymphatic system are 1fluid balance 2 lipid absorption and 3 defence
destroyed microognism
lymphatic systems main function is to transport lymph
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Which of the following accurately describe external respirations
Robin Reply
different between anatomy and physiology
Samwel Reply
anatomy is the study of STRUCTURE of the body while physiology is the study of the function of the part of the body
Anatomy deals with the structure and parts of the body while physiology is the function of the the body parts
not understanding what is a cell
Kesa Reply
its the fundamental unit of life or its the primary step in which two or more cell combine to form a tissue .
Is the smallest structural and functional unit of life
cell is the basic you unit of life
what is osteomalàcia
Ellen Reply
what is hydroxyapitate
Hydroxyapatite, also called hydroxylapatite, is a naturally occurring mineral form of calcium apatite with the formula Ca₅(PO₄)₃, but it is usually written Ca₁₀(PO₄)₆(OH)₂ to denote that the crystal unit cell comprises two entities. Hydroxyapatite is the hydroxyl endmember of the complex apatite gro
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osteomalacia is where inorganic or mineral which is calcium and phosphorus are removed from a bone....this will make the bone become flexible n in children is called ricket
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SULE Reply
what is the meaning of suture?
praba Reply
it is a protective layer of brain
it is a kind of non movable joint present between skull bones
suture is an example of fibrous joint and is synatrosys(is not movable)
a junction between the sclerites of an imsect's body
c organ,organ system, organism
Dorothy Reply
reproductive sytstem
describe structure and functions of spleen
Umesh Reply
why calcium does not prodecessed by biological process?
James Reply
when calcium isn't processed that becomes Calcium Homeostasis
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What are the functions of batholine glan
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what is pathogen
Nafiw Reply
an organism who invades the susceptible host
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Tenderness A She tried to show me how to love D I bit her lips and bruised ...
is not
Pathogens are decease causing agents.... it can be a viral infection or fungai infection... thank u
How do we measure power in skeletal muscles....
Terry Reply
what is sternum?
Lovely Reply
pretty much the body/base that holds the ribs also known as breast bone
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The sternum or breastbone is a long flat bone located in the center of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury.
Is the bone found in between sternal or breast bone.
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its a flat bone between the ribs..
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@Asi Deborah, Do you happen to need any assistance?
A sturnum... is a bone that joins together with the ribs to form the thoracic cage.... it has the shape of a dagger... thank u
please can someone explain how diabetes causes cataract
sternum is a flat bone which is in the centre of the thoracic cavity (chest) it connects the true ribs 1_7 directly through cartilages and it protects the heart and the lungs from injury
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where is the stapes bone is located?
inner ear
*middle ear
how many vertebral column in human body
Name and describe the four special functional characteristics of muscle that are the basic for muscle response
discuss the relative value of immovable, slightly movable and freely moveble joints
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what is the name of the major/largest cavity found in hypogastric region?
is abdominal cavity

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