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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

why the heart is protected with that sac
Joshua Reply
To prevent collision with the lungs, lubricates the heart, protects the heart from infection in the event a peripheral organ is effected, and stabilizes the heart within the mediastinum.
This app should be updated too much as there is very little information for some topics.I hope you will consider my information....
aman Reply
adenohypophysis is made up of what type of cells and what is the name of those cells?
Mannu Reply
armstrong Reply
motor root of the trigeminal nerve
what is the nervous system about
what passes through foramen ovale?
Farah Reply
what are the organelles of a cell
Amina Reply
muscular system
nucleus ribosome Golgi body call membrane cytoplasm
these are the cellular components that functions to provide energy,remove waste and cell division
Organelles of the cell are: Mitochondria,Ribosome,golgi apparatus, nucleus, secretory granules, nuclear e t c
what are local hormones
Local hormones are hormones that effect the cell that released them or cells near the releasing cell and they do not circulate within the blood stream.
the trachea bifurcated into how many branches on the right lung
three lobes
Explain the normal flow of blood.
how can we maintain the internal living things
Choolwe Reply
how many seconds does a human will stop if you sneeze
Kharl Reply
1 sec not specific
physiology of vision
Sudipta Reply
Can pure water become gel like?
ovie Reply
what is Homeostasis
Laura Reply
It is the ability of an organism to co ordinate it's internal environment so as to achieve balance in all areas
what is anatomy
Sandra Reply
what is physiology
The study of how the body works
the branch of biology dealing with the functions and activities of leaving organisms and their parts including all physical and chemical processes
the study of human body . phicically and chemically it's called anotomy physiology
the branch of biology dealing with the functions and activities of living organisms
which vein do we inject to give infusions
it is the scientific study of the body structure
What Choolwe Muselitata said is the definition of anatomy
Physiology can also be described as the way in which a living organism or bodily part functions
Anotomy is the science which we humen body of structure and function know as the anotomy
study of the body funtion and structure
functional study of the body
it is the study about the functions of body organs
what are the vital sign procedure
jeniffer Reply
You start the TPR then BP after explaining the procedure to the patient and your requirements ready.
which type of vein to you inject to give infusions?
how long do u take radial pulse
Flavian Reply
1 minute
You can take it for 15 seconds , the number you got you multiply wth 2
but in the aspect of the multiplying that isn't right
radial pulse kya hai?
for more accurate values you must do the 1 minute
1 minute.
60 seconds
1 minute
yes @sabina
What is homeostasis
Winter Reply
Homeostasis is the state of relative stability of the body's environment
what are fluids
fluid is a substance that has no fixed shape and yield easily to external pressure
Difference between hemostasis and homeostasis
Hemostasis is blocking or stopping blood flow from a damaged blood vessel by coagulation of that vessel or obstructing it.
Homeostasis is a balanced state. An equilibrium. The body does this by regulating itself by using hormones and neurotransmitters to keep chemicals balanced within the body.
Examples of site of homeostasis
For example, to much calcium in the blood would stimulate the release of calcitonin from the thyroid gland. Calcitonin will decrease calcium levels by depositing it into the skeleton. This is known as bone deposition, a homeostatic mechanism. Parathyroid hormone is the opposite to calcitonin.
There are a lot of homeostatic mechanisms in the body. Insulin and glucagon is another one. These two regulate glucose (sugar) levels in the blood. High glucose levels would cause insulin from the pancreas beta cells. Insulin lowers blood sugar. Glucagon increases blood sugar

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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