# 2.1 Atoms, isotopes, ions, and molecules: the building blocks  (Page 8/61)

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Even though all of the reactants and products of this reaction are molecules (each atom remains bonded to at least one other atom), in this reaction only hydrogen peroxide and water are representatives of compounds : they contain atoms of more than one type of element. Molecular oxygen, on the other hand, as shown in [link] ,consists of two doubly bonded oxygen atoms and is not classified as a compound but as a mononuclear molecule.

Some chemical reactions, such as the one shown above, can proceed in one direction until the reactants are all used up. The equations that describe these reactions contain a unidirectional arrow and are irreversible . Reversible reactions are those that can go in either direction. In reversible reactions, reactants are turned into products, but when the concentration of product goes beyond a certain threshold (characteristic of the particular reaction), some of these products will be converted back into reactants; at this point, the designations of products and reactants are reversed. This back and forth continues until a certain relative balance between reactants and products occurs—a state called equilibrium    . These situations of reversible reactions are often denoted by a chemical equation with a double headed arrow pointing towards both the reactants and products.

For example, in human blood, excess hydrogen ions (H + ) bind to bicarbonate ions (HCO 3 - ) forming an equilibrium state with carbonic acid (H 2 CO 3 ). If carbonic acid were added to this system, some of it would be converted to bicarbonate and hydrogen ions.

${{\text{HCO}}_{3}}^{-}{\text{+ H}}^{+}\text{}↔{\text{H}}_{2}{\text{CO}}_{3}$

In biological reactions, however, equilibrium is rarely obtained because the concentrations of the reactants or products or both are constantly changing, often with a product of one reaction being a reactant for another. To return to the example of excess hydrogen ions in the blood, the formation of carbonic acid will be the major direction of the reaction. However, the carbonic acid can also leave the body as carbon dioxide gas (via exhalation) instead of being converted back to bicarbonate ion, thus driving the reaction to the right by the chemical law known as law of mass action    . These reactions are important for maintaining the homeostasis of our blood.

## Ions and ionic bonds

Some atoms are more stable when they gain or lose an electron (or possibly two) and form ions. This fills their outermost electron shell and makes them energetically more stable. Because the number of electrons does not equal the number of protons, each ion has a net charge. Cations are positive ions that are formed by losing electrons. Negative ions are formed by gaining electrons and are called anions. Anions are designated by their elemental name being altered to end in “-ide”: the anion of chlorine is called chloride, and the anion of sulfur is called sulfide, for example.

This movement of electrons from one element to another is referred to as electron transfer    . As [link] illustrates, sodium (Na) only has one electron in its outer electron shell. It takes less energy for sodium to donate that one electron than it does to accept seven more electrons to fill the outer shell. If sodium loses an electron, it now has 11 protons, 11 neutrons, and only 10 electrons, leaving it with an overall charge of +1. It is now referred to as a sodium ion. Chlorine (Cl) in its lowest energy state (called the ground state) has seven electrons in its outer shell. Again, it is more energy-efficient for chlorine to gain one electron than to lose seven. Therefore, it tends to gain an electron to create an ion with 17 protons, 17 neutrons, and 18 electrons, giving it a net negative (–1) charge. It is now referred to as a chloride ion. In this example, sodium will donate its one electron to empty its shell, and chlorine will accept that electron to fill its shell. Both ions now satisfy the octet rule and have complete outermost shells. Because the number of electrons is no longer equal to the number of protons, each is now an ion and has a +1 (sodium cation) or –1 (chloride anion) charge. Note that these transactions can normally only take place simultaneously: in order for a sodium atom to lose an electron, it must be in the presence of a suitable recipient like a chlorine atom.

what are the organelles in cell that involves in protein sythenis
what is a melanin?
Why do parasites take on a parasitic life?
what is cell
A cell is the basic structure unit of an organ
Yaa
what is respiration
Igwe
What will to a cell if the nucleus is removed..?
When a cell nucleus removed then the cell will not be able to function properly, it will not be able to grow .All the metabolic functioning of the cell will stop .Without nucleus the cell will lose it's control. It can not carry out cellular reproduction .
Evelyn
write a short note on how the kidney carry out osmoregulation in man
Kidney play a very large role in human osoregulation by regulating the amount of water reabsorbed from glomerular filtrate in kidney as tubules, which is controlled by hormones such as antrdiuvetic hormone (ADH) , aldosterone and angiotensin.
Evelyn
kidney is capable of osmoregulation in mammal ,
Judith
simple definition for respiratory system
Respiratory system is a network of organs and tissues that helps you to breathe or help in getting the rids of oxygen and discharge of carbon dioxide in the body.
Ruhiyatu
What are uses of respiratory system
Joster
how it functions
Ousman
what is inresparetion
Fikkabex
Why do parasites take on a parasitic life?
how is a aerenchyma tissue adapted to its function
Have large air spaces that store air for gaseous exchange... Large air spaces also facilitate bouyancy.
Goodrick
what does DNA mean
dioxiribo nucleic acid
Lekan
Deoxyribonucleic acid
Goodrick
dioxide nucleic acid
what does RNA mean
Ribonucleic acid
Goodrick
what is DNA and RNA
Miriam
what is a zygote
zygote is developed or fertilized egg cell
Lekan
what is the difference between cell wall and cell membrane
cell wall is found in plant while cell membrane is found in animal cell
Lekan
please this is the main answer to that question okay Cell wall gives shape and support to the cell whiles Cell membrane support the movement of substances into and out of the cell. This question is very tricky that's why I asked.
Ruhiyatu
cell wall makes the cell turgid,in times of flaccidity while cell membrane is a semi permeable tissue
Judith
how cell I form
what difference between animal cell and plant cell
what is animal call
Rita
Animal cell does not have a cell wall but plant cell have a cell wall
Ruhiyatu
plant have cell Wall well animal have not cell wall
Yula
animal cell has a lysosome, while, plant cell does not
Judith
what is fertilization?
Fertilization is the fusion of male sex cell and female sex cell to form a zygote.
Ruhiyatu
What kind of nutrients is composed of plants
phosphorus,potassium,nitrogen
Kritika
NPK
Ruhiyatu