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CHEMICAL BONDS

Companion Text:

Herlihy, B, The Human Body in Health and Illness, 4th Edition, 2011, Saunders Elsevier

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Atoms are attracted to each other because they want to achieve a stable outer electron shell …

In other words, they want either to fill or empty the outer electron shell

Herlihy, B, 2011, p.17

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The force of attraction that occurs between the atoms is like the force of two magnets

When you try to separate the magnets you can still feel the pull

(Herlihy 2011)

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The electrical attraction between atoms is called a chemical bond

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There are three (3) kinds of chemical bond

• Ionic Bonds• Covalent Bonds• Hydrogen Bonds

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IONIC BONDS

An ionic bond happens when electrons are transferred between atoms

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Rather like moving house!

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COVALENT BOND

In a covalent bond, the outer shells of the atoms share electrons

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Covalent bonding is like holding hands

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Water happens because of covalent bonding …

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Covalent Bond

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Covalent bonds are strong and do not break apart in water …

Proteins and Carbohydrates are formed by covalent bonding.

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The strength of these bonds is important because the protein produced by the body must not fall apart when exposed to water

Herlihy, B, p.17 (2011)

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Many proteins, such as hormones, are transported around the body in blood, and blood is made up mainly of water’

If the covalent bonds of the protein broke apart in water the hormones would not be able to accomplish their tasks

Herlihy, 2011, p.17

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Many chemical reactions that happen in the body involve carbon.

Because of this a separate branch of chemistry called organic-chemistry studies only carbon-containing substances

Inorganic chemistry studies non-carbon-containing substances

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HYDROGEN BONDS

This kind of bond is different to ionic and covalent bonds

A hydrogen bond is not caused by either the transfer or the sharing of electrons in the outer shells of the atom

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Hydrogen Bonds are best described as the weak attraction between water molecules

Herlihy, B, P.18 (2011)

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Water is composed of hydrogen and oxygen

The weak positive charge around the hydrogen of one water molecule is attracted to the weak negative charge of the oxygen in a second water molecule

Herlihy, 2011, p.18

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POLARITY

Water engages in hydrogen bonding because it is a polar molecule

A polar molecule has a ‘lop-sided’ charge (a + end and a – end)

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02

H+

Pos

Neg

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Hydrogen Bond

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Several other terms are related to the activity of electrons in the outer shells of the atoms …

Herlihy, 2011, p.19

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Electrons, for instance carry a (-ve) charge, and if they are lost from or gained by the outer shell of an atom, the electrical charge of the atom changes

Herlihy, 2011, p.19

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The electrical charge of the atom or element changes from a neutral charge (no charge) to either a positive (+) or a negative (-) charge

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Atoms that carry an electrical charge are called ions

If an ion is positively charged it is called a cation

If an ion is negatively charged it is called an anion

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An electrolyte is a substance that forms ions when it is dissolved in water

Electrolytes can conduct an electrical current

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For Example …

The electrocardiogram (ECG) records the electrical events of the heart.

It is the movement of ions through the tissues that cause the electrical events to be recorded on the ECG

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So how are ions formed?Ions are formed when the outer shell of the atom loses or gains electrons

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For instance …

The sodium atom has 11 protons (+ve) and 11 electrons (-ve) …

If a single electron is donated the sodium is left with 11 positive (+) charges and only 10 negative (-) charges

The sodium is said to carry a net charge of +1

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The sodium ion is therefore a cation and is represented as Na+

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and …

The chlorine atom has 17 protons (+) and 17 electrons (-ve)

If an electron is gained, the chlorine then contains 17 (+) charges and 18 (-) charges

The chlorine has a net charge of -1, so it is an anion

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Some atoms give up more than one (-ve) electron so they have a more positive charge

Calcium is an example, because it gives up 2 electrons – its chemical symbol is therefore Ca2+

Herlihy, 2011, p.19

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Ionization

Ionization happens when an electrolyte splits or breaks apart in solution

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For Example …

NaCl (sodium chloride) is an electrolyte.

In its solid state it looks like tiny white granules

When it is dissolved in water the table salt dissociates

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The ionic bonds holding the sodium and chloride together weaken and the solid NaCl splits into Na+ (sodium ion) and Cl- (chloride ion).

The NaCl has dissociated and because the products are ions, the process is called ionization

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Remember!Only electrolytes ionize

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What is a Molecule?A molecule is formed when two or more atoms bond together …

Two identical atoms can bond, for instance –one atom of oxygen can bond with another atom of oxygen to form a molecule of oxygen which is called 02

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A molecule can also be formed when atoms of different elements combine.

For example – when 2 atoms of hydrogen combine with 1 atom of oxygen a molecule of water is formed

Herlihy, 2011, p.20

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COMPOUNDSA substance that contains molecules formed by two or more different atoms is called a compound …

We can use the example of water again – when 2 atoms of hydrogen and 1 atom of oxygen combine

Water is considered to be both a molecule and a compound

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Some Important Compounds & Molecules

• Water • Oxygen• Carbon Dioxide

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Water …• Most abundant compound in the body

• Makes up nearly 2/3 of an adult’s body weight

• Essential to life

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UNIVERSAL SOLVENT

Water is the universal solvent because most substances dissolve in water

When substances are dissolved in water they move more easily and this facilitates their involvement in chemical reactions

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REGULATES TEMPERATURE

Water can absorb large amounts of heat without dramatically increasing the temperature of the water itself

This means that heat can be removed from heat-producing tissue, like exercising muscle, while the body maintains a normal body temperature

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WATER AS A LUBRICANT

Water is a major component of mucus and other lubricating fluids

These lubricating fluids decrease friction when two lubricating surfaces slide past each other

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WATER IN CHEMICAL REACTIONS

Water often has a crucial role to play in chemical reactions

We need water to break down carbohydrates during digestion

Herlihy, 2011, p.20

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WATER AS A PROTECTIVE DEVICE

Water also protects important structures.

The cerebrospinal fluid that surrounds and cushions the brain and spinal cord is an example

Amniotic fluid surrounds and cushions the developing infant in its mother’s womb

Herlihy, 2011, p.20

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OXYGEN

Oxygen (02) exists in nature as a gas and is essential to life

There is 21% of Oxygen in the air that we breath

Oxygen is used by the cells to liberate energy from the food we eat. This energy powers the body

If there is no energy, the body like a motor without fuel stops running and we die.

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CARBON DIOXIDE

Carbon Dioxide (CO2) is a compound that consists of one carbon atom and two oxygen atoms …

CO2 is a waste product and must be eliminated from the body.

It is made when food is chemically broken down for energy

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CHEMICAL REACTIONS

A chemical reaction is a process whereby the atoms of molecules or compounds interact and form new chemical combinations

For example, glucose interacts with oxygen to form carbon dioxide, water and energy

Herlihy, 2011, p.21

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CHEMICAL REACTIONS … Cont.

The rate of chemical reactions are importantMost of them take place quite slowly , and chemical substances called catalysts can speed them upEnzymes are proteins that are acting as catalystsMost chemical reactions need a catalyst or an enzyme

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REFERENCEHerlihy, B, The Human Body in Health and Illness, 4th Edition, 2011, Saunders Elsevier