An atom is made of 3 kinds of smaller particles:

• protons are positively charged
• neutrons have no charge and are electrically neutral
• electrons are negatively charged

The nucleus of an atom contains protons and neutrons.

Electrons orbit the nucleus in different energy levels.

Carbon plays a central role in life.

Thus Carbon has an atomic number of 6.

Atoms are electrically neutral: they always have an equal number of protons and electrons.

• How many electrons does Oxygen have?
  • 8 - the same as its atomic number

Lower electron shells possess lower energy, and electrons tend to occupy lower shells first, since they are most stable there.

The 1st shell can hold up to 2 electrons.

The 2nd and 3rd shells can hold up to 8 electrons.

Thus Carbon can form chemical reactions with many kinds of atoms.

This versatility allows Carbon to play a central role in many biological reactions.

A sodium (Na) atom has 11 protons and 11 electrons.

• It becomes a positively charged ion when it loses one electron.

Conversely, a chlorine (Cl) atom has 17 protons and 17 electrons.

• It becomes a negatively charged ion when it gains one electron.

Both ions are stable because they now have a complete complement of electrons (8) in their outermost shells.

The oxygen atom attracts the shared electrons more strongly than the hydrogen atoms; this unequal sharing of electrons makes these covalent bonds polar.

Water is a polar molecule with a partially (δ) positive "pole" at the hydrogen end and a partially (δ) negative "pole" at the oxygen end.

The partial (δ) positive charges of the hydrogen atoms of one water molecule (H₂O) are attracted to the partial (δ) negative charge of the oxygen atom of another molecule.

These weak attractions (dotted lines in the diagram) between polar molecules are hydrogen bonds.

Note that the strong bonds between hydrogen and oxygen atoms within water molecules are covalent bonds.

Tutorial

• High cohesion and surface tension.

• Good solvent for polar and ionic substances.

Cohesion due to hydrogen bonds allows water to be pulled from beneath the soil to great heights in trees.

Note: Trees take up water via roots, while photosynthesis (the process of producing their food with light) occurs in leaves.

This surface tension allows spiders and water striders to walk on water.

Water also adheres to other surfaces, forming a meniscus in a graduated cylinder.

The water molecules separate the ions from the crystal and dissolve the salt to make a solution.

Substances such as NaCl dissolved in liquid are called solutes.

Water is a good solvent for ionic or polar substances, which are said to be "hydrophilic" (water-loving) and form a solution in water.

Pure water has the same concentration of hydrogen and hydroxide ions; this is 7 (neutral) on a pH scale from 0 to 14.

If hydrogen ion concentrations increase, the pH is lower than 7 and is an acid.

If hydrogen ion concentrations decrease, the pH is greater than 7; it is a base.

An acidic solution has a high concentration of hydrogen ions with a low pH.

A basic (alkaline) solution has a low concentration of hydrogen ions with a high pH.

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