Over centuries many theories were developed but today only a few remain valid. One of the most “talked-about” theories was the one made by John Dalton, a physicist, meteorologist and a chemist. One of the main reasons why his work is so popular today is because some of his discoveries have helped in the development of the modern atomic theory. There are however cases were his theory is a little different from modern theories.

John Daltons Theory Stated that:

1. All matter is made up of atoms.
2. Atoms are indestructible and indivisible.
3. Atoms of the same element have identical properties and mass.
4. A chemical reaction is a rearrangement of atoms.
5. A compound is formed by a combination of two or more different kinds of atoms.

Cases where Daltons Theory is different:

• John Dalton’s theory stated that atoms of the same element have identical masses but modern theories have proved that atoms of the same element can have different masses, these atoms are called isotopes however they do have the same chemical properties.
• In modern research it was discovered that atoms can be destroyed by nuclear reactions but not by chemical reactions.

The following table summarizes the changes in theories over time:

Name	Discovery/Theory
Democritus, 460 BC	Came up with the idea of atoms
John Dalton, 1808	After the idea of atoms was dismissed for over 2000 years, he reintroduced the idea of atoms and through many experiments he proved the existence of atoms.
Joseph .J.Thompson, 1898	Stated that positively charged substances were distributed uniformly over the surface of the atom containing electrons embedded within them.
Ernest Rutherford, 1910	From J.Thompson’s model and through his own experiments he was able to determine the true model of the atom. Instead of the positively charged particles being distributed over the atom they were gathered together in the nucleus.
Niels Bohr, 1913	He came up with these rules: Electrons can only orbit a fixed distance from the nucleus. Atoms lose energy when electrons go from higher energy to lower energy orbits and the opposite happens from lower energy to higher energy orbits.
Wolfgang Pauli, 1924	Stated that electrons spin while orbiting the nucleus. He also came up with his own principle known as the Pauli Exclusion Principle which stated that no two electrons of a given atom can have the same set of quantum numbers.

Water plays a vital role in the survival of different organisms. Water is said to occupy more than 70% of the earth’s surface and occupies approximately 60% of volume in plants and animals. Without water human and all life existence would be impossible. The diagram below shows the structure of a molecule of water, it is formed by the bonding of two hydrogen atoms to a central oxygen atom.

Looking at the diagram above one can see how the hydrogen atoms are bonded to the oxygen causing a polarity in the molecule; it is this polarity that causes more molecules to attract to each other through strong molecular bonds. When water changes phase the molecules are arranged differently this is further explained in the topic Different States of matter

Here are some physical properties of water and how they relate to their role and functions:

1.  Water is a universal solvent: - This means it is capable of dissolving a wide variety of chemical substances, this feature allows water to carry solvent nutrients from infiltration, ground water flow, runoff and in different living organisms.
2. Water is a Good Conductor of Heat: - It is said to conduct heat better than any other liquid except mercury. This feature is especially useful to lakes and other large water bodies; this allows them to maintain a uniform vertical temperature profile.
3. Water has a High Surface Tension: - Meaning it has an elastic as well as adhesive structure. This allows it to aggregate in drops and not to spread over a surface as a thin film. It also sticks to the sides of vertical surfaces and not fall due to gravity. This feature is especially useful to plants, it enables them to move water from the roots to their leaves, and also the movement of blood in blood vessels in animals.
4. Water has a high Specific heat: - The specific heat of a substance is the amount of heat required to change its temperature, therefore seeing that it has a high specific heat capacity it has to absorb allot of energy to get hot which takes quite some time and also releases energy slowly when in a cool setting. This helps to explain why sea breeze occurs at day but land breeze occurs at night, because it has a higher specific heat. This also helps in the moderation of the earth’s climate and also in the regulation of body temperature.
5. Water has a neutral pH in the pure state: - This feature enables it to be neither acidic nor basic but changes pH when added to another substance
6. Water is able to remain liquid over a wide range of temperatures 0-100 degrees Celsius: - This feature is of great importance because it is able to remain in the liquid form in most places on earth.

 The late scientists Schwann and Schleiden devised a theory known as the Cell Theory which posed that a cell is the basic unit of structure and function in living organisms. A cell is surrounded by a membrane known as the Cell Surface Membrane which is a thin membrane acting as a barrier for the cell contents, its function is to control what molecules enter and leave the cell. All cells contain a structure known as the nucleus which comprises the chromatin, which is a coiled form of a chromosome, and the nucleolus. Next is the cytoplasm which contains the organelles, and is a jelly-like structure found between the cell surface membrane and the nucleus. Despite these common structures present in both animal and plant cells, they also have distinguishing features.

The table below analyzes the common differences between plant and animal cells.

Plant Cell	Animal Cell
Has a rigid cell wall	Has no cell wall
Has Chloroplasts	Has no chloroplast
Has a Large Central vacuole	May have small vacuoles

The image below shows more differences and similarities as well as the structures of  the plant and animal cell.

Image taken from evolution.berkeley.edu