Atoms and Molecules

Atoms and Molecules; Everything around us in the world we occupy comes from atoms and molecules, including the air we breathe, the water we drink, and ourselves as objects. These building blocks of matter signal how everything is made. By knowing how they work, we can better understand the chemical world in which we live. This page will try to simplify the concepts behind atoms and molecules so you will be able to appreciate them.

Table of Contents

• About Atoms
• Understanding Atomic Size
• About Molecules
• Understanding Molecules with Examples
• An Overview of Dalton's Atomic Theory
• Interactions Between Atoms and Molecules

About Atoms

Atoms are the basic building blocks of matter. They are the smallest units of an element that retain the properties of that element. An atom consists of a central nucleus made up of protons and neutrons, surrounded by electrons in orbitals. 

Understanding Atomic Size

An atom is very small, often measured in nanometers. An atom's size depends on the element and how electrons are arranged around the nucleus. 

An atom's size cannot be measured with a ruler; scientists determine it based on the distance between the centres of two adjacent atoms. By taking half of that distance, we can estimate the atomic radius, which gives one a reasonable feel for how large an atom may be, the size and arrangement of the atoms that defines many properties of materials.

Atomic size is usually expressed in nanometres (nm), which is appropriate for the scale of atoms. 

1 m = 10⁹ nm

Atoms vary in size depending on their atomic number and the number of shells of electrons surrounding the nucleus of the atom. Hydrogen is one of the smallest atoms, while caesium is one of the bigger atoms.

Knowing relative sizes of atoms helps in estimating how atoms might bond and react.

An Atom's Parts

An atom consists of three parts:

• Protons (positive charge)
• Neutrons (neutral)
• Electrons (negative charge)

The protons and neutrons form the nucleus which is dense. The electrons orbit around the nucleus in energy levels.

Now, Atomic mass is the total mass of an atom. To calculate atomic mass you add the number of protons and neutrons in the nucleus together. The mass of electrons is relatively insignificant because it is so small. 

Atomic mass indicates the element and is useful for predicting the behavior of elements in chemical reactions.

About Molecules

A molecule is a bunch of two or more atoms that are chemically bonded. Molecules can be made up of the same type of atom, like O₂ (oxygen) or different atoms, like H₂O (water). 

Molecules are the smallest units of a substance that still have the substance's chemical properties.

Understanding Molecules with Examples

Molecules are formed when atoms bond together. These bonds can be between atoms of the same element or different elements. Examples include:

• O₂:A molecule of oxygen gas
• CO₂: A molecule of carbon dioxide
• H₂O: A water molecule

Molecules are responsible for the chemical nature and reactions of substances.

An Overview of Dalton's Atomic Theory

In the early 1800s, John Dalton proposed a theory explaining the nature of matter. Dalton's theory has the following points:

• All matter is made of atoms.
  
  
• Atoms are indivisible and cannot be destroyed.
• Atoms of a given element are identical to each other in mass and properties.
  
  
• Atoms combine together in fixed ratios to form compounds.

Dalton's theory has since been amended, but it is the basis for atomic theory today.

Interactions Between Atoms and Molecules

Atoms and molecules always interact in many different ways. Ionic or covalent bonding, attraction, and repulsion are all examples of interactions. The atoms and molecules found in substances contain all of the properties that contribute to the physical and chemical properties of that substance. 

Atoms and molecules rarely "float around" on their own as they are constantly interacting with one another. 

Interactions can be broadly classified as 

• intramolecular (inside a molecule) 
• intermolecular (between molecules).

For example, water is formed when many water molecules stick together in a way known as hydrogen bonding, and those bonds are what provide water with its unique physical and chemical properties.

As we have learned,Atoms and molecules are very small, but they have a tremendous influence on everything around us. They determine how substances will form, and drive the chemical reactions that sustain life. Sodium chloride is the chemical structure that describes table salt.