Atoms and Molecules

Atoms

Atoms

• An atom is the smallest particle of an element that retains its (element's) chemical properties.

• An atom of one element is different in size and mass from the atoms of other elements.

• An atom contains electrons surrounding a nucleus that contains protons and neutrons.

• Protons, neutrons and electrons are refered as subatomic particles.

• The protons and neutrons are located at the centre of the atom, which is called the nucleus.

  • The nucleus is positively charged.

Molecules

Molecules

• A molecule is an aggregate of two or more than two atoms of the same or different elements in a definite arrangement.

• These atoms are held together by chemical forces or chemical bonds.

• An atom is the smallest particle of a substance but cannot exist freely.

• Contrary to this, a molecule can be considered as the smallest particle of an element or of a compound which can exist alone or freely under ordinary conditions.

  • Oxygen molecule is made of two atoms of oxygen and therefore it is a diatomic molecule.

  • Phosphorus molecule consists of four atoms.

• A molecule of a substance shows all chemical properties of that substance.

• Molecules of compounds are composed of more than one kind of atoms.

  • Example, In one water molecule there are two atoms of hydrogen and one atom of oxygen.

What is atomic structure?

• Each atom is made of subatomic particles called protons, neutrons and electrons.

• The protons and neutrons are located at the centre of the atom, which is called the nucleus.

• The electrons move very fast around the nucleus in orbital paths called shells.

• The mass of the electron is negligible, hence the mass of an atom in concentrated within the nucleus (where the protons and neutrons are).

  
  

Protons, neutrons and electrons

• The atom is so tiny that we cannot really compare its mass in conventional units such as kilograms or grams; so, a unit called relative atomic mass is used.

• One relative atomic mass unit is equal to 1/12th the mass of a Carbon-12 atom.

• All other elements are measured relative to the mass of a carbon-12 atom, hence relative atomic mass has no units.

  • Hydrogen for example has a relative atomic mass of 1; it means that 12 atoms of hydrogen would have exactly the same mass of that of 1 atom of Carbon-12.

The fundamental subatomic particles

Particle	Symbol	Relative mass	Relative charge
Electron	e	1/1840	-1
Proton	p	1	+1
Neutron	n	1	0(neutral)

What is atomic number?

• The atomic number is the number of protons in the nucleus of an atom.

• The symbol for atomic number is Z.

• It is also the number of electrons present in a neutral atom; it determines the position of the element on the Periodic table.

What is mass number?

• The mass number is the total number of protons and neutrons in the nucleus of an atom.

• The symbol for the mass number is A.

• The mass number minus the atomic number gives the number of neutrons of an atom.

• Protons and neutrons collectively are also called as nucleons.

• The atomic number of an atom and ion determines which element it is:

  • Eg., helium has an atomic number of 2 (2 protons) whereas lithium has atomic number of 3 (3 protons)

• All atoms and ions of the same element have the same number of protons (atomic number) in the nucleus.

How to calculate?

Mass number = number of protons + number of neutrons

Number of protons = mass number - number of neutrons

Number of neutrons = mass number - number of protons

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