What is an atom?

• All substances are made of atoms.
• An atom is the smallest particle of an element.

Structure of Atom

Atoms are made up of subatomic particles called protons, neutrons and electrons.

• Protons and neutrons are packed into a nucleus at the center of the atom.
• Electrons orbit around the nucleus in different shells.

• Protons and neutrons have a relative mass of 1. Compared to protons and neutrons, the mass of electrons is negligibly small.
• Protons have 1+ charge, and electrons have 1- charge.

Atomic number: the number of protons in an atom

• All atoms of the same element element have the same atomic number. In other words, all atoms of an element have the same number of protons.
• Since atoms are electrically neutral, atoms have the same number of electrons and protons.
• Atoms of different elements have different atomic number.

• Different atoms of the same element can have different mass number.
• Since all atoms of the same element have the same number of protons, it means that they can have different number of neutrons.

Atoms can be represented as atomic symbols:

Isotope: Atoms of the same element that have different number of neutrons

• Isotopes have the same atomic number and same number of protons and electrons.
• Isotopes have different mass number and different number of neutrons.
  • Number of neutrons = mass number - atomic number

For example, hydrogen has three common isotopes.

• Hydrogen
  • Number of proton: 1
  • Number of electron: 1
  • Number of neutron: 1 - 1 = 0
• Deuterium
  • Number of proton: 1
  • Number of electron: 1
  • Number of neutron: 2 - 1 = 0
• Tritium
  • Number of proton: 1
  • Number of electron: 1
  • Number of neutron: 3 - 1 = 0

Relative Atomic Mass (A_r): 

• Weighted average of the masses of isotopes of an element
• Takes into account the abundance of isotopes of an element

Calculating relative atomic mass of chlorine:

Chlorine exists as two isotopes in nature, (chlorine-35) and (chlorine-37). The abundance of chlorine-35 is 75%, and the abundance of chlorine-37 is 25%. 

The relative atomic mass of chlorine is:

A_r = 35 x 0.75 + 37 x 0.25 = 35.5

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