Atom Structure

Atom Structure

The atom is the basic building block of matter. It is the smallest unit of a chemical matter and it is composed of three elementary particles: electrons, neutrons and protons.

The protons and neutrons form the core of an atom called the nucleus, while the electrons exist outside of the nucleus and in the orbitals.

Here is a basic summary of the subatomic particles (neutrons, protons and electrons)

Electrons

• Electrons are negatively charged particles
• Electrons are located in a region of space that is outside the nucleus called orbitals.
• The valence electrons are farthest from the nucleus
• They were discovered by Joseph John Thomson
• The mass of the electron is exceedingly small at around 9.10939 x 10-28 g

Neutrons

• Neutrons are electrically neutral (no charge) particles that are located in the nucleus
• The mass of a neutron is slightly greater than that of the protons, approximately 1.67493 x 10-24 g

Protons

• Protons are positively charged particles located in the nucleus
• The mass of the proton is approximately 1.67262 x 10-24 g

Atomic Number

• Atomic Number (Z): number of protons in the nucleus of each atom of a given element
• In a neutral atom:
• Atomic number will also indicate the number of electrons
• The number of protons and neutrons are equal

Mass Number

• Mass Number (A) is the total number of protons and neutrons present in the nucleus of an atom of an element
• Mass Number = atomic number + number of neutrons
• Mass Number = (# of protons) + (# of neutrons)

Atomic Weights and Isotopes

Atomic Weight represents the mass of an atom.

• It is the weight in grams of one mole (mol) of the mass of a given element
• It is expressed in terms of g/mol

Isotopes

• Isotopes are atoms that have the same atomic number but different mass numbers.
• Isotopes have the same number of protons and electrons, but different number of neutrons.
• Example of isotopes (Hydrogen):
• Hydrogen (one proton, no neutrons)
• Deuterium (one proton, one neutron)
• Tritium (one proton, two neutrons)

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