3.5 Fundamental Particles of Atom—Atomic & Mass number

Atoms consist of three fundamental particles: electrons, protons, and neutrons. These particles have different characteristics and roles within the atom.

Electron:

• Charge: Negative (-1.60 × 10^-19 coulombs)

• Mass: 9.11 × 10^-31 g

• Symbol: e

• Relative Charge: -1

• Relative Mass: 0 (since the electron’s mass is approximately 1/1840 of a proton’s mass)

The electron is a fundamental particle of an atom with a negative charge, moving around the nucleus of the atom.

Proton:

• Charge: Positive (+1.60 × 10^-19 coulombs)

• Mass: 1.67 × 10^-24 g

• Symbol: p

• Relative Charge: +1

• Relative Mass: 1

A proton is another fundamental particle that carries a positive charge and resides in the nucleus of the atom.

Neutron:

• Charge: Neutral (no charge)

• Mass: 1.675 × 10^-24 g

• Symbol: n

• Relative Charge: 0

• Relative Mass: 1

Neutrons, like protons, reside in the nucleus but have no charge. Their mass is slightly greater than that of protons. Neutrons are found in all elements except hydrogen.

Table 3.05: Characteristics of Fundamental Particles

Name of Particle Symbol Actual Charge Actual Mass Relative Charge Relative Mass

Electron e -1.60 × 10^-19 coulombs 9.11 × 10^-31 g -1 0

Proton p +1.60 × 10^-19 coulombs 1.67 × 10^-24 g +1 1

Neutron n 0 (neutral) 1.675 × 10^-24 g 0 1

3.5.1 Atomic Number

The atomic number of an element is determined by the number of protons in the nucleus of its atom. It represents the identity of an element.

• Example:

• Helium (He) has 2 protons, so its atomic number is 2.

• Oxygen (O) has 8 protons, so its atomic number is 8.

The atomic number is also called the ID number of an element because it uniquely identifies an atom. The atomic number is represented by the symbol Z.

• Hydrogen: Atomic number 1 (Z = 1)

• Helium: Atomic number 2 (Z = 2)

• Fluorine: Atomic number 9 (Z = 9)

Since atoms are electrically neutral, they contain an equal number of protons and electrons.

3.5.2 Mass Number

The mass number of an atom is the total number of protons and neutrons in its nucleus. It is represented by the symbol A.

• To find the number of neutrons in an atom, subtract the atomic number (number of protons) from the mass number.

• Example:

• Sodium (Na) has a mass number (A) of 23 and a proton number (Z) of 11, so the number of neutrons in sodium is:

• Neutrons = 23 - 11 = 12

In notation, the atomic number is written at the lower left corner of the element’s symbol, and the mass number is written at the upper left corner.

• Sodium (Na): Mass Number (A) = 23, Atomic Number (Z) = 11 → 23Na

Short Form of Different Elements

Atomic/Proton Number (Z) Mass Number (A) Number of Electrons Number of Neutrons (A - Z) Short Form

1 1 1 0 1H

2 4 2 2 4He