1.2.3 Calculating the Particles

Atomic Number

• The atomic number (or proton number) is the number of protons in the nucleus of an atom
• The symbol for this number is Z
• It is also the number of electrons present in an atom and determines the position of the element on the Periodic Table
• The proton number is unique to each element, so no two elements have the same number of protons
• Electrons can be lost, gained, or shared during chemical processes but the proton number of an atom does not change in a chemical reaction

Mass Number

• The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom
• The symbol for this number is A
• The mass number minus the proton number gives you the number of neutrons of an atom
• Note that protons and neutrons can collectively be called nucleons
• The atomic number and mass number for every element is on the Periodic Table

Diagram showing the notation used on the Periodic Table

Isotopes

• Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons
• The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number
• So, C-14 is the isotope of carbon which contains 6 protons and 6 electrons, but the 14 signifies that it has 8 neutrons (14 - 6 = 8)
  • It can also be written as ¹⁴C

• Isotopes display the same chemical characteristics
• This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry
• The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only

The Atomic Structure and Symbols of the Three Isotopes of Hydrogen

Ions

• An ion is an atom or group of atoms that has an electrical charge, either positive and negative
• Atoms have an equal number of protons and electrons and so do not have an overall charge
• Atoms with incomplete outer electron shells are unstable
• By either gaining or losing electrons, atoms can obtain full outer electron shells and become stable
• When this happens, atoms have an unequal number of protons and electrons and so have an overall charge.
• This is how atoms become ions
• An atom that loses electrons has more protons than electrons and so has a positive overall charge
  • This is called a positive ion or cation
• An atom that gains electrons has more electrons than protons and so has a negative overall charge
  • This is called a negative ion or anion

Protons

• The atomic number is equal to the number of protons (p) in an atom
  
• Since atoms are neutral, then it is also the same as the number of electrons (e)
  
• The mass number is the number of protons plus neutrons
  
• The number of neutrons (n) can thus be calculated by subtracting the atomic number from the mass number
  
  • For example, beryllium has an atomic number of 4, therefore it has 4 protons and 4 electrons
    
  • The mass number of beryllium is 9, so it has 9 - 4 = 5 neutrons
    
  • The PEN numbers for beryllium are thus:
    p = 4
    e = 4
    n = (9 - 4 =) 5

The symbol key for beryllium as represented on the periodic table

Worked Example

Answer

Answer 1: The atomic number of a magnesium atom is 12, therefore the number of protons in the magnesium atom is 12

Therefore, the number of protons in a Mg²⁺ ion is also 12 - the number of protons does not change when an ion is formed

Answer 2: The atomic number of a carbon atom is 6 suggesting that a carbon atom has 6 protons in its nucleus

Answer 3: Use the formula to calculate the number of protons

Number of protons = mass number - number of neutrons

Number of protons = 63 - 34

Number of protons = 29

Neutrons

• The mass and atomic numbers can be used to find the number of neutrons in ions and atoms:

Number of neutrons = mass number (A) - atomic number (Z)

Worked Example

Answer

Answer 1: The atomic number of a magnesium atom is 12 and its mass number is 24

Number of neutrons = mass number (A) - atomic number (Z)

Number of neutrons = 24 - 12

Number of neutrons = 12

The Mg²⁺ ion has 12 neutrons in its nucleus

Answer 2: The atomic number of a carbon atom is 6 and its mass number is 12

Number of neutrons = mass number (A) - atomic number (Z)

Number of neutrons = 12 - 6

Number of neutrons = 6

The carbon atom has 6 neutrons in its nucleus

Answer 3: The atomic number of an element X atom is 29 and its mass number is 63

Number of neutrons = mass number (A) - atomic number (Z)

Number of neutrons = 63 - 29

Number of neutrons = 34

The neutral atom of element X has 34 neutrons in its nucleus

Electrons

• An atom is neutral and therefore has the same number of protons and electrons
• Ions have a different number of electrons to the number of protons, depending on their charge
• A positively charged ion has lost electrons and therefore has fewer electrons than protons
• A negatively charged ion has gained electrons and therefore has more electrons than protons

Worked Example

Answer

Answer 1: The atomic number of a magnesium atom is 12 suggesting that the number of protons in the neutral magnesium atom is 12

However, the 2+ charge in Mg²⁺ ion suggests it has lost two electrons. 
It only has 10 electrons left now

Answer 2: The atomic number of a carbon atom is 6 suggesting that the neutral carbon atom has 6 electrons orbiting around the nucleus

Answer 3: The number of protons of element X can be calculated by:

Number of protons = mass number - number of neutrons

Number of protons = 63 - 34

Number of protons = 29

The neutral atom of element X therefore also has 29 electrons