# 1.1.2 Mass Number & Isotopes

### Mass & Charge

• The mass of an atom is concentrated in the nucleus, because the nucleus contains the heaviest subatomic particles (the neutrons and protons)
• The mass of the electron is negligible
• The nucleus is also positively charged due to the protons
• Electrons orbit the nucleus of the atom, contributing very little to its overall mass, but creating a ‘cloud’ of negative charge
• The electrostatic attraction between the positive nucleus and negatively charged electrons orbiting around it is what holds an atom together The mass of the atom is concentrated in the positively charged nucleus which is attracted to the negatively charged electrons orbiting around it

### Subatomic Structure of Atoms & Ions

• An atom is neutral and has no overall charge
• Ions on the other hand are formed when atoms either gain or lose electrons, causing them to become charged
• The number of subatomic particles in atoms and ions can be determined given their atomic (proton) number, mass (nucleon) number and charge

#### Protons

• The atomic number of an atom and ion determines which element it is
• Therefore, all atoms and ions of the same element have the same number of protons (atomic number) in the nucleus
• E.g. lithium has an atomic number of 3 (three protons) whereas beryllium has atomic number of 4 (4 protons)
• The number of protons equals the atomic (proton) number
• The number of protons of an unknown element can be calculated by using its mass number and number of neutrons:

Mass number = number of protons + number of neutrons

Number of protons = mass number – number of neutrons

#### Worked Example

Determine the number of protons of the following ions and atoms:

1.  Mg2+ ion
2.  Carbon atom
3.  An unknown atom of element X with mass number 63 and 34 neutrons

Answer

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

• Therefore the number of protons in a Mg2+ ion is also 12

Answer 2: The atomic number of a carbon atom is 6 indicating 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

• Element X is therefore copper

#### Electrons

• An atom is neutral and therefore has the same number of protons and electrons
• Ions have a different number of electrons to their atomic number 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

#### 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) – number of protons (Z)

### 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
• Eg. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively

### Chemical & Physical Properties of Isotopes

• Isotopes have the same chemical properties but different physical properties

#### Chemical properties

• Isotopes of the same element display the same chemical characteristics
• This is because they have the same number of electrons in their outer shells
• Electrons take part in chemical reactions and therefore determine the chemistry of an atom

#### Physical properties

• The only difference between isotopes is the number of neutrons
• Since these are neutral subatomic particles, they only add mass to the atom
• As a result of this, isotopes have different physical properties such as small differences in their mass and density
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