Atoms, isotopes and ions

Atoms are the building blocks of all matter, forming the foundation of everything in the universe. Understanding their structure and properties is essential for exploring the principles of physics and chemistry.

The structure of an atom

Size of an Atom:

• Atoms are incredibly small, with a typical radius of about 1×10-10 meters.

• The nucleus, which is at the center of the atom, is even smaller, with a radius less than 1/10,000 of the overall atom's radius. Despite its tiny size, it contains almost all the mass of the atom.

Structure:

• An atom consists of three types of subatomic particles:

  • Protons: Positively charged particles located in the nucleus.

  • Neutrons: Neutral particles also found in the nucleus.

  • Electrons: Negatively charged particles orbiting the nucleus in defined energy levels.

• The nucleus is positively charged due to the protons, while the electrons give the atom its overall negative outer structure.

Electron Arrangement:

• Electrons are arranged in shells (or energy levels) around the nucleus.

• Electrons can gain or lose energy:

  • Absorbing energy (e.g., from light or heat) causes electrons to jump to a higher energy level (further from the nucleus).

  • Emitting energy (e.g., as electromagnetic radiation) causes electrons to move closer to the nucleus.

The structure of an atom.

Key Skills:

• Recognising expressions in standard form:

  • Example: The size of an atom can be written as 1×10-10 meters, where the power of 10 makes it easier to work with very small numbers.

  • Practice converting numbers into standard form, such as 0.00000005 to 5×10-8.

Atomic number and mass number

Atomic Number:

• The atomic number of an atom is the number of protons in its nucleus.

• In a neutral atom, the number of electrons equals the number of protons, meaning there is no overall electrical charge.

• Example: Carbon has an atomic number of 6, which means it has 6 protons and 6 electrons.

Mass Number:

• The mass number is the total number of protons and neutrons in an atom.

• Example: A sodium atom has 11 protons and 12 neutrons (23 - 11 = 12).

Representation of Atoms:

• Atoms can be represented using the mass number and atomic number. 

• For example

• • Mass number: 23 (total protons and neutrons).

  • Atomic number: 11 (protons, which also equals the number of electrons in a neutral atom).

Isotopes and ions

Isotopes:

• Isotopes are variants of the same element that have the same number of protons but a different number of neutrons.

  • Example: Carbon has three main isotopes:

    • Carbon-12: 6 protons and 6 neutrons.

    • Carbon-13: 6 protons and 7 neutrons.

    • Carbon-14: 6 protons and 8 neutrons.

  • Although isotopes of the same element have identical chemical properties, their physical properties (like mass) can vary.

Ions:

• • Atoms can lose or gain electrons to form ions:

    • Positive ions are formed when atoms lose electrons.

      • Example: Sodium loses one electron to form Na+.

    • Negative ions are formed when atoms gain electrons.

      • Example: Chlorine gains one electron to form Cl-.

Key Skills:

• Interpreting differences between isotopes:

  • Example: Understanding that Carbon-12 and Carbon-14 differ in the number of neutrons but have the same atomic number.

  • In exam questions, isotopes might be presented in nuclear notation. Be ready to calculate the number of neutrons (Mass number - Atomic number).

• Understanding how charge affects the atom:

  • Example: Na+ has lost one electron, so it has 11 protons but only 10 electrons, giving it a positive charge.

• Representing elements conventionally:

  • Practice writing atoms in nuclear form, e.g., oxygen with 8 protons and 8 neutrons: