Atomic structure — KCSE Chemistry

In chemistry, several key concepts help us understand atomic structure:

• Atomic Number (Z): This is the number of protons in an atom's nucleus. It defines the element. For example, carbon has an atomic number of 6.
• Mass Number (A): This is the total number of protons and neutrons in an atom's nucleus. For instance, the mass number of carbon-12 is 12 (6 protons + 6 neutrons).
• Isotopes: These are atoms of the same element that have the same atomic number but different mass numbers due to varying numbers of neutrons. For example, carbon-12 and carbon-14 are isotopes of carbon.
• Relative Atomic Mass: This is the weighted average mass of an atom compared to 1/12th of the mass of carbon-12. It considers the isotopic composition of an element.

Understanding these terms allows you to perform calculations, such as determining the number of neutrons in an isotope.

For example, if you have an isotope of oxygen with a mass number of 18, the atomic number is 8. Thus, the number of neutrons is calculated as:

Neutrons = Mass Number - Atomic Number = 18 - 8 = 10.

The electronic configuration of an element describes the distribution of electrons in its atomic orbitals. For elements with atomic numbers 1 to 20, we follow the Aufbau principle, which states that electrons occupy the lowest energy orbitals first. The order of filling is:

• 1s,
• 2s,
• 2p,
• 3s,
• 3p,
• 4s,
• 3d,
• 4p.

Example configurations:

• Hydrogen (H, atomic number 1): 1s¹
• Helium (He, atomic number 2): 1s²
• Lithium (Li, atomic number 3): 1s² 2s¹
• Beryllium (Be, atomic number 4): 1s² 2s²
• Boron (B, atomic number 5): 1s² 2s² 2p¹
• Carbon (C, atomic number 6): 1s² 2s² 2p²

Continue this pattern up to atomic number 20, ensuring to follow the order of orbitals. This understanding is crucial for predicting chemical behavior and bonding.

Atoms are the basic units of matter, consisting of three main subatomic particles: protons, neutrons, and electrons.

• Protons are positively charged particles found in the nucleus.
• Neutrons are neutral particles, also located in the nucleus.
• Electrons are negatively charged and orbit the nucleus in energy levels.

The number of protons in an atom determines its atomic number, which defines the element. For example, hydrogen has one proton and an atomic number of 1. The mass number of an atom is the sum of protons and neutrons.

To describe the atomic structure of an element, we can use the notation:

A/Z X Where:

• X is the chemical symbol of the element.
• Z is the atomic number (number of protons).
• A is the mass number (total of protons and neutrons).

For instance, for Carbon (C):

• It has 6 protons and 6 neutrons, giving it a mass number of 12.
• Its atomic notation is 12/6 C.