Recap of Atomic Structure and the Periodic Table - SS2 Chemistry Lesson Note
Atoms are the fundamental units of matter. They consist of three main subatomic particles:
a. Protons: Positively charged particles found in the nucleus of an atom. They have a relative mass of 1 atomic mass unit (AMU).
b. Neutrons: Neutral particles are also located in the nucleus. They also have a relative mass of approximately 1 (AMU).
c. Electrons: Negatively charged particles that orbit the nucleus in energy levels or shells. They have a much smaller mass compared to protons and neutrons.
The atomic number (Z) of an atom is determined by the number of protons it contains. It defines the element's identity. For example, hydrogen (H) has an atomic number of 1, carbon (C) has an atomic number of 6, and gold (Au) has an atomic number of 79.
The mass number (A) of an atom is the sum of protons and neutrons in its nucleus. It represents the total number of particles in the atom's nucleus. Isotopes are atoms of the same element with different numbers of neutrons, leading to different mass numbers.
The electrons in an atom occupy different energy levels or shells. The innermost shell can hold a maximum of 2 electrons, while the subsequent shells can hold more. The shells are labelled as K, L, M, N, and so on, starting from the innermost K shell.
The valence electrons are the electrons in the outermost shell (valence shell) of an atom. They play a crucial role in determining the atom's chemical properties and how it interacts with other atoms.
The Periodic Table:
The periodic table is a tabular arrangement of elements based on their atomic number, electron configuration, and chemical properties. It is organised into periods (rows) and groups (columns). There are currently seven periods, numbered from 1 to 7, and multiple groups, labelled from 1 to 18. Elements in the same period have the same number of electron shells, while elements in the same group have similar valence electron configurations and exhibit similar chemical behaviour.
The periodic table is divided into several main blocks:
a. S-block: Group 1 (alkali metals) and Group 2 (alkaline earth metals). These elements have their outermost electrons in the s subshell.
b. P-block: Groups 13-18. These elements have their outermost electrons in the p subshell.
c. D-block: Transition metals. These elements have their outermost electrons in the d subshell.
d. F-block: Lanthanides and actinides. These elements have their outermost electrons in the f subshell.
The periodic table is a valuable tool for predicting an element's properties, including its atomic mass, atomic radius, electronegativity, and reactivity. Elements are arranged in order of increasing atomic number, from left to right and top to bottom, which results in a periodic pattern of properties. This pattern is known as periodicity and is due to the periodic recurrence of similar electron configurations and valence shells across the table. Some elements in the periodic table have special names for historical or practical reasons. For example, the first period starts with hydrogen (H) and helium (He), which are unique in many aspects.