Atomic Structure
Atomic Structure
- Atoms are the smallest part of an element that can exist while still retaining its identity.
- Every atom consists of a nucleus and one or more electrons bound to the nucleus.
Components of an Atom
- The nucleus is made up of protons and neutrons.
- Protons are positively charged particles whilst neutrons have no charge.
- Electrons are negatively charged particles and exist in an area outside the nucleus called the electron cloud.
Atomic and Mass Number
- The atomic number (Z) of an atom is the number of protons in the nucleus. It defines the chemical properties of the atom and it also determines the identity of the element.
- The sum of protons and neutrons in an atom is known as the mass number (A).
- Atoms of the same element with different mass numbers are called isotopes.
Electron Configuration
- Electrons occupy energy levels, also known as shells around the nucleus. These are ordered from closest to the nucleus (lowest energy) to furthest away (highest energy).
- The first energy level can hold up to 2 electrons, the second can hold up to 8, and the third and any subsequent levels can hold up to 8 as well.
- The electron configuration of an atom represents the distribution of electrons in these energy levels.
Ions and Isotopes
- If an atom gains or loses electrons, it becomes an ion. A positive ion (has lost electrons) is called a cation, while a negative ion (has gained electrons) is an anion.
- Isotopes are atoms of the same element with the same atomic number but different mass numbers due to different numbers of neutrons.
- The properties of isotopes may vary due to their different masses, but their chemical properties remain the same as they have the same electronic configuration.
Relative Atomic Mass
- Relative atomic mass (Ar) is the weighted mean mass of an atom of an element compared with one-twelfth of the mass of an atom of carbon-12. This takes account of the percent abundance of the isotopes of the element.