Atomic Structure

Atomic Structure

Atoms consist of three basic particles - protons, neutrons and electrons.

Protons bear a positive charge, are found within the nucleus, and determine the atomic number of an element.
Electrons bear a negative charge, orbit the nucleus in shells and play a crucial role in bonding and chemical reactions.
Neutrons are neutral particles, located within the nucleus - their number can vary, creating various isotopes of an element.

Atomic Number and Mass Number

The atomic number or proton number of an atom, represented by ‘Z’, is equal to the number of protons in the nucleus.
The mass number or nucleon number, represented by ‘A’, is the sum of the protons and neutrons in the nucleus of an atom.

Electron Configurations

Electron configuration is the arrangement of electrons in an atom’s electron shells.
The first shell can hold up to 2 electrons, the second shell can hold up to 8, and the third can hold up to 8.
Electrons inhabit the lowest available energy shell first, filling up in order of increasing energy level.

Isotopes

Isotopes are variants of a particular element that have the same number of protons, but different numbers of neutrons.
The chemical properties of isotopes remain largely unchanged, but their physical properties can vary, due to the difference in mass.

Ion Formation

Atoms can lose or gain electrons to form ions in order to achieve a stable octet electron arrangement.
Atoms that lose electrons become positively charged, forming cations.
Atoms that gain electrons become negatively charged, forming anions.

Understanding Electronic Configurations from the Periodic Table

The periodic table can predict the electronic configuration of an atom.
The period number refers to the number of electron shells.
The group number (for groups 1 and 2 and the transition metals) often corresponds to the number of electrons in the outer shell.
For groups in the p-block (groups 13-18), the group number minus 10 gives the number of electrons in the outer shell.