Electron Configuration
Electrons don't orbit randomly — they fill defined shells and subshells around the nucleus, starting from the lowest energy level and working outwards. This arrangement is called an atom's electron configuration.
The electrons in the outermost shell, called valence electrons, are the ones involved in chemical bonding, which is why electron configuration is the starting point for predicting how an element reacts.
Protons, Neutrons and Electrons
Every atom is built from three subatomic particles: protons and neutrons, which sit together in the nucleus, and electrons, which occupy the space around it. Protons carry a positive charge, electrons a negative charge, and neutrons no charge at all.
The number of protons in an atom is its atomic number, and it's what defines which element the atom is. An atom is electrically neutral when it has equal numbers of protons and electrons.