Electrons, reactivity and ions

The outer shell decides

When atoms react, the nucleus doesn't change. Only the electrons do, and almost always only the electrons in the outer shell, called valence electrons. That is why elements in the same group behave alike: lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) each have one valence electron, and fluorine (2,7) and chlorine (2,8,7) each have seven.

The noble gases are the clue to what atoms "want". Helium (2), neon (2,8) and argon (2,8,8) have full outer shells and almost never react. A full outer shell is a very stable arrangement. Other atoms tend to react in ways that give them the same arrangement as the nearest noble gas, by losing, gaining or sharing electrons.

Forming ions

An atom has equal numbers of protons and electrons, so it has no overall charge. If it loses or gains electrons, the charges no longer balance and it becomes an ion, a charged particle.

• Metals lose electrons. A sodium atom (2,8,1) loses its one valence electron to become 2,8, the same arrangement as neon. It now has 11 protons but only 10 electrons, so it has a charge of 1+. Positive ions are called cations.
• Non-metals gain electrons. A chlorine atom (2,8,7) gains one electron to become 2,8,8, like argon. It now has 17 protons and 18 electrons, a charge of 1−. Negative ions are called anions.

An atom will lose or gain the smallest number of electrons that gives it a full shell. Magnesium (2,8,2) loses two electrons, not six, so it forms Mg²⁺. Oxygen (2,6) gains two rather than losing six, so it forms O²⁻. That makes the charge predictable from the group:

Writing ions. The charge is written as a superscript after the symbol, number first then sign: Mg²⁺, O²⁻. A charge of one is written as just + or −: Na⁺, Cl⁻.

Naming ions. Metal ions keep the element's name: a sodium ion, a calcium ion. Simple non-metal ions change their ending to -ide: chlorine becomes chloride, oxygen becomes oxide, sulfur becomes sulfide, nitrogen becomes nitride, fluorine becomes fluoride. So "chloride" in a list of ingredients means Cl⁻ ions, not poisonous chlorine gas.

Many transition metals can form more than one ion. Iron forms Fe²⁺ and Fe³⁺, and copper forms Cu⁺ and Cu²⁺. Roman numerals show which: iron(III) means Fe³⁺, copper(II) means Cu²⁺.

Explaining reactivity trends

Electrons explain the trends you saw in 2C.

Group 1 gets more reactive going down. An alkali metal reacts by losing its one outer electron. Going down the group, each atom has one more shell, so the outer electron is further from the nucleus and has more inner electrons between it and the nucleus, shielding it. The positive nucleus holds it less tightly, so it is lost more easily and the metal reacts faster. Potassium's outer electron is in the fourth shell; sodium's is in the third.

Group 17 gets less reactive going down. A halogen reacts by gaining an electron. In a small atom like fluorine, the incoming electron can get close to the nucleus and is pulled in strongly. In a large atom like iodine, the outer shell is far from the nucleus, so the attraction is weaker and the electron is gained less readily. Fluorine is the most reactive non-metal, so reactive that it attacks glass.

The same idea explains metals in general. Metals on the left of the table have few outer electrons to lose, and those lower in a group lose them most easily. That is why the most reactive metals are at the bottom left (caesium, francium) and the most reactive non-metal is at the top right (fluorine, ignoring the noble gases).

Ions in your body

Ions behave very differently from the atoms they came from, and your body depends on them. Nerve signals are carried by sodium ions (Na⁺) and potassium ions (K⁺) moving in and out of nerve cells. Calcium ions (Ca²⁺) make muscles contract and build bones. Chloride ions (Cl⁻) help make stomach acid. When you sweat heavily you lose these ions, which is why sports drinks contain "electrolytes": dissolved salts that put them back.