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A short class with Sarah and her students. The other ways are under More.
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Sodium is 2,8,1 and chlorine is 2,8,7. What is the easiest way for both to get a full outer shell?
Sodium can lose its one outer electron, and chlorine can gain one.
So sodium gives its electron to chlorine. What ions do we get?
Na⁺ with 2,8 and Cl⁻ with 2,8,8. Those are like neon and argon.
Why do the ions stay together?
Opposite charges attract. That electrostatic attraction is the ionic bond.
Magnesium is 2,8,2. How many electrons must it lose?
Two. But a chlorine atom only needs one.
So what has to happen?
Magnesium gives one electron to each of two chlorine atoms, so we get MgCl₂.
Now try sodium and oxygen.
Oxygen needs two electrons, but each sodium only gives one, so two sodium atoms are needed: Na₂O.
Aluminium forms Al³⁺ and oxygen forms O²⁻. How do we balance the charges?
We need a total of 6 each way: two Al³⁺ make +6 and three O²⁻ make −6. So Al₂O₃.
How many electrons are transferred altogether?
Six, three from each aluminium atom.
Why does MgO melt at about 2800 °C but NaCl at 801 °C?
Mg²⁺ and O²⁻ have double charges, so they attract each other much more strongly.
metal atoms lose outer electrons and non-metal atoms gain them, giving both a full outer shell.
Group 1 forms 1+, Group 2 forms 2+, aluminium 3+, Group 15 forms 3−, Group 16 forms 2− and Group 17 forms 1−.
the total positive charge equals the total negative charge, which fixes formulas such as MgCl₂, Na₂O and Al₂O₃.
the metal keeps its name and the non-metal ends in "-ide", as in sodium chloride or magnesium oxide.
ionic compounds form lattices, not molecules, and the formula gives the ratio of ions.
ions with larger charges, such as Mg²⁺ and O²⁻, attract more strongly, giving higher melting points.
Sodium chloride: Na (2,8,1) gives one electron to Cl (2,8,7), forming Na⁺ (2,8) and Cl⁻ (2,8,8) in a 1 : 1 ratio, NaCl.
Magnesium oxide: Mg (2,8,2) gives two electrons to O (2,6), forming Mg²⁺ (2,8) and O²⁻ (2,8), MgO.
Magnesium chloride: one Mg atom gives one electron to each of two Cl atoms, forming Mg²⁺ and 2Cl⁻, MgCl₂.
Aluminium oxide: two Al atoms each lose three electrons (6 in total) to three O atoms, each gaining two, forming 2Al³⁺ and 3O²⁻, Al₂O₃.
Metals give and non-metals take, full outer shells are what they make; plus and minus pull so tight, the charges balance, the formula's right.
Sodium has one coin it does not want, and chlorine is one coin short of a full set. Sodium hands it over, and now one has a positive balance and the other a negative one, and they stick together for good.
"Lose, Gain, Balance, Name" - the four steps for any ionic compound.
O.I.L. R.I.G. - Oxidation Is Loss, Reduction Is Gain: the metal is oxidised as it loses electrons and the non-metal is reduced as it gains them.
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What ions form when sodium reacts with chlorine?
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How many electrons are transferred when one magnesium atom reacts with one oxygen atom?
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What is the formula of magnesium chloride, and why?
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What charge do Group 16 elements have as ions?
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Why does MgO have a much higher melting point than NaCl?
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The strong electrostatic attraction between oppositely charged ions, formed by the transfer of electrons from a metal to a non-metal.
The number of electrons in each shell of an atom or ion, such as 2,8,1 for sodium.
A positive ion, formed when an atom loses electrons, such as Na⁺ or Mg²⁺.
A negative ion, formed when an atom gains electrons, such as Cl⁻ or O²⁻.
A diagram showing the outer electrons of each atom as dots or crosses, so the transfer of electrons can be seen.
A regular three-dimensional arrangement of many oppositely charged ions held together by ionic bonds.
The simplest ratio of ions in an ionic compound, such as one Na⁺ to one Cl⁻ in NaCl.
This lesson works through examples of ionic bonding, starting with sodium chloride: sodium (2,8,1) gives one electron to chlorine (2,8,7), forming Na⁺ (2,8) and Cl⁻ (2,8,8), which attract each other. It then applies the same steps to magnesium oxide, magnesium chloride, sodium oxide and aluminium oxide.
Ionic bonds form when metal atoms transfer electrons to non-metal atoms, producing positive and negative ions with full outer shells that are held together by electrostatic attraction. Using electron arrangements, this lesson shows exactly how many electrons move in each example: one in sodium chloride (NaCl), two in magnesium oxide (MgO), one to each of two chlorine atoms in magnesium chloride (MgCl₂), one from each of two sodium atoms in sodium oxide (Na₂O), and six in aluminium oxide (Al₂O₃). It explains how to predict ion charges from group numbers, how to balance charges to write formulas, how to name ionic compounds, and why the compounds form giant ionic lattices rather than molecules.
The pattern in every example. A metal atom loses its outer electrons and becomes a positive ion (cation); a non-metal atom gains electrons and becomes a negative ion (anion). Both ions end up with a full outer shell, like a noble gas. The oppositely charged ions then attract each other strongly: this electrostatic attraction is the ionic bond.
Example 1: sodium chloride, NaCl.
In a dot-and-cross diagram, sodium's outer electron is drawn as a cross that ends up in chlorine's outer shell alongside chlorine's seven dots. Each ion is drawn in square brackets with its charge written outside.
Example 2: magnesium oxide, MgO. Magnesium (2,8,2) transfers two electrons to oxygen (2,6). This forms Mg²⁺ (2,8) and O²⁻ (2,8). A charge of 2+ balances 2−, so the ratio is 1 : 1 and the formula is MgO.
Example 3: magnesium chloride, MgCl₂. Magnesium must lose two electrons, but each chlorine atom can accept only one. So magnesium gives one electron to each of two chlorine atoms, forming one Mg²⁺ and two Cl⁻. The charges balance (+2 and 2 × −1), so the formula is MgCl₂.
Example 4: sodium oxide, Na₂O. Oxygen needs two electrons, but each sodium atom gives only one. Two sodium atoms each transfer one electron to one oxygen atom, forming two Na⁺ and one O²⁻, so the formula is Na₂O.
Example 5: aluminium oxide, Al₂O₃. Aluminium (2,8,3) forms Al³⁺ and oxygen forms O²⁻. The smallest numbers that balance the charges are two Al³⁺ (+6) and three O²⁻ (−6), so the formula is Al₂O₃, and six electrons are transferred in total.
Predicting ion charges from the periodic table.
Writing formulas. The total positive charge must equal the total negative charge. Find the smallest numbers of each ion that balance, and write them as subscripts. For example, Ca²⁺ and F⁻ need one calcium ion to two fluoride ions, giving CaF₂.
Naming. The metal is named first, unchanged, and the non-metal's name ends in "-ide": chlorine becomes chloride, oxygen becomes oxide, sulfur becomes sulfide, fluorine becomes fluoride and nitrogen becomes nitride.
Lattices, not molecules. Ionic compounds do not form separate molecules. In solid sodium chloride, the ions are arranged in a giant ionic lattice in which each Na⁺ is surrounded by six Cl⁻ and each Cl⁻ by six Na⁺. The formula NaCl gives only the ratio of the ions.
Why the examples differ. Magnesium oxide melts at about 2800 °C, far higher than sodium chloride at 801 °C. Its ions carry double charges (2+ and 2−) and are smaller, so the attraction between them is much stronger and more energy is needed to separate them. The general properties of ionic compounds are covered in the next lesson.