Physics and chemistry Year 8: Atoms, ions and chemical formulas
Physics and chemistry, Year 8 (Portugal): What an atom is made of, what makes an element, how molecules and ions arise, and how to read or write the formula of an ionic compound.
The atom and the element
An atom has a nucleus with protons (positive charge) and neutrons (no charge), and electrons (negative charge) around it. In an atom the number of protons equals the number of electrons, so the atom is neutral. What defines the chemical element is the number of protons: every atom with 11 protons is sodium, every atom with 17 is chlorine. Each element has a symbol: Na, Cl, O, H, Ca.
Molecules, ions and formulas
Atoms join to form bigger units. A molecule is a group of bonded atoms: water, H₂O, has 2 hydrogen atoms and 1 oxygen atom. An ion is an atom, or a group of atoms, that lost or gained electrons, so it carries a charge: losing electrons makes it positive, gaining makes it negative. A chemical formula says which elements a substance has and how many atoms of each. The structural unit can be an atom (Fe), a molecule (CO₂) or a group of ions (NaCl).
From atom to compound: CaCl₂
Calcium has 20 protons; losing 2 electrons leaves 18, so the charge is 20 − 18 = +2: the ion Ca²⁺. Chlorine has 17 protons; gaining 1 electron gives 18, charge 17 − 18 = −1: the ion Cl⁻. A compound must be neutral, so the charges add to zero: 1 × (+2) + 2 × (−1) = 0. One Ca²⁺ needs two Cl⁻: CaCl₂, calcium chloride. The 2 gives the ratio of ions, not a molecule of three atoms.
NaCl is not a molecule
It is common to hear «a molecule of salt». But table salt, NaCl, is not made of molecules: it is an enormous orderly group of Na⁺ and Cl⁻ ions in a 1 to 1 ratio, and its formula gives only that ratio. In H₂O or CO₂ the formula does describe a real molecule. And never change a subscript to «make it fit»: CO and CO₂ are different substances, and so are H₂O (water) and H₂O₂ (hydrogen peroxide).
Ions on labels
Read the label of a mineral water: calcium, magnesium, sodium, potassium, chlorides, sulfates, bicarbonates. Those are dissolved ions. A toothpaste may list sodium fluoride, NaF, made of Na⁺ and F⁻. With an ion table you can write formulas: Mg²⁺ and Cl⁻ give MgCl₂, and Na⁺ with SO₄²⁻ gives Na₂SO₄, because 2 × (+1) + 1 × (−2) = 0. Check the charges add to zero, and the formula is right.
Test what you learned
What decides which chemical element an atom belongs to?
- a) The number of electrons it happens to have
- b) The number of protons in its nucleus
- c) How big the atom is
- d) Its electric charge
Correct answer: b) The number of protons in its nucleus — Right: 11 protons is sodium, 17 is chlorine. The symbol (Na, Cl) names the element.
Aluminium forms the ion Al³⁺ and sulfate forms the ion SO₄²⁻. What is the formula of aluminium sulfate?
- a) AlSO₄
- b) Al₃(SO₄)₂
- c) Al₂(SO₄)₃
Correct answer: c) Al₂(SO₄)₃ — Right: 2 × (+3) + 3 × (−2) = 0. The parentheses show that the 3 applies to the whole SO₄²⁻ ion.
Which statement about table salt, NaCl, is correct?
- a) It is a large ordered group of Na⁺ and Cl⁻ ions in a 1 to 1 ratio
- b) It is made of molecules, each with one sodium atom and one chlorine atom
- c) It is made of neutral sodium atoms and neutral chlorine atoms
Correct answer: a) It is a large ordered group of Na⁺ and Cl⁻ ions in a 1 to 1 ratio — Right: NaCl is a group of ions, and its formula gives only the ratio between them.
Keep exploring
Other languages
Loading MyLeoNes™…