4.1.1.1 - Atoms, elements, compounds and equations

Atoms and element symbols

Every substance is made of atoms, and the kind of atom determines which element it belongs to.

There are about 100 different elements, displayed in the periodic table. Copper contains copper atoms; oxygen contains oxygen atoms. Oxygen gas usually has its atoms joined in pairs, but it is still an element because both atoms in each pair are oxygen. “Two atoms” does not automatically mean “two elements”.

Each element has a chemical symbol. A symbol has one capital letter, sometimes followed by a lower-case letter: $\ce{O}$ is oxygen and $\ce{Na}$ is sodium. The case matters: $\ce{Co}$ is cobalt, whereas $\ce{CO}$ is a formula containing carbon and oxygen. Some symbols come from older names, so sodium is not S; S represents sulfur.

The periodic table connects element names with their symbols. These are the first twenty elements, in atomic-number order:

Group 1 adds rubidium $\ce{Rb}$, caesium $\ce{Cs}$ and francium $\ce{Fr}$ to lithium, sodium and potassium. Hydrogen is shown above Group 1 in the supplied table, but is a non-metal and is not an alkali metal. Group 7 contains fluorine, chlorine, bromine $\ce{Br}$, iodine $\ce{I}$, astatine $\ce{At}$ and tennessine $\ce{Ts}$. Later lessons use the measured behaviour of the familiar halogens; a table position alone is not experimental evidence about the rarest elements.

Other symbols used in this course include chromium $\ce{Cr}$, manganese $\ce{Mn}$, iron $\ce{Fe}$, cobalt $\ce{Co}$, nickel $\ce{Ni}$, copper $\ce{Cu}$, zinc $\ce{Zn}$, silver $\ce{Ag}$, barium $\ce{Ba}$, lead $\ce{Pb}$, tin $\ce{Sn}$, gold $\ce{Au}$, platinum $\ce{Pt}$ and mercury $\ce{Hg}$. Use the periodic table to connect an unfamiliar symbol to its element; the surrounding formula tells you how it is combined.

Compounds and chemical formulae

A compound contains atoms of different elements chemically combined in a fixed proportion.

When magnesium reacts with oxygen, a new substance, magnesium oxide, forms. Its properties differ from those of magnesium metal and oxygen gas. A chemical reaction always makes one or more new substances and often produces a detectable energy change, such as heating or light. Warming alone does not prove that a reaction happened: melting a solid changes its state without making a new substance.

The formula shows the elements and their proportions. In $\ce{H2O}$, each water molecule contains two hydrogen atoms and one oxygen atom. No written subscript means one. In $\ce{CO2}$, one carbon atom is combined with two oxygen atoms. These fixed ratios distinguish compounds from mixtures, whose proportions can vary. Salt cannot be turned into sodium and chlorine by filtering it; physical separation does not break its chemical combination.

Not every compound consists of separate molecules. Sodium chloride has a repeating structure: $\ce{NaCl}$ represents its one-to-one ratio of sodium to chlorine, rather than a separate NaCl molecule. You will learn why when studying ionic bonding.

For many compounds of two elements, name the first element followed by the second with an -ide ending: $\ce{MgO}$ is magnesium oxide, $\ce{NaCl}$ is sodium chloride and $\ce{CaS}$ is calcium sulfide. Common compounds containing oxygen with another non-metal use names such as sulfate, nitrate and carbonate: $\ce{Na2SO4}$ is sodium sulfate, $\ce{KNO3}$ is potassium nitrate and $\ce{CaCO3}$ is calcium carbonate. The oxygen is part of that named group. A hydroxide contains $\ce{OH}$, so $\ce{NaOH}$ is sodium hydroxide. Do not call a sulfate a sulfide: $\ce{Na2S}$ and $\ce{Na2SO4}$ are different compounds.

Prefixes can distinguish molecular compounds: carbon monoxide is $\ce{CO}$, while carbon dioxide is $\ce{CO2}$. To write a formula, preserve the stated or established ratio, write the correct element symbols and attach subscripts to the atoms they count. Aluminium oxide has an aluminium-to-oxygen ratio of two to three, so its formula is $\ce{Al2O3}$. A bracket multiplies a whole group: $\ce{Ca(OH)2}$ contains one calcium, two oxygen and two hydrogen atoms. Rules for deriving ionic ratios are taught in the bonding lessons.

Word and balanced symbol equations

An equation records which substances react and which new substances form, while a balanced symbol equation also accounts for every atom.

The reactants go before the arrow and the products after it. A plus sign separates substances on the same side. The word equation for burning magnesium is:

magnesium + oxygen → magnesium oxide

Replace each name with its correct formula. Magnesium is $\ce{Mg}$, oxygen gas is $\ce{O2}$ and magnesium oxide is $\ce{MgO}$. The first attempt, $\ce{Mg + O2 -> MgO}$, has two oxygen atoms on the left but one on the right. Ordinary chemical reactions rearrange atoms; they do not create or destroy them.

Put a coefficient 2 before magnesium oxide, then a coefficient 2 before magnesium:

There are now two magnesium atoms and two oxygen atoms on each side. A coefficient multiplies the whole formula. Never balance by changing $\ce{MgO}$ into $\ce{MgO2}$: that changes the substance rather than its amount.

A second example is hydrogen reacting with oxygen to make water. Start with $\ce{H2 + O2 -> H2O}$. Two water molecules are needed to use the two oxygen atoms, and this requires four hydrogen atoms:

State symbols add physical information where it is known: (s) means solid, (l) liquid, (g) gas and (aq) dissolved in water. Thus burning solid magnesium in oxygen gas produces solid magnesium oxide: $\ce{2Mg(s) + O2(g) -> 2MgO(s)}$. “Aqueous” does not mean a pure liquid compound. Check each element separately, including atoms inside brackets, and use the smallest whole-number coefficients.