Atoms, Elements, Compounds and Formulas

A gold ring contains an enormous number of gold atoms. An atom is the smallest particle of an element that is still that element. Atoms are too small to see with an ordinary light microscope. Special microscopes can make images of them.

How small is an atom?

A typical atom is about 0.1 to 0.5 nanometers across. One nanometer is one billionth of a meter. A row just 1 millimeter long would contain roughly 2 million to 10 million atoms. Drawings enlarge atoms so we can follow what happens to them.

From an idea to a model

Around 400 BCE, the Greek thinker Democritus suggested that matter could not be cut into smaller pieces forever. He called the smallest pieces “atomos,” meaning “uncuttable.” This was an idea based on reasoning.

In the early 1800s, John Dalton used measurements of substances combining. He built an atomic model and published it in 1808. A scientific model is a simplified description that explains observations and makes predictions.

Dalton pictured atoms as tiny solid balls. His model said:

• Matter is made of atoms that cannot be divided, created, or destroyed.
• All atoms of one element are identical. Different elements have different atoms.
• Atoms join in fixed, simple whole-number ratios to form compounds.
• Chemical reactions rearrange atoms into new combinations.

He drew different markings inside circles to show different elements. He also compared atomic masses with hydrogen, which he gave a value of 1.

What the model explains

Water always has two hydrogen atoms for each oxygen atom. A larger sample has more atoms, but the same ratio. Fixed ratios explain why a compound has a fixed composition.

When carbon burns completely, each carbon atom joins two oxygen atoms. The product is carbon dioxide. The atoms are rearranged, so the product's mass equals the mass of the carbon and oxygen that reacted. Oxygen from the air must be included in that total.

Why models change

Later evidence showed that atoms contain smaller particles. Atoms of the same element can also have different masses. Dalton had guessed that water was HO; experiments later showed H₂O. These findings changed parts of his model. His ideas about fixed ratios and atoms being conserved in chemical reactions still work.

A model connects tiny particles with things we can measure. But one particle does not have every property of a sample. One copper atom is not a shiny orange piece of metal. A single water molecule is not wet and has no boiling point. Such properties belong to many particles together.

An element contains only one kind of atom. A copper sample contains copper atoms; a helium sample contains helium atoms. No chemical reaction can break an element into simpler substances. Copper can react to make compounds, but those compounds are not simpler parts of copper.

Elements around us

There are 118 known elements, listed in the periodic table. More than 90 occur in nature; the rest have been made in laboratories. Most are solids at 25 °C. Oxygen, nitrogen, hydrogen, chlorine, and helium are gases. Mercury and bromine are liquids at this temperature.

Elements with similar properties appear in the same columns of the table. A useful first comparison is between metals and non-metals.

About three-quarters of the elements are metals. A malleable metal can be hammered into shape. A ductile metal can be pulled into a wire. Mercury is a liquid metal, so not every metal is a solid.

Properties help explain uses

Copper carries electricity and can be drawn into wires. These two properties make it useful inside electrical cables. Aluminum has low density and resists corrosion, making it useful in aircraft and foil. Gold stays shiny because it reacts very little with air or water. Helium is less dense than air and does not burn, so it is used in balloons.

These properties help sort a sample already known to be an element. A shiny, bendable, conducting element is a metal. Shine alone cannot establish that a material is an element: mixtures such as brass can be shiny too.

One element, different forms

Carbon is a non-metal, but its atoms can form different structures. Diamond is very hard and does not conduct electricity. Graphite is soft, slippery, and conducts electricity. Both contain only carbon atoms. Graphite is a useful exception to the usual non-metal pattern.

Oxygen has two familiar forms too. Oxygen gas is O₂, while ozone is O₃. Both are elements because they contain only oxygen atoms. The number and arrangement of atoms affect the substance's properties.

A substance containing iron is not always the element iron. Rust contains iron chemically joined with other elements, mainly oxygen and hydrogen. Rust is usually a mixture of several iron compounds.

Chemical symbols are short labels agreed across languages. The first letter is capitalized. A second letter is lowercase. For example, C is carbon, Ca is calcium, and Cl is chlorine.

Capitals change the meaning

Each capital letter begins a new element symbol. Co means cobalt. CO contains the symbols C and O, so it means carbon monoxide. Similarly, No is nobelium, while NO contains nitrogen and oxygen. Writing “CL” or “cl” changes the correct chlorine symbol, Cl.

Build a useful symbol bank

Some symbols come from older names. Na comes from natrium, K from kalium, and Fe from ferrum. Copper's Cu comes from cuprum. Silver, tin, gold, mercury, and lead use Ag, Sn, Au, Hg, and Pb. Tungsten's W comes from the German name wolfram.

Keep similar-looking symbols separate. Ag is silver, Ar is argon, and Au is gold. P means phosphorus; potassium is K. Iron is Fe, while Ir is a different element, iridium.

A symbol and a substance's formula

A symbol can name an element or represent one atom. O represents one oxygen atom. Oxygen gas consists of pairs of joined oxygen atoms, so its formula is O₂. The small 2 counts the atoms in one molecule. The writing 3O instead represents three separate oxygen atoms.

Seven elements commonly form two-atom molecules:

H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂.

The noble gases helium, neon, and argon exist as separate atoms: He, Ne, and Ar. Metals and carbon have extended structures, so their formulas use the symbol alone, such as Cu or C. Cu does not mean a copper wire contains just one atom.

White phosphorus forms P₄ molecules and sulfur forms S₈ molecules, although school work often writes these elements as P and S.

Letter symbols replaced Dalton's marked circles. In the 1810s, Jöns Jacob Berzelius proposed letters taken from element names. They were easier to write and print, and became the basis of today's formulas.

A molecule is a group of two or more atoms joined by chemical bonds. A bond holds atoms together. Oxygen gas contains O₂ molecules: each molecule has two oxygen atoms. Water contains H₂O molecules: each has two hydrogen atoms and one oxygen atom.

Molecules can be elements or compounds

O₂, N₂, and H₂ each contain one kind of atom. They are molecules of elements. H₂O, CO₂, NH₃, and CH₄ contain different kinds of atoms. They are molecules of compounds.

A water molecule has a bent shape. Carbon dioxide has a carbon atom between two oxygen atoms in a straight line. Ammonia, NH₃, has four atoms in each molecule. Methanol, CH₃OH, has six: one carbon, four hydrogen, and one oxygen. Three hydrogens join the carbon, and the fourth joins the oxygen.

Molecules can be much larger. Sucrose, C₁₂H₂₂O₁₁, has 45 atoms per molecule. Protein, DNA, and plastic molecules can contain thousands of atoms or more.

Choose what a model should show

A ball-and-stick model uses balls for atoms and sticks for bonds. It makes the joins easy to follow. A space-filling model uses overlapping spheres to show more of the molecule's overall shape. Flat circle drawings are easy to record, while computer models can be turned around.

Model kits often use this color key:

Colors are labels in a model, not the colors of individual atoms. A diagram needs its own key. Real bonds are not sticks, and ball sizes or spaces may not be to scale. A still model also leaves out molecular motion.

Some structures keep repeating

Not every substance has separate molecules. In an extended structure, a repeating pattern continues through the sample. Diamond has carbon atoms joined in all directions. Each carbon joins four others. Copper also has an extended structure.

Sodium chloride has a repeating pattern of sodium and chlorine, with equal numbers of each. Its formula, NaCl, gives a 1 : 1 ratio, not a two-atom molecule. Silicon dioxide, SiO₂, has an extended structure with two oxygen atoms for each silicon atom.

Physical changes keep molecules whole

When water boils, whole H₂O molecules spread farther apart and move differently. They do not split into hydrogen and oxygen. Sugar molecules also stay sugar molecules when sugar dissolves. Breaking the bonds inside molecules to make new substances requires a chemical reaction.

A compound contains two or more different elements chemically joined in a fixed ratio. Water, H₂O, contains hydrogen and oxygen. Sodium chloride, NaCl, contains sodium and chlorine.

A fixed recipe of atoms

Every water molecule contains two hydrogen atoms and one oxygen atom. This is true in a small drop and in a large container of pure water. Changing that ratio makes a different substance. H₂O₂ is hydrogen peroxide, used in bleaches and disinfectants. It is not another way to write water.

Compounds have new properties. Sodium is a soft metal that reacts violently with water. Chlorine is a poisonous yellow-green gas. Their compound, sodium chloride, is a white solid used as table salt. Its properties are not a blend of the elements' properties.

Making a compound

During a chemical reaction, atoms join in new combinations. Making a compound often releases heat or light. Burning magnesium produces bright white light and leaves white magnesium oxide.

A word equation names the substances:

magnesium + oxygen → magnesium oxide

The starting substances are reactants, written on the left. The new substance is the product, written on the right. The arrow means “react to form.” Other examples are iron + sulfur → iron sulfide, and carbon + oxygen → carbon dioxide.

Breaking a compound down

Decomposition breaks a compound into simpler substances through a chemical reaction. Different compounds respond to different energy sources:

• Heat: green copper carbonate forms black copper oxide and carbon dioxide gas.
• Electricity: water forms hydrogen and oxygen. The hydrogen gas volume is twice the oxygen gas volume.
• Light: white silver chloride darkens as silver and chlorine form.

Filtering, boiling, or using a magnet cannot remove the elements from a compound. Boiling water makes steam, which is still H₂O. Splitting water into hydrogen and oxygen changes the substance.

Properties you can observe

A physical property can be observed without making a new substance. Examples include color, state, melting point, density, hardness, solubility, and electrical conductivity. A chemical property describes a reaction that makes new substances, such as burning or reacting with acid.

Baking soda, NaHCO₃, is a white powder that dissolves in water. These are physical properties. Its reaction with vinegar releases carbon dioxide: that is a chemical property. Heating it also releases carbon dioxide, helping baked foods rise.

Sodium chloride dissolves in water and melts at 801 °C. Sugar, sucrose, dissolves too, but heating it strongly causes decomposition. Its blackening and release of steam involve new substances. Carry out chemical experiments only with a teacher's supervision.

A chemical formula records elements and their amounts. Read each capital letter with any lowercase letter that follows it as one symbol. Then read the small numbers, called subscripts.

A subscript belongs to what comes before it

In CO₂, the 2 counts oxygen atoms only. One molecule contains one carbon and two oxygen atoms: three atoms in total. No subscript means one. CH₄ has one carbon and four hydrogen atoms.

Count kinds of element separately from total atoms. NaHCO₃ contains four elements: sodium, hydrogen, carbon, and oxygen. One formula unit contains six atoms: one sodium, one hydrogen, one carbon, and three oxygen.

Parentheses group atoms

A subscript outside parentheses multiplies everything inside them. In Ca(OH)₂, the OH group appears twice:

• Calcium: 1.
• Oxygen: 2 × 1 = 2.
• Hydrogen: 2 × 1 = 2.

The total is five atoms per formula unit. Calcium is outside the parentheses, so the 2 does not multiply it.

In Mg(NO₃)₂, there are two NO₃ groups. Each group contains one nitrogen and three oxygen atoms. This gives one magnesium, two nitrogen, and six oxygen atoms, or nine atoms altogether.

A number in front counts whole units

The full-size 3 in 3H₂O means three water molecules. Each contains two H and one O. Together they contain six hydrogen atoms and three oxygen atoms.

Compare 2O and O₂. The first means two separate oxygen atoms. The second means one molecule with two joined oxygen atoms. A front number counts units; a subscript describes each unit.

Write a formula from a model

An ethanol model has two black balls, six white balls, and one red ball. Its key says black = carbon, white = hydrogen, and red = oxygen.

1. Count each kind: two C, six H, one O.
2. Write the symbols in the usual order: carbon, hydrogen, oxygen.
3. Put counts after their symbols as subscripts. Leave out a count of 1.

The formula is C₂H₆O. It is also written C₂H₅OH to show the hydrogen joined to oxygen.

A description works the same way. One sulfur atom joined with three oxygen atoms gives SO₃. Two aluminum atoms for every three oxygen atoms gives Al₂O₃. Metal compounds normally put the metal first.

Keep each molecule's actual counts

Hydrogen peroxide is H₂O₂. Do not reduce it to HO: its molecule really has two H and two O atoms. Also, the two oxygen atoms in CO₂ are each joined to carbon. An O₂ molecule is not hidden inside it. When typing without subscripts, H2O is understood as H₂O.

A mixture contains two or more substances together without being chemically joined to each other. The amounts can change. Salt water can contain a little salt or a lot; it is still a mixture of salt and water.

Mixtures and compounds behave differently

A mixture of iron filings and sulfur has visible gray and yellow specks. A magnet can pull out the iron. Changing the amounts does not turn the mixture into a compound.

Heating iron and sulfur makes black iron sulfide, FeS. The atoms have chemically joined in a fixed 1 : 1 ratio. A magnet cannot pull iron out of that compound. Formation gives out energy, seen as a glow. An energy change can support evidence of a reaction, but classification depends on the substances and their particles.

What “pure” means in science

A pure substance is one element or one compound, with nothing else mixed in. Distilled water is a pure compound. Pure copper is an element. “Pure orange juice” is an everyday label, but chemically it is a mixture of water, sugars, acids, and other substances.

At a given pressure, a pure substance has a fixed melting or boiling temperature. Pure water melts at 0 °C and boils at 100 °C at sea-level air pressure. Mixtures usually change state over a temperature range. Salt water boils above 100 °C at that pressure, and its boiling temperature changes as water leaves.

A mixture can look uniform

A solution is a mixture that is the same throughout. Salt disappears from view when it dissolves, but sodium chloride is still present. Evaporating the water recovers the salt. Sugar in tea and air are also uniform mixtures. Sand in water is non-uniform because different parts can be seen.

Dry air is about 78% nitrogen, 21% oxygen, 0.9% argon, and 0.04% carbon dioxide by volume. Real air also has a changing amount of water vapor. Its substances do not all join into one kind of molecule.

Metals can form mixtures too

An alloy is a mixture of a metal with other elements. Brass contains copper and zinc; bronze contains copper and tin. Steel contains iron and a little carbon. Stainless steel contains chromium, which helps resist rusting, and many kinds also contain nickel.

Alloys are made for useful properties such as greater hardness. Pure gold is 24 karat. In 18-karat gold, 18 of every 24 parts by mass are gold: 75%. The remaining elements make it an alloy.

Particle diagrams let you classify a sample even when its name is unfamiliar. Use the diagram's key to identify each kind of atom. Touching or connected circles show joins only when the key says they do.

Start with the kinds of particle

For separate atoms or molecules, ask:

1. Is there only one kind of particle? If yes, the diagram shows a pure substance. Two or more different substances make a mixture.
2. For a pure substance, how many kinds of atom occur? One kind gives an element. Different kinds chemically joined give a compound.

Imagine identical pairs of red circles. The key says red = oxygen, and each pair is joined. These are O₂ molecules. There is only oxygen, so the sample is an element, despite each molecule containing two atoms.

Now imagine identical groups with one black circle joined to two red circles. Black = carbon and red = oxygen. Every group is CO₂. The sample is a compound because two elements are joined in each molecule.

A box holding separate helium and neon atoms shows a mixture of two elements. A box containing O₂ and H₂O molecules shows a mixture of an element and a compound. Count kinds of substance, not just circle colors.

Read an extended structure differently

A metal diagram may show a repeating grid of identical atoms. It is an element. A sodium chloride diagram has two kinds in one repeating 1 : 1 pattern. It is a compound, even though the grid is not divided into identical little molecules.

Seeing two colors alone is not enough. They might show different elements joined in a compound, or separate substances in a mixture. Follow the joins and the repeating pattern.

Use formulas as evidence

A formula describes a substance. A real product may contain more substances than its main ingredient. Store-bought table salt often contains iodine compounds or anti-caking agents. That product is a mixture, mostly sodium chloride.

Link an observation to a reason

A white solid melts sharply at 801 °C. Electricity passed through the melted solid produces a metal and a gas. The splitting into simpler substances is chemical decomposition, so the original substance is a compound. The sharp melting point alone would not tell an element from a compound.

Give both a class and its reason. For example: “The sample is a mixture because it contains separate oxygen molecules and water molecules.” Copper, diamond, and oxygen gas are elements. Distilled water, sucrose, and pure baking soda are compounds. Tap water, milk, soil, sea water, and steel are mixtures.

A compound's name tells you which elements or groups it contains. Begin by recognizing the symbols in its formula. For simple compounds of a metal and a non-metal, name the metal first.

Two elements: use -ide

The metal keeps its name. Change the non-metal's ending to -ide. NaCl is sodium chloride. MgO is magnesium oxide. CaCl₂ is calcium chloride; the subscript does not add a number word to this type of name.

KBr becomes potassium bromide, and Al₂O₃ becomes aluminum oxide. “Sodium chlorine” uses the element name where the compound needs “chloride.”

Groups with oxygen: -ate and -ite

Some names contain an oxygen-bearing group. Learn these common groups:

• Carbonate, CO₃: one carbon and three oxygen atoms.
• Sulfate, SO₄: one sulfur and four oxygen atoms.
• Nitrate, NO₃: one nitrogen and three oxygen atoms.

CaCO₃ is calcium carbonate. MgSO₄ is magnesium sulfate. KNO₃ is potassium nitrate. The -ate ending points to a group containing oxygen.

An -ite group has less oxygen than its matching -ate group. Sodium sulfite is Na₂SO₃; sodium sulfate is Na₂SO₄. Sodium nitrite is NaNO₂; sodium nitrate is NaNO₃. The small change in the name identifies a different compound.

Three useful names to recognize

Hydroxide ends in -ide but contains oxygen and hydrogen, OH. NaOH is sodium hydroxide. Ca(OH)₂ is calcium hydroxide. So an -ide name does not always mean just two elements.

Hydrogen carbonate, HCO₃, contains hydrogen, carbon, and oxygen. NaHCO₃ is sodium hydrogen carbonate, also called sodium bicarbonate or baking soda.

Ammonium, NH₄, can stand at the front of a name in place of a metal. NH₄Cl is ammonium chloride.

Follow a name through a reaction

For iron reacting with sulfur, keep the element names on the left. Give the product its compound name:

iron + sulfur → iron sulfide

Compare copper sulfide, CuS, with copper sulfate, CuSO₄. Sulfide has sulfur without oxygen; sulfate has the SO₄ group. Neither name tells you to copy the other compound's formula.

For these metal compounds, names identify the parts but do not give every atom count. Magnesium chloride is MgCl₂, while sodium chloride is NaCl. Use the formula or model to find the ratio.

Carbon and oxygen can form CO or CO₂. These are different compounds, so their names need to show the difference. Number prefixes count atoms in many compounds of two non-metals.

Learn the number words

The first element keeps its name. Give it a prefix only when more than one atom is present. The second element takes a prefix, including mono- for one atom, and ends in -ide. Mono- and oxide combine as “monoxide.”

From formula to name

Read SO₃ in two parts. S has no subscript, so there is one sulfur atom: write “sulfur.” O₃ means three oxygen atoms: write “trioxide.” Together, the name is sulfur trioxide.

CO is carbon monoxide. CO₂ is carbon dioxide. N₂O is dinitrogen monoxide: two nitrogen atoms and one oxygen atom. The prefix belongs to the element it comes before. In carbon dioxide, di- counts oxygen, not carbon.

More examples show the larger prefixes:

• CCl₄: carbon tetrachloride.
• PCl₅: phosphorus pentachloride.
• SF₆: sulfur hexafluoride.
• CS₂: carbon disulfide.

SiO₂ is silicon dioxide, the main compound in quartz and many kinds of sand. Its name uses the same number prefix, although its structure is extended.

From name to formula

“Phosphorus trichloride” starts with phosphorus and no prefix, giving one P. Trichloride means three chlorine atoms, giving Cl₃. The formula is PCl₃. Do not write a full-size 3 at the front: that would count whole units.

A small name change can signal very different properties. Carbon monoxide is colorless, odorless, and poisonous. It forms when fuels burn without enough oxygen. Carbon dioxide is the gas we breathe out and plants take in during photosynthesis. The extra oxygen is part of a different molecule.

Common names stay useful

Some formulas are normally read using familiar names:

Hydrogen chloride, HCl, and hydrogen sulfide, H₂S, are usually named without number prefixes. Metal compounds follow their usual names too: MgCl₂ is magnesium chloride, not “magnesium dichloride.”

An atom is far too light for an ordinary balance. A hydrogen atom has a mass of about 1.7 × 10⁻²⁴ grams. Chemists therefore use a scale that compares atomic masses without writing such tiny measurements each time.

A comparison scale

Relative atomic mass, written $A_r$, compares the average mass of an element's atoms with a shared standard. The scale gives the most common kind of carbon atom a mass of exactly 12. Hydrogen is about 1 on this scale.

Relative atomic mass has no unit. A comparison such as “twice as heavy” has no grams attached. Some books instead give atomic masses in atomic mass units, written u or amu. Those are masses with a unit; relative atomic mass is the comparison number.

Read the right value

A periodic-table box often shows a symbol, a name, and two numbers. Read the labels or the table's key. The relative atomic mass is often a decimal and usually the larger number. The atomic number is a different number that gives the element's position in the table.

Oxygen has atomic number 8 and relative atomic mass about 16. Use 16 when comparing atomic masses. Chlorine's table value may be about 35.45, rounded to 35.5 for school calculations.

These rounded values will be useful:

A question or data sheet may use a different rounding, such as copper = 64. Use that supplied value throughout the calculation.

Compare by dividing

To compare magnesium with carbon, divide 24 by 12:

A magnesium atom has about twice the mass of a carbon atom. To match one sulfur atom using helium atoms, divide 32 by 4. Eight helium atoms have about the same total mass as one sulfur atom.

The word average matters. Not every atom of the same element has exactly the same mass. This is why an element's relative atomic mass can be a decimal, such as chlorine's 35.5. It is not a count of part of an atom.

Relative atomic mass compares mass, not size. An atom with twice the mass does not have to be twice as wide. Write $A_r$ with the small r, or use the full name. Ar without the small r is the chemical symbol for argon.

A formula tells you which atoms contribute to a substance's relative formula mass. Relative formula mass, written $M_r$, is the sum of their relative atomic masses. For molecules, it is also called relative molecular mass. It has no unit.

Count, multiply, add

Use four steps:

1. Write the formula.
2. Count each kind of atom, including any parentheses.
3. Multiply each count by that element's relative atomic mass.
4. Add all the contributions.

For water, H₂O, use H = 1 and O = 16. Two hydrogens contribute 2 × 1 = 2. One oxygen contributes 16. Adding gives $M_r=2+16=18$.

For CO₂, carbon contributes 12. Two oxygens contribute 2 × 16 = 32. The total is $M_r=12+32=44$. The 2 multiplies oxygen only. Multiplying both carbon and oxygen by 2 would count an extra carbon atom.

Work through parentheses

Find the relative formula mass of Ca(OH)₂, using Ca = 40, O = 16, and H = 1.

Add the contributions: $M_r=40+32+2=74$.

Both O and H are inside the parentheses, so both are doubled. For Mg(NO₃)₂, there are one Mg, two N, and six O. Using Mg = 24, N = 14, and O = 16 gives 24 + 28 + 96 = 148.

The same method works for other structures

Sodium chloride has no separate NaCl molecules. Use its formula ratio anyway: Na contributes 23 and Cl contributes 35.5, giving 58.5.

An element can have a relative molecular mass too. Oxygen gas is O₂, so its relative molecular mass is 2 × 16 = 32. The relative atomic mass of oxygen remains 16. One value refers to an atom; the other refers to a molecule.

Glucose, C₆H₁₂O₆, gives a larger sum. Carbon contributes 6 × 12 = 72. Hydrogen contributes 12 × 1 = 12. Oxygen contributes 6 × 16 = 96. The total is 72 + 12 + 96 = 180.

Compare the right particles

A CO₂ molecule has relative mass 44, so it has more mass than an O₂ molecule at 32. In 3H₂O, the front number counts three water molecules. Water's relative molecular mass is still 18; the three molecules together have relative mass 54.

Water also shows a fixed mass proportion. Its oxygen contributes 16 and its hydrogen contributes 2. Oxygen therefore contributes eight times as much mass as hydrogen, even though there are fewer oxygen atoms. Atom counts and mass contributions answer different questions.