Chemistry 1: Matter, Atoms, and Bonds

What this guide is for

Chemistry is about what things are made of and how they change. This guide starts at the beginning. It explains what matter and energy are, then the building blocks of all matter: elements, atoms, molecules, and compounds. Then it goes inside the atom, to the three particles it is made of, and shows you how to count them, including how to work out the number of neutrons. From there it explains how an atom arranges its electrons, how to read the periodic table, how to read a chemical formula, and the two main ways atoms join together: by sharing electrons and by handing them over.

Part of this guide moves. In Part 7 you can add electrons to an atom one at a time and watch its shells fill, and Parts 11 and 12 show a water molecule and a crystal of table salt forming, step by step, with a voice that reads each step aloud. You can turn the voice off and read instead.

This guide gives you a little more than the GED test strictly asks for, where the extra makes the main ideas easier to understand. Boxes marked Going further hold extra material you can skip. The next guide, Chemistry 2, takes up mixtures and solutions, solids, liquids, and gases, physical and chemical changes, chemical reactions and how to balance their equations, the energy that reactions give off or take in, and acids and bases.

Goes with: the Chemistry reading · Science Practices

In this guide:

1. Matter and energy
2. The building blocks: elements, atoms, molecules, compounds
3. Inside the atom: protons, neutrons, and electrons
4. Counting particles: atomic number, mass number, and neutrons
5. Isotopes and ions
6. What holds the nucleus together
7. Electrons live in shells
8. Full shells and valence electrons
9. The periodic table
10. Chemical symbols and formulas
11. Covalent bonds: sharing electrons
12. Ionic bonds: handing electrons over
13. Which kind of bond? A rule of thumb
14. Check yourself

Matter and energy

Everything around you, the air, the water, your chair, your own body, is made of matter. In science, matter means anything that has mass and takes up space.

Mass is the amount of matter in something. It is measured in grams and kilograms. Volume is the amount of space something takes up. It is measured in liters and milliliters, or in cubic centimeters. A brick has mass and volume. So does the air in a balloon: air is thin, but it is still matter, which is why a balloon stays puffed up when you blow air into it.

Some things that are real are not matter. Light, heat, and sound have no mass and take up no space. They are forms of energy.

Energy is the ability to do work or to cause a change: to move something, heat it, light it up, or change it into something else. Energy comes in several forms:

• Chemical energy, stored in the bonds that hold atoms together. Food, gasoline, wood, and batteries all hold chemical energy.
• Heat (thermal energy), the energy of moving particles. The faster the particles of a substance move, the hotter it is.
• Light and other radiant energy, such as the energy that reaches us from the Sun.
• Motion (kinetic energy), the energy of anything that moves.
• Electrical energy, carried by moving electric charges.

Energy can change from one form to another. When you eat, your body turns the chemical energy in food into motion and heat. A car engine turns the chemical energy in gasoline into motion and heat. But energy is never made out of nothing and never destroyed. It only changes form. Scientists call this the law of conservation of energy. (The Physics guides look at energy more closely.)

Chemistry is the study of matter: what it is made of, how it is put together, how it changes, and the energy that goes in or comes out when it changes. Burning wood, rusting iron, baking bread, and digesting lunch are all chemistry.

The building blocks: elements, atoms, molecules, compounds

All matter is built from a fairly small number of basic substances called elements. An element is a pure substance that cannot be broken down into anything simpler by ordinary chemical means, such as heating it or mixing it with other chemicals. Gold, oxygen, carbon, iron, and hydrogen are elements.

Scientists know of 118 elements. About 90 of them occur in nature; the rest have been made in laboratories, and most of those last only a moment. A handful of elements make up most of the living world. About 96 percent of the mass of your body is just four elements: oxygen, carbon, hydrogen, and nitrogen.

Atoms

If you could cut a piece of gold in half, and then in half again, over and over, you would finally reach a piece that could not be cut any smaller and still be gold. That smallest piece is an atom. An atom is the smallest piece of an element that is still that element. Atoms are far too small to see. A single grain of table salt holds about a billion billion of them.

Each element has its own kind of atom. All the atoms of gold are alike in one important way (Part 4 explains which way), and they are different from the atoms of every other element.

Molecules and compounds

Atoms can join together. The connection that holds two atoms together is called a chemical bond. When two or more atoms are bonded together into a unit, that unit is called a molecule. The oxygen you breathe is made of molecules of two oxygen atoms bonded together. A water molecule is two hydrogen atoms bonded to one oxygen atom.

A compound is a substance made of two or more different elements bonded together, always in the same proportion. Water is a compound: every bit of pure water, anywhere, has exactly two hydrogen atoms for each oxygen atom. Carbon dioxide, table salt, and sugar are compounds too.

So a molecule of oxygen gas is a molecule, but it is not a compound, because both of its atoms are the same element. A water molecule is both a molecule and a compound.

A compound is something new

When elements combine into a compound, the result can be completely unlike the elements that made it. Sodium is a soft, silvery metal that bursts into flame in water. Chlorine is a poisonous yellow-green gas. Bonded together, they make sodium chloride: ordinary table salt, which you sprinkle on your food. Hydrogen and oxygen are both gases, and hydrogen burns easily; together they make water, which puts fires out.

Inside the atom: protons, neutrons, and electrons

For a long time, people thought atoms were the smallest things there are. Around 1900, scientists found that atoms are made of even smaller particles. There are three of them, called subatomic particles (sub means under or below):

• Protons, which carry a positive electric charge.
• Neutrons, which carry no charge. They are neutral, which is where the name comes from.
• Electrons, which carry a negative charge.

The protons and neutrons are packed together in the center of the atom, in a tiny, heavy core called the nucleus. The electrons move around the nucleus, far outside it.

Charge

Electric charge comes in two kinds, positive and negative. The rule is simple: opposite charges attract and like charges repel (push each other away). The negative electrons are attracted to the positive protons in the nucleus. That attraction is what keeps the electrons around the atom.

A proton’s charge is written +1 and an electron’s is written −1. They are exactly equal in size and opposite in sign. An ordinary atom has the same number of electrons as protons, so the charges cancel, and the atom as a whole has no charge. Scientists say the atom is neutral.

Mass

Atoms are so light that grams are far too big a unit for them. Scientists use the atomic mass unit (amu) instead. A proton has a mass of about 1 amu. A neutron is just slightly heavier, also about 1 amu. An electron is much, much lighter: it would take about 1,836 electrons to match the mass of one proton. For counting purposes, an electron’s mass is treated as zero.

So almost all of an atom’s mass is in its nucleus. Yet the nucleus is unbelievably small compared with the whole atom. If an atom were blown up to the size of a baseball stadium, the nucleus would be about the size of a pea at the center, and the electrons would be moving around somewhere out in the seats. Nearly all of an atom is empty space.

Counting particles: atomic number, mass number, and neutrons

Three numbers describe an atom. Once you know two of them, you can work out the third.

The atomic number

The atomic number is the number of protons in the atom’s nucleus. This number is what makes an element that element. Every atom with 6 protons is a carbon atom. Every atom with 8 protons is oxygen. Every atom with 79 protons is gold. If an atom gained or lost a proton, it would become a different element.

In a neutral atom, the number of electrons equals the number of protons. So the atomic number also tells you the number of electrons in a neutral atom.

The mass number

The mass number is the number of protons plus neutrons in the nucleus. Protons and neutrons each have a mass of about 1, and electrons have almost none, so the mass number is very close to the mass of the whole atom in atomic mass units.

Scientists show the mass number by writing it after the name of the element, with a dash. Carbon-12 is a carbon atom with a mass number of 12. Uranium-238 is a uranium atom with a mass number of 238. Another way to write this puts the mass number above and to the left of the symbol, sometimes with the atomic number below it: ¹²C, or ¹²₆C.

Working out the number of neutrons

Since the mass number counts protons and neutrons together, you can find the neutrons by taking away the protons:

Reading one box of the periodic table

The periodic table (Part 9) lists every element in a box like this one:

The whole number at the top is the atomic number, 11 for sodium. The decimal number at the bottom, 22.99, is the atomic mass. It is a decimal and not a whole number because it is an average: the atoms of an element do not all have the same number of neutrons (Part 5 explains this), and the atomic mass averages them, counting the common kinds more heavily than the rare ones.

To find the mass number of an element’s most common atom, round the atomic mass to the nearest whole number. Sodium’s 22.99 rounds to 23, so the most common sodium atom is sodium-23, with 23 − 11 = 12 neutrons. This shortcut works for most of the elements you will meet. A few are exceptions, but a test question will give you the numbers it expects you to use.

Isotopes and ions

Isotopes: same element, different neutrons

All atoms of an element have the same number of protons. But they do not all have the same number of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes of that element.

Carbon is a good example. Every carbon atom has 6 protons. Most carbon atoms (about 99 in every 100) have 6 neutrons: carbon-12. About 1 in 100 has 7 neutrons: carbon-13. A very few have 8 neutrons: carbon-14.

Isotopes of an element behave the same way in chemical reactions, because chemical behavior depends on the electrons, and the isotopes all have the same number of electrons. What differs is the mass, and sometimes how stable the nucleus is. Carbon-14 is radioactive: its nucleus slowly breaks down, at a steady, known rate. Living things take in carbon, including a little carbon-14, as long as they live. After they die, the carbon-14 in them slowly disappears. By measuring how much is left, scientists can tell how long ago a bone, a piece of wood, or a scrap of cloth was part of a living thing. This is called carbon dating, and it works for things up to about 50,000 years old.

Ions: atoms with a charge

Protons stay locked in the nucleus during chemical reactions. But electrons, out at the edge of the atom, can be gained or lost. When an atom gains or loses electrons, its protons and electrons no longer balance, and the atom has an electric charge. A charged atom is called an ion.

• An atom that loses electrons has more protons than electrons, so it becomes a positive ion.
• An atom that gains electrons has more electrons than protons, so it becomes a negative ion.

Sodium has 11 protons and 11 electrons. When it loses 1 electron, it has 11 protons and 10 electrons: 11 − 10 = +1. This sodium ion is written Na⁺. Chlorine has 17 protons and 17 electrons. When it gains 1 electron, it has 17 protons and 18 electrons: 17 − 18 = −1. This chloride ion is written Cl⁻. The charge is written small and raised, after the symbol. An atom that loses or gains two electrons gets a charge of 2+ or 2−: a magnesium ion is Mg²⁺ and an oxide ion is O²⁻.

What changes, and what you get

What holds the nucleus together

Here is a puzzle. Every proton carries a positive charge, and like charges push each other away. Packed together in a nucleus, protons push on each other very hard. So why doesn’t every nucleus fly apart?

What holds a nucleus together is a different force, called the strong nuclear force: a powerful pull that works only over the tiny distances inside a nucleus. Protons and neutrons both feel this pull. A neutron adds to the pull without adding any push, because it has no charge. So neutrons help hold the protons together, the way extra glue helps hold a stack of magnets that are trying to fly apart.

This explains why the most common kind of hydrogen has no neutron at all. Its nucleus is a single proton. With only one proton, there is no other proton pushing it away, so there is nothing to hold together. Helium, with 2 protons, has 2 neutrons. Bigger atoms need more and more neutrons for each proton: iron has 26 protons and 30 neutrons, and uranium has 92 protons and 146 neutrons. In the very largest atoms, even the extra neutrons are not quite enough, and the nucleus slowly breaks apart. That is why uranium is radioactive.

Neutrons do not change how an atom bonds. Bonding depends on the electrons, and a neutral atom has as many electrons as protons. That is why the drawings in the rest of this guide show the nucleus as one colored circle with its number of protons, and leave the neutrons out.

Electrons live in shells

The electrons in an atom are not scattered just anywhere. They are arranged in layers around the nucleus, called shells or energy levels. Scientists number the shells starting from the inside: shell 1 is closest to the nucleus, then shell 2, then shell 3, and so on.

Two things increase as you go outward. The shells get farther from the nucleus, and the electrons in them have more energy. An electron in an outer shell is held less tightly than one close to the center, the way a magnet holds a paper clip more weakly the farther away the clip is.

An electron is always in one shell or another. It cannot sit in the space between two shells. One way to picture this is a building: a person can stand on the first floor or the second floor, but not halfway between them. That is why the shells are also called energy levels.

How many electrons each shell holds

Each shell has room for only so many electrons. For the first 20 elements, the ones you will meet most on the GED test, the rule is:

The shells fill from the inside out. An atom puts its first 2 electrons in shell 1. When shell 1 is full, the next electrons go into shell 2, up to 8. When shell 2 is full, the next ones go into shell 3. Sodium, with 11 electrons, has 2 in shell 1, 8 in shell 2, and 1 in shell 3. Scientists write this as 2, 8, 1.

Full shells and valence electrons

A full outer shell means stable

Look at three elements in the builder above: helium (2), neon (2, 8), and argon (2, 8, 8). Each one has a full outer shell. These three are called noble gases, and they almost never react with anything. They do not need to gain, lose, or share electrons, because their outer shell is already complete. Scientists call an atom like this stable.

Every other atom has an outer shell that is not full. Atoms react with each other in ways that leave them with a full outer shell: 8 electrons in most cases, or 2 for the smallest atoms, hydrogen and helium, whose only shell is shell 1. This is sometimes called the rule of eight, or the octet rule (octet means a group of eight).

Valence electrons: the ones that matter for bonding

The electrons in an atom’s outer shell are called its valence electrons. They are the farthest from the nucleus and the most loosely held, so they are the electrons that take part when atoms bond. The inner electrons stay out of it.

Sodium (2, 8, 1) has 1 valence electron. Chlorine (2, 8, 7) has 7. Oxygen (2, 6) has 6. Carbon (2, 4) has 4.

The number of valence electrons tells you what an atom is likely to do:

• 1, 2, or 3 valence electrons (most of these are metals): it is easier to give away a few electrons than to find 5 or more. Sodium gives up its one.
• 5, 6, or 7 valence electrons (nonmetals): it is easier to take or share a few more than to give away so many. Chlorine takes one.
• 4 valence electrons (carbon and silicon): halfway there, so it tends to share. Carbon shares its 4, which is why it can build the long chains in sugar, fat, and DNA.
• A full outer shell (noble gases): nothing to do. It stays alone.

Notice what happens to the ions from Part 5. When sodium (2, 8, 1) loses its one valence electron, it is left with 2, 8: a full outer shell, like neon. When chlorine (2, 8, 7) gains one, it has 2, 8, 8: a full outer shell, like argon. Atoms form the ions that give them full outer shells.

The periodic table

The periodic table arranges all 118 elements in order of their atomic numbers, in rows and columns. It was first worked out in 1869 by a Russian chemist, Dmitri Mendeleev. He arranged the elements known in his day so that elements with similar properties fell in the same column, and where the pattern called for an element no one had found yet, he left a gap. He even predicted what the missing elements would be like. When gallium and germanium were discovered in the years that followed, they matched his predictions closely.

Rows and columns

The rows are called periods. Every element in a row has the same number of electron shells: the elements in period 2 have 2 shells, and the elements in period 3 have 3. The columns are called groups or families. The elements in a column have the same number of valence electrons, and that is why they behave alike.

For the tall columns, you can read the number of valence electrons right off the table:

For groups 13 to 18, take away 10 from the group number. Some tables number these columns 1A to 8A instead; then the number in front of the A is the number of valence electrons. Helium is the one exception in group 18: its only shell is shell 1, so it is full with 2. The short columns in the middle of the table, groups 3 to 12, follow a more complicated pattern, and the GED test does not ask about them.

Families that behave alike

• Group 1 (lithium, sodium, potassium): 1 valence electron each. All are soft metals that react strongly with water, and the reaction gets stronger going down the column. (Hydrogen sits at the top of group 1 because it has 1 valence electron, but it is a gas and a nonmetal, not a metal.)
• Group 2 (beryllium, magnesium, calcium): 2 valence electrons each. Also reactive metals, but less so than group 1.
• Group 17 (fluorine, chlorine, bromine, iodine): 7 valence electrons each. All grab one more electron easily, and all pair up with group 1 metals to make salts like sodium chloride.
• Group 18 (helium, neon, argon): full outer shells. None of them reacts in everyday conditions.

Metals, nonmetals, and metalloids

A heavy staircase line runs down the right side of the table. The elements to its left are metals, and the elements to its right are nonmetals. The elements that sit along the line are metalloids.

Patterns across a row

Going from left to right across a row, each element has one more proton and one more valence electron than the one before. The elements on the left are metals that give electrons away easily. The elements on the right are nonmetals that pull electrons in. The row ends with a noble gas, whose outer shell is full. Then the next row starts a new shell, and the pattern repeats. That repeating pattern is why the table is called periodic.

Chemical symbols and formulas

Chemical symbols

Each element has a chemical symbol of one or two letters. The first letter is always a capital; the second, if there is one, is always small. Most symbols come from the English name: H for hydrogen, O for oxygen, C for carbon, Ca for calcium, Cl for chlorine. A few come from the old Latin names: Na for sodium (natrium), K for potassium (kalium), Fe for iron (ferrum), Au for gold (aurum), Ag for silver (argentum), and Pb for lead (plumbum).

Chemical formulas

A chemical formula shows which elements are in a substance and how many atoms of each. The small number written low after a symbol is called a subscript. It tells how many atoms of the element just before it. When there is no subscript, there is one atom.

Parentheses and big numbers in front

Sometimes a group of atoms is put in parentheses with a subscript after it. The subscript then multiplies everything inside. Calcium hydroxide, Ca(OH)₂, has 1 calcium, 2 oxygen, and 2 hydrogen atoms.

A large number written in front of a formula is called a coefficient. It tells how many molecules there are. 2H₂O means two water molecules: 2 × 2 = 4 hydrogen atoms and 2 × 1 = 2 oxygen atoms. You will use coefficients when you balance chemical equations in Chemistry 2.

Covalent bonds: sharing electrons

Atoms bond in ways that leave them with full outer shells. There are two main ways to do it, and which one happens depends on what kind of atoms meet.

• When two nonmetals meet, neither one gives up electrons easily, so they share. A shared pair of electrons is a covalent bond. This part is about covalent bonds.
• When a metal meets a nonmetal, the metal hands over its few valence electrons and the nonmetal takes them. The atoms become ions with opposite charges, and the charges attract. That attraction is an ionic bond. Part 12 is about ionic bonds.

Sharing: the covalent bond

In a covalent bond, two atoms each put one electron into a pair, and both atoms count that pair as their own. The shared pair sits between the two nuclei, and both nuclei pull on it. That shared pull is what holds the atoms together. (Co- means together, as in co-workers; the atoms hold their valence electrons together.)

Water is the classic example. Oxygen has 6 valence electrons and needs 2 more. Each hydrogen has 1 and needs 1 more to fill shell 1. Oxygen shares one pair with each hydrogen. Then oxygen counts 8 in its outer shell, and each hydrogen counts 2. Everyone is full.

More than one shared pair

Sometimes two atoms share more than one pair. In oxygen gas, O₂, each oxygen atom has 6 valence electrons and needs 2 more. The two atoms share two pairs, four electrons in all. Two shared pairs make a double bond.

In carbon dioxide, CO₂, carbon has 4 valence electrons and needs 4 more. Each oxygen needs 2. Carbon shares two pairs with each oxygen: two double bonds. Carbon now counts 8, and so does each oxygen.

In methane, CH₄, carbon shares one pair with each of four hydrogen atoms: four single bonds. Look back at the drawing of four molecules in Part 10 to see all of these.

Carbon’s 4 valence electrons let it form four bonds at once, with hydrogen, oxygen, nitrogen, and above all other carbon atoms. That is how carbon builds the long chains and rings in sugar (C₆H₁₂O₆), fat, protein, and DNA, the molecules of life. All of their bonds are covalent.

Substances held together by covalent bonds are made of separate molecules. Many of them are gases or liquids at room temperature (oxygen, carbon dioxide, water), or solids that melt fairly easily (sugar, wax).

Ionic bonds: handing electrons over

When a metal meets a nonmetal, they do not share. The metal has only 1, 2, or 3 valence electrons, held loosely, and the nonmetal pulls on electrons strongly. So the metal hands over its valence electrons, and the nonmetal takes them.

Sodium and chlorine show how this works. Sodium (2, 8, 1) gives its one valence electron to chlorine (2, 8, 7). Now sodium is 2, 8, with a full outer shell, and chlorine is 2, 8, 8, also full. But the trade has left them charged. Sodium has 11 protons and only 10 electrons: it is a positive ion, Na⁺. Chlorine has 17 protons and 18 electrons: it is a negative ion, Cl⁻.

Opposite charges attract. The pull between the positive sodium ion and the negative chloride ion is called electrostatic attraction, and it is what holds them together. This attraction between oppositely charged ions is an ionic bond.

Ionic compounds form crystals

An ionic compound is not made of separate molecules. A grain of salt is a huge, orderly stack of sodium ions and chloride ions, one after another, in every direction. Each sodium ion is surrounded by six chloride ions, and each chloride ion by six sodium ions. A regular, repeating arrangement like this is called a crystal. That is why salt grains, seen under a magnifying glass, are little cubes.

The formula NaCl does not describe one molecule. It describes the ratio of ions in the crystal: one sodium ion for each chloride ion.

Ionic compounds share some properties. They are hard but break easily, and they melt only at high temperatures (table salt melts at 801°C, about 1,474°F). Many dissolve in water, and when they do, the ions come apart and move freely, so the solution conducts electricity. That is why salt water conducts electricity and pure water hardly does.

Other ionic compounds

• Magnesium oxide, MgO. Magnesium (2 valence electrons) gives both of them to oxygen (6 valence electrons, needs 2). The ions are Mg²⁺ and O²⁻. Magnesium oxide is the bright white powder left when magnesium metal burns, as it does in some flares and fireworks.
• Calcium chloride, CaCl₂. Calcium has 2 valence electrons to give away, but each chlorine atom takes only one. So it takes two chlorine atoms to take both: one Ca²⁺ ion for every two Cl⁻ ions. That is what the subscript 2 says. Calcium chloride is spread on icy sidewalks.
• Calcium carbonate, CaCO₃, the material of chalk, limestone, seashells, and eggshells, and an ingredient in some antacid tablets. Here the calcium ion, Ca²⁺, is bonded to a group of atoms, the carbonate group, CO₃. Inside the carbonate group, the carbon and oxygen atoms share electrons: their bonds are covalent. The group as a whole carries a charge of 2−, and it is held to the calcium ion by an ionic bond. So one compound can contain both kinds of bond.
• Sodium bicarbonate, NaHCO₃, baking soda. In the same way, a sodium ion, Na⁺, is held by an ionic bond to the bicarbonate group, HCO₃⁻, whose own atoms are joined by covalent bonds.

Which kind of bond? A rule of thumb

You can usually tell which kind of bond two elements will form just by finding them on the periodic table.

Remember that hydrogen counts as a nonmetal, even though it sits at the top of group 1.

Like most rules of thumb, this one has exceptions, but the GED test sticks to clear cases like the ones above.

The terms in this guide

15 questions on this guide

After this guide

• Study guide: Chemistry 2: Mixtures, Changes, and Reactions The next guide: mixtures and solutions, states of matter, density, chemical reactions and their energy, and acids and bases.
• Chemistry The short reading on chemistry, and the door to the other chemistry pages.
• Physical Science Chemistry and physics together.
• Study guide: How Life Is Organized From atoms and molecules up to cells, organs, and whole living things.
• Study guide: Photosynthesis and Cellular Respiration Chemical energy at work in living things: the bonds in sugar, made and broken.
• Study guide: Science Practices How to read the tables, diagrams, and data that chemistry questions use.