Matter and Atoms — How to Read the Periodic Table

1.The structure of the atom

All matter is made of extremely small particles called atoms. At the center of an atom is the nucleus, made of protons, which carry a (+) charge, and neutrons, which carry no charge; around it are electrons, which carry a (−) charge. An atom has equal numbers of protons and electrons, so overall it is electrically neutral.

The nucleus is tiny compared with the whole atom: an atom is tens of thousands of times larger than its nucleus, or more. Even so, almost all of the mass is concentrated in the nucleus, because a single proton has about 1836 times the mass of an electron.

The textbook picture of an atom as planets orbiting a sun is a model to aid understanding. According to modern science, an electron is not a little ball circling on a fixed orbit; it exists in a state where we can only state the probability of finding it somewhere around the nucleus. Still, the model that places electrons in separate shells (electron shells) remains useful for explaining chemical properties.

2.Elements, compounds, and mixtures

An element is a basic substance that cannot be broken down into other substances. Like hydrogen, oxygen, carbon, and iron, it is made of only one kind of atom. There are 118 known elements; only some of them are found in nature, and the rest have been made in the laboratory.

A compound is a substance in which two or more elements are chemically bonded in a fixed ratio. Water (H₂O) consists of hydrogen and oxygen bonded in a 2 : 1 ratio by number of atoms, and sodium chloride (NaCl), the main ingredient of table salt, consists of sodium and chlorine bonded 1 : 1. A compound has properties completely different from those of its elements. Sodium is a metal that reacts violently with water and chlorine is a toxic gas, but bonded together they form sodium chloride, the salt we eat.

A mixture is two or more substances mixed together without chemical bonding. Salt water, air, milk, and soil are examples. The proportions are not fixed, and each component keeps its own properties, so mixtures can be separated by physical methods such as evaporation, filtering, and distillation. Getting salt by evaporating seawater is one example.

3.Reading the periodic table

The periodic table lists the elements in order of atomic number, arranged so that elements with similar properties fall in the same vertical column. The horizontal rows are called periods (periods 1–7), and the vertical columns are called groups (groups 1–18). Each box usually shows the atomic number, the element symbol, the element name, and the atomic weight.

Elements in the same period have the same number of shells containing electrons, and elements in the same group have the same number of electrons in the outermost shell (valence electrons). Because it is mainly these outer electrons that take part in chemical reactions, elements in the same group have similar properties.

Metals are mostly on the left and in the middle, and nonmetals are mostly in the upper right. In group 1, lithium, sodium, and potassium are highly reactive metals (hydrogen sits in group 1 but is a nonmetal); in group 17, fluorine and chlorine are highly reactive nonmetals; and in group 18, helium, neon, and argon are noble gases that hardly react at all. It is also worth remembering that lithium, used in phone batteries, is a group 1 metal.

4.Ions and isotopes

When an atom loses or gains electrons, its numbers of protons and electrons no longer match, so it becomes charged. This is called an ion. Losing electrons makes a positively charged (+) cation, and gaining electrons makes a negatively charged (−) anion. Only the number of electrons changes; the number of protons stays the same, so becoming an ion does not change which element it is.

Conversely, there are atoms with the same number of protons but different numbers of neutrons. These are called isotopes. Because they have the same number of protons, they are the same element and have nearly the same chemical properties, but their mass numbers differ. Besides carbon-12 and carbon-13, carbon has a radioactive isotope, carbon-14, and the fact that carbon-14 decreases at a steady rate is used to estimate the age of ancient artifacts. The atomic weights on the periodic table are not whole numbers because they are averages over mixtures of several isotopes like these.

5.States of matter and changes of state

Matter usually exists in three states: solid, liquid, and gas. In a solid, the particles only vibrate in place, so its shape and volume are fixed; in a liquid, the particles can swap places, so its volume is fixed but its shape changes with the container; and in a gas, the particles fly around freely, so neither its shape nor its volume is fixed.

When the state changes, the particles themselves don't change — only the distance between them and their arrangement — so a change of state is a physical change, not a chemical one. Ice, water, and steam are all the same water molecules. At 1 atmosphere of pressure, water freezes at 0 °C and boils at 100 °C. High in the mountains, the air pressure is lower, so water boils at a temperature below 100 °C.

Heat flows in or out when a state changes. Melting and evaporating absorb heat, while freezing and condensing release it. Sprinkling water on a yard in summer cools it down, and drying sweat lowers your body temperature, because evaporation absorbs heat from the surroundings. Refrigerators and air conditioners also rely on a refrigerant absorbing heat as it evaporates.