原子、元素与化合物

All substances are made from about 100 simple building blocks. Knowing how they are joined lets you sort every substance into one of three groups.

• An element 元素 is a substance made of only one type of atom 原子. You cannot break it into anything simpler by a chemical reaction. Examples: copper, oxygen, carbon.
• A compound 化合物 is two or more elements chemically joined (bonded) together. The atoms are joined in a fixed ratio. Examples: water, carbon dioxide. A compound has different properties from the elements in it.
• A mixture 混合物 is two or more substances that are just mixed, not chemically joined. The parts keep their own properties and can be separated by physical methods. Example: air.

The key difference: in a compound the elements are bonded and can only be separated by chemical reactions; in a mixture they are not bonded and are easy to separate.

Inside the atom

Every atom has a small, dense centre called the nucleus 原子核. Around it, electrons 电子 move in shells 电子层 (energy levels). The nucleus contains protons 质子 and usually neutrons 中子. The common hydrogen-1 nucleus has one proton and no neutron.

Each particle has a relative mass and a relative charge 电荷. You must learn these values:

An atom has no overall charge because it has equal numbers of protons ($+1$ each) and electrons ($-1$ each).

Proton number and mass number

Two numbers describe an atom:

• The proton number 质子数 (also called the atomic number 原子序数) is the number of protons in the nucleus. It tells you which element the atom is.
• The mass number 质量数 (also called the nucleon number 核子数) is the total number of protons and neutrons in the nucleus.

So the number of neutrons $=$ mass number $-$ proton number.

Electronic configuration

The electrons fill the shells from the inside out. The first shell holds up to 2 electrons; the next shells hold up to 8 each (for the first 20 elements). The electronic configuration 电子排布 lists how many electrons are in each shell, starting from the inside.

For example, an atom with 13 electrons has the configuration $2,8,3$. Sodium (proton number 11) is $2,8,1$. Calcium (proton number 20) is $2,8,8,2$.

The configuration links to the Periodic Table 周期表:

• A Group 族 number (Groups I to VII) equals the number of electrons in the outer shell. So $2,8,1$ is in Group I.
• A Period 周期 number equals the number of shells that hold electrons. So $2,8,1$ has three shells, so it is in Period 3.
• The noble gases 稀有气体 in Group VIII (or 0) have a full outer shell, which makes them very unreactive.

Isotopes 同位素 are atoms of the same element that have the same number of protons but different numbers of neutrons. Because the proton number is the same, they are the same element. Because the neutron number is different, they have different mass numbers.

You write an atom with its mass number on top and proton number below, like $^{12}_{6}\text{C}$. For an ion you add the charge, like $^{35}_{17}\text{Cl}^{-}$.

Isotopes of an element have the same chemical properties. This is because chemical reactions only involve electrons, and isotopes have the same number of electrons and the same electronic configuration. (The extra neutrons change only the mass.)

Calculating relative atomic mass

The relative atomic mass 相对原子质量 ($A_r$) of an element is the average mass of its atoms, taking into account how common each isotope is. The abundance 丰度 is the percentage of each isotope.

For example, chlorine is 75% $^{35}\text{Cl}$ and 25% $^{37}\text{Cl}$:

Worked example. Boron has two isotopes, $^{10}\text{B}$ and $^{11}\text{B}$, and its $A_r$ is 10.8. Find the percentage of each. Let the abundance of $^{11}\text{B}$ be $x$, so the abundance of $^{10}\text{B}$ is $(100 - x)$. Then

so $11x + 1000 - 10x = 1080$, giving $x = 80$. Boron is 80% $^{11}\text{B}$ and 20% $^{10}\text{B}$. Check it against common sense: 10.8 lies closer to 11, so the heavier isotope must be the more common one. If your answer gives the majority to the isotope further from $A_r$, you have them the wrong way round.

An ion 离子 is an atom (or group of atoms) that has lost or gained electrons, so it has an electric charge.

• A metal atom loses electrons to form a positive ion, called a cation 阳离子.
• A non-metal atom gains electrons to form a negative ion, called an anion 阴离子.

Atoms do this to get a full outer shell, like a noble gas.

How an ionic bond forms

An ionic bond 离子键 is a strong electrostatic attraction 静电引力 between oppositely charged ions (a $+$ ion and a $-$ ion pull together).

Many common ionic compounds form from a metal 金属 and a non-metal 非金属. Take sodium chloride, $\text{NaCl}$. Sodium ($2,8,1$) gives its one outer electron to chlorine ($2,8,7$). Now sodium is $\text{Na}^{+}$ ($2,8$) and chlorine is $\text{Cl}^{-}$ ($2,8,8$). Both have full outer shells, and the opposite charges attract.

You can show this with a dot-and-cross diagram: draw each atom's outer-shell electrons as dots for one element and crosses for the other, then show the electron moving from the metal to the non-metal.

The structure and properties of ionic compounds

An ionic compound 离子化合物 is not made of separate molecules 分子. The ions pack together into a giant lattice 晶格 — a regular pattern of huge numbers of alternating 交替 positive and negative ions.

This structure explains the properties:

A covalent bond 共价键 forms when two atoms share a pair of electrons. In the examples here, sharing gives the atoms full outer shells (noble gas configurations). Covalent bonds form between non-metal atoms.

Some molecules to know:

• $\text{H}_2$ — two hydrogen atoms share one pair of electrons (a single bond).
• $\text{Cl}_2$, $\text{HCl}$ — one shared pair each.
• $\text{H}_2\text{O}$ — oxygen shares one pair with each of two hydrogen atoms.
• $\text{NH}_3$ — nitrogen shares a pair with each of three hydrogen atoms.
• $\text{CH}_4$ — carbon shares a pair with each of four hydrogen atoms.
• $\text{O}_2$ and $\text{CO}_2$ have double bonds (two shared pairs); $\text{N}_2$ has a triple bond (three shared pairs); $\text{C}_2\text{H}_4$ and $\text{CH}_3\text{OH}$ also use shared pairs.

In a dot-and-cross diagram for a molecule, you draw the outer electrons of each atom and show which pairs are shared in the overlap between the atoms.

Properties of simple molecular compounds

These substances are made of small, separate molecules.

• They have low melting points and boiling points. The covalent bonds inside each molecule are strong, but the intermolecular forces 分子间作用力 (the forces between one molecule and the next) are weak, so little energy is needed to separate the molecules.
• They are poor conductors of electricity, because the molecules have no overall charge and no free electrons or ions to carry charge.

Some covalent substances are not small molecules. Instead, millions of atoms are joined by covalent bonds into one giant covalent structure 巨型共价结构. The two you must know are both forms of carbon.

Diamond 金刚石: each carbon atom is bonded to four other carbon atoms. This makes a very strong, rigid 3-D network. Diamond is extremely hard, so it is used in cutting tools 切割工具.

Graphite 石墨: each carbon atom is bonded to only three others, forming flat layers 层. There are weak forces between the layers, so the layers can slide over each other — this makes graphite a good lubricant 润滑剂. The fourth outer electron of each carbon is free; these delocalised electrons 离域电子 can move, so graphite conducts electricity and is used as an electrode 电极.

Silicon(IV) oxide 二氧化硅 ($\text{SiO}_2$) has a giant covalent structure like diamond, so it is also very hard and has a very high melting point.

A metal is a giant structure of positive ions surrounded by a 'sea' of delocalised electrons that are free to move through the whole metal. Metallic bonding 金属键 is the strong electrostatic attraction between these positive ions and the sea of electrons.

This explains two key properties of metals:

• Good electrical conductivity: the delocalised electrons are free to move and carry charge through the metal.
• Malleability 展性 (can be hammered into sheets) and ductility 延性 (can be pulled into wires): the layers of positive ions can slide over each other without breaking the metallic bond, so the metal changes shape instead of shattering.

Common mistakes.

• Saying a simple molecular substance melts easily because its covalent bonds are weak. The covalent bonds inside each molecule are strong; it is the intermolecular forces between molecules that are weak and are overcome.
• Saying solid sodium chloride does not conduct because it has no ions, or saying molten sodium chloride conducts by moving electrons. It always has ions; when molten or aqueous the ions are free to move and carry the charge.
• Calling a lattice of $\text{Na}^{+}$ and $\text{Cl}^{-}$ ions "NaCl molecules". An ionic compound has no molecules; it is a giant lattice of ions.
• Saying graphite conducts because its layers slide. It conducts because each carbon has one delocalised electron that can move; the sliding layers explain why it is soft and slippery.
• Drawing a metal atom gaining electrons. Metals lose electrons to form positive ions; non-metals gain electrons.
• Saying isotopes have different numbers of protons or electrons. Isotopes differ only in the number of neutrons.

• Number of neutrons = mass number − proton number. The proton number is what decides which element an atom is.
• Isotopes have the same chemical properties because reactions involve only electrons, and isotopes share the same electron configuration; only the mass differs.
• Match structure to properties: ionic = giant lattice (high melting point; conducts only when molten or aqueous); simple molecular = weak forces between molecules (low melting point; no conduction); giant covalent = very hard, very high melting point.
• Graphite conducts and is slippery (one free electron per carbon; layers slide); diamond does neither (all four electrons bonded, so hard and non-conducting). Metals conduct and bend thanks to the sea of delocalised electrons.
• For relative atomic mass, take the weighted average: $A_r = \dfrac{\sum(\text{isotope mass} \times \text{abundance})}{100}$.