Atomic Structure and Chemical Bond

An element is a pure substance that can’t be broken down into other substances with different properties. Iron is an element: however you break it, crush it or melt it, you only ever get iron. An element is made of a huge number of atoms of one kind.

An atom is the smallest particle of an element that can take part in a chemical reaction.

• All the atoms of one element are alike, and they are different from the atoms of every other element. Every atom in a gold ring is a gold atom; a silver atom is different from it, and a copper atom is different again.
• Most atoms readily react with the atoms of other elements. A few, the atoms of the inert gases (helium, neon, argon and their family), hardly react at all.
• A compound is made of the atoms of two or more elements joined together: water is hydrogen and oxygen, common salt is sodium and chlorine.

Atoms are tiny. The average diameter of an atom is about 10⁻¹⁰ m, one ten-billionth of a meter. Line up ten million atoms and the row would be just 1 mm long.

An atom is made of three even smaller particles, the subatomic particles: protons, neutrons and electrons.

• Protons and neutrons are packed together in a tiny center called the nucleus.
• Electrons move around the nucleus at high speed in fixed paths called orbits or shells.

Particle	Where it is	Charge	Mass (in amu)
proton (p⁺)	in the nucleus	+1 (positive)	1
neutron (n⁰)	in the nucleus	0 (neutral)	1
electron (e⁻)	in the shells, around the nucleus	−1 (negative)	about 1/1840

An atom has as many electrons as protons, so their charges cancel and the whole atom is electrically neutral.

• The number of protons in an atom is its atomic number. It decides which element the atom is: every atom with 6 protons is carbon, every atom with 8 is oxygen.
• The number of protons plus neutrons is its mass number.

Shells and their energy

The electrons can’t be just anywhere. In 1913 the Danish scientist Niels Bohr showed that electrons move only in certain shells, each with a fixed energy. The shells are named K, L, M, N… going outwards (or numbered n = 1, 2, 3, 4…). The K shell, nearest the nucleus, has the lowest energy.

Each shell holds at most 2n² electrons, where n is the shell’s number:

Shell	n	Most electrons, 2n²
K	1	2 × 1² = 2
L	2	2 × 2² = 8
M	3	2 × 3² = 18
N	4	2 × 4² = 32

Electrons fill the shells from the inside out, K first. There is one more rule for the first 20 elements: the outermost shell never holds more than 8 electrons. That is why potassium (19 electrons) is 2, 8, 8, 1 and not 2, 8, 9: its 19th electron starts the N shell even though the M shell could hold 18. Writing the electrons shell by shell like this is called the atom’s electronic configuration.

Bohr’s shells are the picture you will use to count electrons, and they work well for that. But no one has ever seen an electron going round a track. Since 1926 scientists have known that an electron has no fixed path at all: we can only say where it is likely to be found. So its place is pictured as an electron cloud, thick where the electron is often found and thin where it rarely is. Each shell is, roughly, a cloud at its own distance from the nucleus.

Helium has only 2 electrons, and both are in its one and only shell, the K shell. The K shell holds at most 2 (2 × 1² = 2), so helium’s shell is full.

Having two electrons in the K shell, when it is the only shell, is called the duplet state. It is a very stable state. Because of it, helium doesn’t take part in any chemical reaction, and it is found in nature as free, single atoms.

Hydrogen (1 electron), lithium (2, 1) and beryllium (2, 2) are close to helium. In reactions they lose, gain or share electrons so that their outer shell ends up as a duplet, like helium’s: hydrogen by sharing (or by losing its electron), lithium by losing 1 and beryllium by losing 2. The tendency of these atoms to get two electrons in the K shell is called the duplet rule.

Neon (2, 8), argon (2, 8, 8), krypton, xenon and radon, the other members of the inert gas group (group 18 of the modern periodic table), all have 8 electrons in their outermost shell. This state is called the octet state (octa means eight).

Atoms in the octet state, like those in the duplet state, are stable: they don’t take part in chemical reactions under ordinary conditions, and they are found as single atoms, as gases. That is why these elements are called inert gases (or noble gases).

Every other atom has fewer than 8 electrons in its outermost shell. These atoms react: they lose, gain or share electrons until their outermost shell holds 8, like an inert gas. This tendency is the octet rule.

Sodium has 11 electrons, but only the 1 electron in its outermost shell, the M shell, takes part in its chemical reactions. The inner shells are full and stay as they are.

• The outermost shell of an atom is its valence shell. For sodium and chlorine it is the M shell.
• The electrons in the valence shell are the valence electrons. Sodium has 1 valence electron; chlorine has 7.

In a reaction, an atom loses, gains or shares some of its valence electrons. The number of electrons an atom loses, gains or shares when it forms a compound decides how many other atoms it can join with. The combining capacity of an atom with other atoms is called its valency.

• An atom with 1, 2 or 3 valence electrons usually loses them; its valency is 1, 2 or 3.
• An atom with 5, 6 or 7 valence electrons usually gains 3, 2 or 1; its valency is 8 minus the number of valence electrons.
• An atom with 4 valence electrons (carbon, silicon) shares them; its valency is 4.
• Inert gases have valency 0.

Here are the first 20 elements:

Element	Symbol	Atomic number	K	L	M	N	Valency
hydrogen	H	1	1				1
helium	He	2	2				0
lithium	Li	3	2	1			1
beryllium	Be	4	2	2			2
boron	B	5	2	3			3
carbon	C	6	2	4			4
nitrogen	N	7	2	5			3
oxygen	O	8	2	6			2
fluorine	F	9	2	7			1
neon	Ne	10	2	8			0
sodium	Na	11	2	8	1		1
magnesium	Mg	12	2	8	2		2
aluminium	Al	13	2	8	3		3
silicon	Si	14	2	8	4		4
phosphorus	P	15	2	8	5		3, 5
sulphur	S	16	2	8	6		2, 6
chlorine	Cl	17	2	8	7		1
argon	Ar	18	2	8	8		0
potassium	K	19	2	8	8	1	1
calcium	Ca	20	2	8	8	2	2

Phosphorus and sulphur have two valencies: in some compounds they share all their valence electrons (5 or 6), in others only enough to complete an octet (3 or 2).

Reactivity and valency

Among the metals of one row of the periodic table, the one with fewer electrons to lose usually reacts more vigorously. Sodium (valency 1) needs to lose only one electron, and it reacts violently with cold water; magnesium (valency 2) must lose two, and it reacts only slowly with cold water. (This is a rough guide for metals, not a law: fluorine, with valency 1, is the most reactive non-metal of all, while nitrogen, with valency 3, hardly reacts. Grade 10 explains reactivity more fully, in How properties change across periods and down groups.)

Variable valency

Some elements have more than one valency, depending on the compound they form. When only the valence electrons take part, the valency is the lower one; when an electron from the shell inside also takes part, the valency is higher.

Element	Valencies	Examples
iron (Fe)	2 and 3	FeO, ferrous oxide; Fe₂O₃, ferric oxide (rust)
copper (Cu)	1 and 2	Cu₂O, cuprous oxide (red); CuO, cupric oxide (black)
gold (Au)	1 and 3	AuCl, aurous chloride; AuCl₃, auric chloride
mercury (Hg)	1 and 2	Hg₂Cl₂, mercurous chloride; HgCl₂, mercuric chloride

The ending tells you which: -ous for the lower valency, -ic for the higher. Silver has just one usual valency, 1.

An atom has equal numbers of protons (+) and electrons (−), so it has no overall charge. When an atom, or a group of atoms, loses or gains electrons from its valence shell, the numbers no longer match, and it becomes charged. A charged atom or group of atoms is called an ion.

The charge is written at the top right of the symbol: Na⁺, K⁺, Cu⁺, Mg²⁺, Ca²⁺, Zn²⁺, Al³⁺, Cl⁻, O²⁻, N³⁻. The number of charges on an ion is equal to its valency. (Some books write Mg²⁺ as Mg⁺⁺ and O²⁻ as O⁻⁻; they mean the same.)

Positively charged ions (cations)

When an atom loses one or more electrons from its valence shell, it has more protons than electrons, so it becomes positively charged. An ion formed by losing electrons is a positive ion or cation. Metals form cations.

$$\text{Na} - e^- \rightarrow \text{Na}^+ \qquad \text{(11 protons, now 10 electrons: charge +1)}$$

Valency 1	Valency 2	Valency 3
hydrogen, H⁺	magnesium, Mg²⁺	aluminium, Al³⁺
lithium, Li⁺	calcium, Ca²⁺	ferric (iron(III)), Fe³⁺
sodium, Na⁺	beryllium, Be²⁺	auric (gold(III)), Au³⁺
potassium, K⁺	zinc, Zn²⁺
silver, Ag⁺	manganese, Mn²⁺
cuprous (copper(I)), Cu⁺	cupric (copper(II)), Cu²⁺
mercurous (mercury(I)), Hg⁺ (really a pair, Hg₂²⁺)	mercuric (mercury(II)), Hg²⁺
aurous (gold(I)), Au⁺	ferrous (iron(II)), Fe²⁺
ammonium, NH₄⁺

Boron also has valency 3, but it shares its electrons instead of forming B³⁺ ions.

Negatively charged ions (anions)

When an atom gains electrons from other atoms, it has more electrons than protons, so it becomes negatively charged. An ion formed by gaining electrons is a negative ion or anion. Non-metals form anions.

$$\text{Cl} + e^- \rightarrow \text{Cl}^- \qquad \text{(17 protons, now 18 electrons: charge −1)}$$

Valency 1	Valency 2	Valency 3
fluoride, F⁻	oxide, O²⁻	nitride, N³⁻
chloride, Cl⁻	sulphide, S²⁻	phosphate, PO₄³⁻
bromide, Br⁻	sulphate, SO₄²⁻
iodide, I⁻	carbonate, CO₃²⁻
nitrate, NO₃⁻
hydroxide, OH⁻
bicarbonate, HCO₃⁻
bisulphate, HSO₄⁻
chlorate, ClO₃⁻
nitrite, NO₂⁻

Some ions, like NH₄⁺, OH⁻, NO₃⁻, SO₄²⁻, CO₃²⁻ and PO₄³⁻, are groups of atoms that stay together and carry a charge as one unit. Such a group is called a radical (or a polyatomic ion). The names of anions made from one atom end in -ide: chloride, oxide, sulphide.

The atoms of a compound are held together by a force called a chemical bond. In table salt, for example, sodium and chlorine are held together by chemical bonds.

Why do atoms bond at all? Apart from the inert gases (He, Ne, Ar, Kr, Xe and Rn), no atom has a duplet or an octet in its outermost shell: its outer shell is incomplete. Atoms take part in chemical reactions, forming bonds, to get a duplet or octet.

Not every compound has the same kind of bond. There are several kinds; in this unit you will study the two most important, the electrovalent (ionic) bond and the covalent bond.

Metal atoms usually have 1, 2 or 3 electrons in their valence shell. The easiest way for them to reach an octet (or a duplet) is to lose those electrons.

Non-metal atoms usually have 5, 6 or 7 valence electrons. The easiest way for them to reach an octet is to gain 3, 2 or 1 electrons.

So when a metal meets a non-metal, the metal gives its valence electrons to the non-metal. The metal becomes a positive ion and the non-metal a negative ion. Opposite charges attract each other strongly (like the north and south poles of two magnets), and this electrostatic attraction holds the ions together.

The bond formed by the transfer of electrons from the valence shell of a metal atom to the valence shell of a non-metal atom is called an electrovalent bond or ionic bond. A compound held together by electrovalent bonds is an electrovalent compound (or ionic compound): sodium chloride (NaCl), magnesium oxide (MgO) and calcium chloride (CaCl₂) are examples.

a. Formation of sodium chloride (NaCl)

Sodium is 2, 8, 1: it has 1 valence electron. Chlorine is 2, 8, 7: it has 7. Sodium gives its one valence electron to chlorine.

$$\text{Na} - e^- \rightarrow \text{Na}^+ \qquad\qquad \text{Cl} + e^- \rightarrow \text{Cl}^-$$

Now Na⁺ is 2, 8 and Cl⁻ is 2, 8, 8: both have an octet. The opposite charges attract, and an electrovalent bond holds them together:

$$\text{Na}^+ + \text{Cl}^- \rightarrow \text{NaCl}$$

b. Formation of magnesium oxide (MgO)

Magnesium (a metal) is 2, 8, 2; oxygen (a non-metal) is 2, 6. Magnesium loses its 2 valence electrons, and an oxygen atom takes both:

$$\text{Mg} - 2e^- \rightarrow \text{Mg}^{2+} \qquad\qquad \text{O} + 2e^- \rightarrow \text{O}^{2-}$$

Mg²⁺ (2, 8) and O²⁻ (2, 8) both have an octet, and they attract each other:

$$\text{Mg}^{2+} + \text{O}^{2-} \rightarrow \text{MgO}$$

c. Formation of calcium chloride (CaCl₂)

Calcium is 2, 8, 8, 2: it has 2 valence electrons to lose. But a chlorine atom needs only 1. So two chlorine atoms are needed: calcium gives one electron to each.

$$\text{Ca} - 2e^- \rightarrow \text{Ca}^{2+} \qquad\qquad 2\text{Cl} + 2e^- \rightarrow 2\text{Cl}^-$$

One Ca²⁺ ion and two Cl⁻ ions attract each other, and calcium chloride is formed:

$$\text{Ca}^{2+} + 2\text{Cl}^- \rightarrow \text{CaCl}_2$$

Magnesium chloride (MgCl₂) forms in exactly the same way: magnesium (2, 8, 2) gives one electron to each of two chlorine atoms.

Characteristics of electrovalent compounds

• a.They are formed between a metal and a non-metal.
• b.Most of them dissolve in water, and when they dissolve, they separate into positive and negative ions. That is why salt water conducts electricity. (A few, like calcium carbonate, the stuff of marble, hardly dissolve.)
• c.They have high melting and boiling points, because the attraction between millions of oppositely charged ions in a crystal is very strong. Common salt melts at 801 °C.
• d.They are usually hard, brittle crystalline solids at room temperature.
• e.They conduct electricity when melted or dissolved in water (the ions can move), but not as solids.

Non-metals (except hydrogen) have 5, 6 or 7 electrons in their valence shell, so each wants to gain 3, 2 or 1 electrons. When two non-metal atoms meet, neither will give its electrons away: both want more. So they share.

Each atom puts one or more of its electrons into a shared pair, which belongs to both atoms at once. Counting the shared electrons for each atom, both get a duplet or an octet.

A bond formed between two atoms of non-metals (the same or different elements) by sharing pairs of electrons, each atom giving equally to each pair, is called a covalent bond. A compound with covalent bonds is a covalent compound: water (H₂O), hydrogen chloride (HCl), methane (CH₄) and ammonia (NH₃) are examples.

A covalent bond is drawn as a line between the atoms: one line for each shared pair. Atoms can share one pair (a single bond, H–H), two pairs (a double bond, O=O) or three pairs (a triple bond, N≡N).

Formation of hydrogen chloride (HCl)

A hydrogen atom has 1 electron in its K shell; chlorine has 7 in its valence shell. Each puts one electron into a shared pair. Now hydrogen counts 2 (a duplet) and chlorine 8 (an octet). This makes a molecule of hydrogen chloride, a covalent compound.

Formation of a water molecule (H₂O)

A water molecule has one oxygen atom and two hydrogen atoms.

Atom	K	L
hydrogen	1
oxygen	2	6

Oxygen has 6 valence electrons and needs 2 more; each hydrogen needs 1 more. So oxygen shares one pair with each of two hydrogen atoms. Oxygen now counts 8 (an octet) and each hydrogen 2 (a duplet). Water is a covalent compound: H–O–H.

Formation of methane (CH₄)

Carbon has 4 valence electrons; each hydrogen atom has 1. One carbon atom shares one pair with each of four hydrogen atoms: four shared pairs. Carbon gets an octet and each hydrogen a duplet. Methane, the main gas in biogas, is a covalent compound.

Formation of ammonia (NH₃)

Nitrogen has 5 valence electrons and needs 3 more. It shares one pair with each of three hydrogen atoms. Nitrogen gets an octet (3 shared pairs plus its own lone pair), and each hydrogen a duplet.

Double and triple bonds

Two oxygen atoms (6 valence electrons each) each need 2 more: they share two pairs, a double bond. Two nitrogen atoms (5 each) each need 3: they share three pairs, a triple bond.

Characteristics of covalent compounds

• a.They are formed when two or more non-metals combine.
• b.Most of them don’t dissolve in water (wax, oil, petrol). Some do, like sugar and alcohol.
• c.They don’t split into ions, so most of them don’t conduct electricity.
• d.They have low melting and boiling points: many are gases or liquids at room temperature (methane, ammonia, water), or soft solids that melt easily (wax).

The smallest particle of a substance that can exist on its own is a molecule. A molecule is made of two or more atoms joined together. A molecule of water (H₂O) is two hydrogen atoms and one oxygen atom; a molecule of hydrogen (H₂) is two hydrogen atoms; a molecule of oxygen (O₂) is two oxygen atoms.

Just as an element has a symbol, a molecule is written as a group of symbols. The group of symbols that shows the atoms in one molecule of a substance is its molecular formula.

No.	Substance	Molecular formula
1	hydrochloric acid	HCl
2	sulphuric acid	H₂SO₄
3	sodium hydroxide	NaOH
4	magnesium hydroxide	Mg(OH)₂
5	ammonia	NH₃
6	sodium chloride	NaCl
7	magnesium carbonate	MgCO₃
8	magnesium chloride	MgCl₂
9	water	H₂O
10	calcium carbonate	CaCO₃
11	carbon dioxide	CO₂
12	potassium nitrate	KNO₃
13	sodium bicarbonate	NaHCO₃
14	potassium sulphate	K₂SO₄
15	ammonium phosphate	(NH₄)₃PO₄

Writing a molecular formula: the criss-cross method

You don’t have to memorize formulae. If you know the valencies, the criss-cross method gives the formula:

1. Write the name of the compound.
2. Write the symbols of its elements or radicals under the name, the positive one (usually the metal) first.
3. Write the valency of each just above its symbol.
4. Cross the valencies over: each valency goes to the bottom right of the other symbol.
5. Simplify: if both numbers can be divided by the same number, divide them; don’t write a 1; and if a radical (a group like SO₄ or NO₃) has a number 2 or more, put it in brackets.

Here are more, worked the same way:

Compound	Symbols	Valencies	Crossed over	Formula
sodium chloride	Na, Cl	1, 1	Na₁Cl₁	NaCl
calcium chloride	Ca, Cl	2, 1	Ca₁Cl₂	CaCl₂
aluminium chloride	Al, Cl	3, 1	Al₁Cl₃	AlCl₃
carbon tetrachloride	C, Cl	4, 1	C₁Cl₄	CCl₄
boron oxide	B, O	3, 2	B₂O₃	B₂O₃
ammonium sulphate	NH₄, SO₄	1, 2	(NH₄)₂(SO₄)₁	(NH₄)₂SO₄
calcium sulphate	Ca, SO₄	2, 2	Ca₂(SO₄)₂ → divide by 2	CaSO₄
aluminium nitrate	Al, NO₃	3, 1	Al₁(NO₃)₃	Al(NO₃)₃
calcium bicarbonate	Ca, HCO₃	2, 1	Ca₁(HCO₃)₂	Ca(HCO₃)₂

What a molecular formula tells you

1. The elements in the molecule (their names and symbols).
2. The valency of each element or radical (by crossing back).
3. The number of atoms of each element in one molecule. H₂SO₄ has 2 hydrogen, 1 sulphur and 4 oxygen atoms, 7 atoms in all.

Atomic mass

Almost all the mass of an atom is in its nucleus, in its protons and neutrons; electrons are so light that we can leave them out. Atomic and molecular masses are measured in atomic mass units (amu); a proton and a neutron each have a mass of about 1 amu. So, for one atom,

$$\text{atomic mass} = \text{number of protons} + \text{number of neutrons}$$

• An oxygen atom has 8 protons and 8 neutrons: atomic mass = 8 + 8 = 16 amu.
• A sodium atom has 11 protons and 12 neutrons: atomic mass = 11 + 12 = 23 amu.

For calculations, use these atomic masses:

H	C	N	O	Na	Mg	Al	P	S	Cl	K	Ca
1	12	14	16	23	24	27	31	32	35.5	39	40

Molecular mass

The sum of the atomic masses of all the atoms in a molecule is its molecular mass (or molecular weight).

The nucleus is packed with protons, all positively charged, and like charges repel. What stops a nucleus from flying apart? A very strong, very short-range force, the nuclear force, which pulls protons and neutrons together. Neutrons add this pull without adding any repulsion, so they act like glue.

Whether a nucleus is stable (stays as it is for ever) depends on its balance of neutrons and protons:

• Light nuclei are stable with about as many neutrons as protons: a neutron-to-proton ratio of about 1. Carbon-12 has 6 and 6, oxygen-16 has 8 and 8.
• Heavier nuclei have more protons pushing each other apart, so they need more neutrons to hold together: iron-56 has 26 protons and 30 neutrons; lead-208 has 82 protons and 126 neutrons, a ratio of about 1.5.
• A nucleus with too many or too few neutrons for its protons is unstable.
• Beyond bismuth (83 protons), no number of neutrons is enough: every such nucleus is unstable.

So the larger the atomic number, the harder it is for a nucleus to be stable. An unstable nucleus sooner or later changes towards a stable one, and as it does, it throws out energetic radiation. This is radioactivity.

Unstable nuclei, mostly those with a high atomic number, give out powerful rays to become more stable. Elements whose nuclei do this are called radioactive elements: uranium, plutonium, thorium, radium and radon are examples. Most have an atomic number above 83. The giving out of these rays is radioactive emission (radioactivity).

There are three kinds of radiation:

Radiation	What it is	Charge	How far it gets
alpha (α)	a helium nucleus: 2 protons and 2 neutrons	+2	a few cm of air; stopped by a sheet of paper or by skin
beta (β)	a fast electron, made in the nucleus	−1	a meter or so of air; stopped by a few mm of aluminium
gamma (γ)	an electromagnetic ray, like very energetic X-rays	none	passes through the body; reduced only by thick lead or concrete

The radiation from radioactive elements is harmful: it damages the cells of the body and their DNA, causing burns, sickness and cancer. Radioactive materials are handled from behind shields, and their containers carry the warning sign ☢.

Nuclear fission

The splitting of a heavy, unstable nucleus into two smaller nuclei is called nuclear fission. It happens when a nucleus of uranium-235 or plutonium-239 is hit by a neutron. The nucleus absorbs the neutron, wobbles and splits into two middle-sized nuclei, such as barium and krypton, and throws out 2 or 3 new neutrons and a huge amount of energy:

$${}^{235}_{\;92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{141}_{\;56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3\,{}^{1}_{0}\text{n} + \text{energy}$$

(In this way of writing, the top number is the mass number and the bottom number the atomic number. Check: 235 + 1 = 141 + 92 + 3, and 92 = 56 + 36.)

Where does the energy come from? Weigh everything after the split, and it is very slightly lighter than the uranium and neutron before it. That lost mass (m) has turned into energy (E), by Einstein’s famous E = mc², where c is the speed of light (3 × 10⁸ m/s). Because c² is enormous, a tiny loss of mass gives a vast amount of energy. The energy comes out mostly as the speed of the flying fragments (which becomes heat) and as gamma rays.

The new neutrons can split more uranium nuclei, which give more neutrons, which split more nuclei: a chain reaction. In a nuclear power station it is controlled, so that each fission causes just one more; in an atom bomb it is not.

Nuclear fusion

Joining light nuclei, such as hydrogen, to make a heavier nucleus, such as helium, is called nuclear fusion. Nuclei are all positive and repel each other, so they fuse only when they crash together at tremendous speed: that needs a temperature of millions of degrees and a huge pressure. Those conditions exist in the core of the Sun and the other stars, which also have plenty of hydrogen. There, in several steps, four hydrogen nuclei join to make one helium nucleus, and an enormous amount of energy is released:

$$4\,{}^{1}_{1}\text{H} \rightarrow {}^{4}_{2}\text{He} + 2\,{}^{\;0}_{+1}e + \text{energy}$$

(The two ⁰₊₁e particles are positrons, particles like electrons but positively charged. Check: mass numbers 4 = 4 + 0, charges 4 = 2 + 2.)

The energy released from the nucleus is called atomic energy or nuclear energy.

Electricity from fission. In a nuclear power plant, controlled fission happens in a strong steel vessel called a nuclear reactor. The heat boils water into steam; the steam turns a turbine, and the turbine turns a generator that makes electricity, just as in a coal power plant, but without burning anything. About 30 countries make electricity this way; altogether it gives about a tenth of the world’s electricity (as of 2024), and about two-thirds of France’s.

Electricity from fusion? Not yet. Fusion needs temperatures of more than 100 million °C on Earth, hotter than the Sun’s core, and no material can hold anything that hot: in most designs the gas is held away from the walls by magnetic fields. In December 2022 a laboratory in the USA, for a fraction of a second, got more energy out of a fusion reaction than its lasers put in. A huge experimental reactor, ITER, is being built in France. A fusion power station is still years away (as of 2026).

Other uses of radiation. The radiation from radioactive substances is used to:

• treat cancer (radiotherapy), by aiming radiation at a tumor to kill its cells; in Nepal, at hospitals such as the B.P. Koirala Memorial Cancer Hospital in Bharatpur;
• sterilize (kill all germs on) syringes, bandages and surgical instruments, and keep some foods from spoiling;
• find the age of old objects (carbon-14 dating), check welds and pipes for cracks, and much more.

Dangers. Nuclear energy has a dark side too:

• Nuclear weapons. The atom bombs dropped on Hiroshima and Nagasaki in Japan in August 1945 killed more than 100,000 people, many of them later, from radiation.
• Accidents and leaks. At Chernobyl (in Ukraine) in 1986 and at Fukushima (Japan) in 2011, reactors failed and released radiation; thousands of people had to leave their homes for good.
• Waste. Used fuel stays dangerously radioactive for thousands of years, and must be stored safely all that time.

1. Tick the correct answer

2. Fill in the blanks

3. Write the differences between

4. Give reasons

5. Answer the following questions