Chapter – 3
Atoms and Elements
ATOMS
ATOMS
• Atoms are the smallest particles of matter, that we cannot
break down further by chemical means.
• Single atoms are far too small to see. Perhaps a million sodium
atoms could fit in a line across this full stop. So you can see
sodium only if there are enough sodium atoms together in one
place!
• In fact atoms are mostly empty space. Each consists of a nucleus
and a cloud of particles called electrons that whizz around it. This
drawing shows how a sodium atom might look, magnified many
millions of times.
Structure of an atom
The elements
• Sodium is made of sodium atoms only, so it is an element.
• An element contains only one kind of atom.
• Around 90 elements have been found in the Earth and
atmosphere.
• Scientists have made nearly 30 others in the lab. Many of the
‘artificial’ elements are very unstable, and last just a few
seconds before breaking down into other elements. (That is why
they are not found in nature.)
Symbols of elements
Q1. What is: a an atom? b an element?
Q2. If you could look inside an atom, what would you see?
Q3. The symbols for some elements come from their Latin names.
See if you can identify the element whose Latin name is:
a natrium b ferrum c plumbum d argentum
Q4. Which element has this symbol? a Ca b Mg c N
Q5. See if you can pick out an element named after the famous
scientist Albert Einstein.
Q.6 From the Periodic Table, name a three metals a three nonmetals that you expect to behave in a similar way.
• Atoms consist of a nucleus and a cloud of
electrons that move around the nucleus.
• The nucleus is itself a cluster of two kinds
of particles, protons and neutrons.
• All the particles in an atom are very light.
So their mass is measured in atomic
mass units, rather than grams.
• Every atom has an equal number of protons and electrons. So
atoms have no overall charge.
• The total number of protons and neutrons in an atom is called its
nucleon number.
• The nucleon number for the sodium atom is 23. (11 + 12 = 23).
• The lower number is always the proton number. The other number
is the nucleon number. So you can tell straight away that sodium
atoms have 12 neutrons.
Q1. Name the particles that make up the atom.
Q2. Which particle has:
a positive charge? b no charge? c almost no mass?
Q3. An atom has 9 protons. Which element is it?
Q4. Why do atoms have no overall charge?
Q5. What does this term mean?
a proton number b nucleon number
Q6. Name each of these atoms, and say how many protons,
electrons, and neutrons it has:
12
6
C 16 8 O 2 4 12 Mg 2 7 13 Al 64 29 Cu
ISOTOPES
RADIOACTIVITY
Radioactivity is harmful but we can
make use of radioisotopes
• To check for leaks Engineers can check oil and gas pipes for leaks by
adding radioisotopes to the oil or gas. If a Geiger counter detects
radiation outside the pipe, it means there is a leak. Radioisotopes
used in this way are called tracers.
• To treat cancer Radioisotopes can cause cancer. But they are also
used in radiotherapy to cure cancer – because the gamma rays in
radiation kill cancer cells more readily than healthy cells. Cobalt-60 is
usually used for this. The beam of gamma rays is aimed carefully at
the site of the cancer in the body.
• To kill germs and bacteria Gamma rays kill germs too. So they
are used to sterilize syringes and other disposable medical
equipment. They also kill the bacteria that cause food to decay. So in
many countries, foods like vegetables, fruit, spices, and meat, are
treated with a low dose of radiation. Cobalt-60 and cesium-137 are
used for this.
Q1. a What are isotopes?
b Name the three isotopes of carbon, and write symbols for
them.
Q2. Carbon-14 is radioactive. What does that mean?
Q3. What is a radioisotope? Give two examples.
Q4. a Radiation can kill us. Why?
b So why are radioisotopes used to treat cancer?
Q5. Radioisotopes can be used to check pipes for leaks.
a Explain how this works.
b How could you tell that a pipe had no leak?
How electrons are arranged ?
• Electrons are arranged in shells around the
nucleus.
• The first shell, closest to the nucleus, is the
lowest energy level.
• The further a shell is from the nucleus, the
higher the energy level.
• Each shell can hold only a certain number
of electrons.
Patterns in the Periodic Table
Group O: Noble
gases
Q1. The element rubidium, Rb, is the 37th element in the
Periodic Table. It is in Group I, Period 5. Its proton number is
37. What is its electron distribution?
Q2. One element has atoms with 13 electrons.
a Draw a diagram to show the electron distribution.
b Write the electron distribution in this form: 2+ …
c Name the element.
Q3. The electron distribution for boron is 2, 3. What is it for:
a lithium? b magnesium? c hydrogen?
Q4. An element has 5 valency electrons. Which group is it in?
Solvent:-The part of the
solution which is present in
a larger amount and
contains solute.
Solute:-The part of the
solution which has a low
amount by mars. It is
present in Solvent.
How our model of the atom developed?
• The two big ideas
• All chemistry depends on these two big ideas:
everything is made of particles, and …
atoms are the simplest particles of an element, that cannot be broken down
in a chemical reaction.
It began with the Ancient Greeks
In Ancient Greece (around 750 BC – 150 BC), the philosophers
thought hard about the world around them.
The philosopher Democritus came up with an answer: everything is
made of tiny particles that cannot be divided. He called them
atoms.
Aristotle (384–270 BC) believed that everything was made of four
elements – earth, air, fire, and water – mixed in different amounts.
A stone has a lot of earth but not much water. No matter how much
you crush it, each tiny bit will still have the properties of stone.
From about 600 AD, the practice of
alchemy spread to many
countries, including Persia (Iran),
India, China, Greece, France, and
Britain.
The alchemists did not succeed in
making gold. But they made
many substances look like gold, by
using secret recipes to coat them
with other substances. They also
developed many of the techniques
we use in the lab today, such as
distillation and crystallisation.
By now the idea of atoms was almost forgotten. But in 1661 the scientist Robert
Boyle showed that a gas can be compressed into a smaller space.
He deduced that gas is made of particles with empty space between them.
In 1799, over 130 years later, the French chemist Joseph Louis Proust showed
that copper(II) carbonate always contained the same proportions by mass of
copper, carbon, and oxygen, no matter how it was made: 5.3 parts of copper to 1
of carbon to 4 of oxygen. This suggested that copper, carbon, and oxygen were
made of particles, and these always combined in the same ratios.
The English chemist John Dalton puzzled over these discoveries. In 1803 he concluded that if elements really were made of
indivisible particles then everything made sense. He called the particles atoms, as a tribute to the Greek philosophers. He
suggested that atoms of one element could combine with atoms of another element only in a fixed ratio.
This time the idea of atoms caught on really fast, because it fitted with the results from so many experiments.
in 1827, a Scottish botanist called Robert Brown was studying some pollen grains in water, under a microscope. He saw them
jiggling around. He deduced that they were being struck by water particles. That meant tiny separate particles really did exist.
They were not just theory.
And then …
•In 1955 Erwin Müller, an American, developed a machine called a field-ion microscope. It could ‘picture’ the tip of a needle, magnified 5 million times! The
atoms in the needle showed up as dots. Today, microscopes are much more powerful. The scanning tunneling microscope gives us images of individual
atoms, magnified by up to 100 million times.
The atom: the inside story
By 200 years ago, chemists had accepted that everything was indeed
made of tiny indivisible particles: atoms.
In 1897, the English physicist J.J. Thomson was investigating
cathode rays.
He deduced that these rays were streams of charged particles,
much smaller than atoms. In fact they were bits from atoms. He
called them corpuscles, but soon the name got changed to electrons.
Thomson imagined that electrons were stuck on the atoms like
raisins on a bun.
Radioactive particles
• A year earlier, a French physicist called Becquerel had
been working with crystals of a uranium salt. He found
that they glowed in the dark.
• By accident, he left some in a drawer, wrapped in thick
paper, on top of a photographic plate. To his surprise,
he found an image of the crystals on the plate. They
had given out rays of some kind, that could pass
through paper! He had discovered radioactivity.
• Later, the English physicist Ernest Rutherford found that
radiation could be separated into alpha particles, beta
particles, and gamma rays.
• Alpha particles were found to be 7000 times heavier
than electrons, with a positive charge. You could speed
them up and shoot them like tiny bullets!
The nucleus and
protons
At last, the neutrons
• In 1930, two German physicists, Bothe and Becker,
shot alpha particles at
• beryllium – and knocked a stream of new particles
from it. In 1932 the
• English physicist James Chadwick found that these
particles had the same
• mass as protons, but no charge. He named them
neutrons.
• So finally, 129 years after Dalton proposed the
atom, the chemist’s model
• of it was complete.
The metals and nonmetals
Making use of the metals
Because metals are generally hard and strong, and good conductors, we make
great use of them. For example:
Iron is the most-used metal in the world. It is used in buildings, bridges, cars,
tin cans (coated with tin), needles, and nails.
Copper is used for electrical wiring in homes.
Aluminium is strong but light. So it is used in planes and space rockets.
Non-metals are everywhere
• There are far fewer non-metals than metals. But they are all
around us – and inside us.
Air is almost 80% nitrogen, and about 20% oxygen.
Water is a compound of hydrogen and oxygen.
Our bodies are mostly water, plus hundreds of carbon compounds.
Many of these contain atoms of other non-metals too, such as
nitrogen, phosphorus, and iodine. (Plus metals such as calcium and
iron.)
Sand is mainly the compound silicon dioxide, formed from silicon
and oxygen.
Q1. Without looking at the Periodic Table, see if you can quickly list
30 elements, and give their symbols. Then underline the metals.
Q2. Explain what these terms mean.
Conductor, ductile, malleable, brittle, sonorous, density
Q3. Aluminium is used for outdoor electricity cables. See if you can
suggest three reasons why.
Q4. Write down what you think are the three main general properties
that distinguish metals from non-metals.
Q5. Give one example of a physical property, and one of a chemical
property, for non-metals.