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160 Science and Technology

A substance made up of one kind of atom is an element.

Gold is an element because it only contains gold atoms.

Water is not an element because it contains hydrogen

and oxygen atoms. Scientists list the elements by atomic

number in the. PERIODIC TABLE. The atomic number

is the number of protons in the atom’s nucleus.

Over 60 elements are metals. They are

elements with only one or two electrons

in their outer shell. They all share similar

properties (characteristics) – they are shiny

and strong. Metals also conduct heat and

electricity well – they are useful for

cooking utensils and electric wires.

HOW MANY ELEMENTS ARE THERE?

Scientists have identified 92 elements that occur

naturally. Over three-quarters of the natural elements

are. METALS. The heaviest natural element is the

metal uranium. Scientists have created heavier

elements in nuclear reactors and particle accelerators.

WHY DO METALS CONDUCT ELECTRICITY?

A metal’s outer electrons become detached from their

atoms and wander freely through the atoms that make

up the metal substance. These free electrons can carry

heat and electricity, which explains why metals are

good electrical and thermal (heat) conductors.

1 METAL AND NON-METAL

Sodium is a silvery-white metal. It reacts with other elements by giving

them its single outer electron. Chlorine is a non-metal. It reacts by

accepting an electron. It needs just one more electron to make a stable

outer shell of eight. Elements, such as the group of elements called the

noble gases, that have eight outer electrons, are particularly stable. They

do not lose or gain electrons from other atoms to form chemical bonds.

2 SUPERNOVA

All the natural elements were

created by nuclear reactions that

happened inside stars. After

burning for billions of years,

stars explode as spectacular

supernovas, scattering atomic

nuclei through space.

2 GOLD

Gold is a transition metal. The transition metals have metallic

properties, but they are less reactive than the groups of elements

called alkali and alkali-earth metals.

ELEMENTS METALS

4 CHLORINE

This non-metal has

7 electrons in the

outer shell.

The middle atomic

shell can hold 8

electrons. The inner

shell can hold 2

4 SODIUM

This metal has

one electron in

its outer shell.

The inner shell

holds 2 electrons.

The middle holds 8

7 electrons

in outer shell

Single electron

in outer shell

Nucleus

Nucleus