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Chapter 2

Atomic

Structure


2.1 Atoms and

Elements

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16-09-2021

Thursday


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1 Atoms are made up of three

different particles:

• protons, which are positively charged

• neutrons, which have no charge

• electrons, which are negatively charged.

The negatively charged particles are arranged in different shells

(energy levels) around the nucleus of the atom. The particles

with a negligible mass are the electrons. All atoms of the same

element contain the same number of protons and neutrons.

Atoms of the same element with different numbers of neutrons

are known as isotopes.


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2 a 3

b 4

c 7

d


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The electrons in an atom are arranged in a series of shells

around the central nucleus. These shells are also called

energy levels. In an atom, the shell closest / nearest to the

nucleus fills first, then the next shell, and so on. There

is room for:

• up to two electrons in the first shell

• up to eight electrons in the second shell

• up to eight electrons in the third shell.


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The elements in the Periodic Table are organized in the

same way as the electrons fill the shells. Shells fill from left

to right across the rows of the Periodic Table.

• The first shell fills up first, from hydrogen to helium.

• The second shell fills next, from lithium to neon.

• Eight electrons go into the third shell, from sodium to

argon.

• Then the fourth shell starts to fill, from potassium.


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4 a Mg (magnesium)

b F (fluorine)

c K (potassium)


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5 a boron / 2,3

b phosphorus / 2,8,5


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b one (the atoms of element B)

c B and C


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d 3


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8 a 38

b 53

c 78

d 137 – 55 = 82

e Isotopes are different atoms of the same

element that have the same proton

number but different nucleon numbers.


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9 a B

b E

c A and C

d B and D


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10 a 2 protons, 2 neutrons, +2


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10 b The gold atoms are packed together in a

regular arrangement (lattice) / in layers / the

atoms can only vibrate about fixed positions.

c This suggests that the atoms were largely empty

space through which the α-particles passed.


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d These α-particles had made direct hits on

the nuclei of the gold atoms. They are

repelled backwards because the nucleus of

the atom is positively charged and so are the

α-particles.


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b The chemical properties of isotopes of the same element

are the same because the number and arrangement of

electrons in the isotopes are the same / the atoms of the

isotopes all have the same number of outer electrons.


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