Atomic Structure – Learning Outcomes Describe the structure of the atom.

State the location, relative charge, and atomic mass of

the sub-atomic particles.

HL: Define atomic number.

HL: Draw the Bohr structure of the first 20 elements.

HL: Define isotope.

1

Describe the Structure of the Atom An atom is the smallest particle of an element which still

retains the properties of that element.

Atoms have a solid core called the nucleus.

The nucleus contains two types of sub-atomic particles –

protons and neutrons.

The nucleus is surrounded by mostly empty space.

The empty space contains shells where another sub-

atomic particle, electrons are found.

2

State Facts About Sub-Atomic Particles The location, charge, and atomic mass of sub-atomic

particles are how they are distinguished.

These masses and charges are so small (e.g. mass of

proton = 0.000 000 000 000 000 000 000 000 001 67 kg),

that we use new units more suitable.

For mass, we use atomic mass units (u) and for charge,

we use elementary charge.

3

Particle Location Charge Atomic Mass

Proton Nucleus +1 1

Neutron Nucleus 0 1

Electron Outside the

nucleus

-1 1

1850

Define Atomic Number4

by 2

012rc

–C

C-B

Y-S

A-3

.0

Define Atomic Number The atomic number of an atom is the

number of protons in its nucleus.

Most of the time, this is also the

number of electrons the atom has

orbiting it.

The mass number of an atom is the

sum of protons and neutrons in its

nucleus.

i.e. #neutrons + #protons = mass

number

5

Draw Bohr Structure Protons and neutrons always exist in the nucleus.

6

Draw Bohr Structure Electrons are more complicated:

They exist outside the nucleus.

7

Draw Bohr Structure They exist in pairs where possible.

8

Draw Bohr Structure Shells have a maximum amount of space. The first shell

takes two electrons. A new shell is needed to add more.

9

Draw Bohr Structure Pairs on each shell spread away from each other.

10

Draw Bohr Structure The second and third shells hold 8 electrons each.

Remember the first shell can only hold 2.

11

Draw Bohr Structure These rules combined give the Bohr structure of the

atom.

On our course, we need to be able to draw the Bohr

structure of the first 20 elements.

Draw the Bohr structure for:

12

Hydrogen (𝐻11)

Helium 𝐻𝑒42

Lithium (𝐿𝑖73)

Oxygen 𝑂168

Neon (𝑁𝑒2010)

Sodium 𝑁𝑎2311

Silicon 𝑆𝑖2814

Argon 𝐴𝑟4018

Potassium 𝐾3919

Calcium (𝐶𝑎4020)

Define Isotope Each element is defined by its atomic number.

Notice that the mass number on the periodic table

comes with decimals.

e.g. the mass number of hydrogen is given as 1.00794.

Every hydrogen has 1 proton (that’s what makes it

hydrogen), but some hydrogens have different numbers

of neutrons in their nucleus.

Hydrogen-1 has 1 proton, 0 neutrons (total mass 1)

Hydrogen-2 has 1 proton, 1 neutron (total mass 2)

Hydrogen-3 has 1 proton, 2 neutrons (total mass 3)

13

Define Isotope Isotopes are atoms with the same atomic number but

different mass numbers.

The 1.00794 given as the mass of hydrogen on the

periodic table is an average value for the mass of

hydrogen.

Since hydrogen-1 is much, much more common than

hydrogen-2 or hydrogen-3, the average mass (relative

atomic mass) is very close to 1.

14