### Learning check

Once you have watched the video, check your learning with this quiz.

## Atomic number

Each element has its own number: an atom number.

• The number of protons in the nucleus decides which element it is.
• The atom number = the number of protons.

Examples

 Element Atomic number Number of p+ Hydrogen (H) 1 1 Helium (He) 2 2 Carbon (C) 6 6 Chlorine (Cl) 17 17

## Mass number

Every element (except hydrogen) also contains neutrons.

• Mass number = number of protons + number of neutrons
• $$A = Z + N$$

Two definitions

• Nuclide: An atomic species with a certain number of p+ and n.
• Isotopes: Different nuclides of the same element

The universe's most simple nuclide.

### Some isotopes and their mass numbers

 Isotope Nuclide Atomic nr. p+ n mass number Hydrogen $$^1_1$$H 1 1 0 1 Deuterium $$^2_1$$H 1 1 1 2 Tritium $$^3_1$$H 1 1 2 3 Helium $$^4_2$$He 2 2 2 4 Carbon-12 $$^{12}_6$$C 6 6 6 12 Carbon-13 $$^{13}_{\text{ }\text{ }6}$$C 6 6 7 13 Carbon-14 $$^{14}_{\text{ }\text{ }6}$$C 6 6 8 14 Chlorine-35 $$^{35}_{17}$$Cl 17 17 18 35 Chlorine-37 $$^{35}_{17}$$Cl 17 17 20 37

## Atomic mass

A single carbon-12 atom weighs 1.994×10–23 g

• Cumbersome figure!

The unified atomic mass unit ”u” (sometimes "amu") has been defined:

• A single carbon-12 atom weighs exactly 12u:
• $$m_{^{12}_{\text{ }\text{ }6}\text{C}}\equiv 12\text{u}$$

Also note: 1u = 1Da

How much do the building blocks of the atom weigh?

• 1 proton weighs approx. 1 u
• 1 neutron weighs approx. 1 u
• 1 electron weighs approx. 1/1800 u

The atomic mass of chlorine

• The atomic mass of chlorine: 35.453u ≈ 35.5u
• There are two chlorine isotopes: chlorine-35 (75 %) and chlorine-37 (25 %)
• The average atomic mass of chlorine:

$0.75 \times 35\text{u} + 0.25 \times 37\text{u} = 35.5\text{u}$
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