Atomic number, mass number and isotopesIB MYP Chemistry: Revision notes

Section 1

Atomic number and mass number

Every atom is described by two numbers.

The atomic number is the number of protons in the nucleus. It decides which element the atom is.
The mass number is the number of protons plus neutrons in the nucleus.

In notation the mass number is written at the top and the atomic number at the bottom, for example $^{23}_{11}\mathrm{Na}$ . Elements in the periodic table are arranged in order of atomic number.

Key termsatomic numbermass number

Common mistake

The mass number is not the relative atomic mass. The mass number is always a whole number; relative atomic mass can be a decimal.

Section 2

Calculating the numbers of particles

Use these rules for a neutral atom.

Protons = atomic number
Electrons = atomic number (a neutral atom has equal protons and electrons)
Neutrons = mass number - atomic number

Worked example: $^{37}_{17}\mathrm{Cl}$ has 17 protons, 17 electrons and 37 - 17 = 20 neutrons.

Key termsneutral atom

Exam tip

Check your answer: protons + neutrons must equal the mass number.

Section 3

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They therefore have the same atomic number but different mass numbers.

For example, carbon-12 and carbon-14 both have 6 protons. Carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. Because isotopes have the same number of electrons, they have the same chemical properties.

Key termsisotope

Common mistake

Isotopes differ in neutrons, not protons. If the number of protons changes, it is a different element.

Section 4

Relative atomic mass

Most elements are a mixture of isotopes, so we use an average mass. The relative atomic mass is the weighted average of the mass numbers of the isotopes, taking account of how much of each there is.

Worked example: a sample of chlorine is 75% chlorine-35 and 25% chlorine-37.

(35 × 75 + 37 × 25) / 100 = 3550 / 100 = 35.5

The answer lies between the two mass numbers and is closer to the more abundant isotope.

Key termsrelative atomic massabundance

Exam tip

Multiply each mass number by its percentage, add the results, then divide by 100.

Must know

Atomic number = number of protons; mass number = protons + neutrons
Neutrons = mass number - atomic number
Neutral atom: electrons = protons
Isotopes: same protons, different neutrons
Isotopes have the same chemical properties
Relative atomic mass is a weighted average of the isotope masses

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Atomic number, mass number and isotopes

A student is studying sodium, which is written in notation as $^{23}_{11}\mathrm{Na}$ . She is learning what the two numbers in the notation tell her about the particles inside a sodium atom.
Explain why a neutral sodium atom has 11 electrons.2 marks
Chemists describe atoms by the numbers of particles they contain. Four atoms are described below. Atom W has 17 protons, 18 neutrons and 17 electrons. Atom X has 18 protons, 18 neutrons and 18 electrons. Atom Y has 17 protons, 20 neutrons and 17 electrons. Atom Z has 19 protons, 20 neutrons and 19 electrons.
Explain why atoms W and Y are the same element but have different masses.2 marks
Chlorine has atomic number 17. A typical sample of chlorine contains two isotopes, $^{35}\mathrm{Cl}$ and $^{37}\mathrm{Cl}$ . In the sample, 75% of the atoms are $^{35}\mathrm{Cl}$ and 25% of the atoms are $^{37}\mathrm{Cl}$ .
State the number of protons, neutrons and electrons in a neutral atom of $^{37}\mathrm{Cl}$ .3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).