Mass number, atomic number and isotopes

StarterOn MWB:

Draw and label a diagram of an atom of helium with its sub-atomic particles (with relative masses and charges)

Learning outcomes:Define the terms mass number and atomic

number (Grade C)

Define an isotope and give examples (Grade B)

Explain why the isotopes of the same element have identical chemical properties (Grade A)

Specification reference 3.1.1

Atomic number, ZAtoms consist of a tiny nucleus made up of

protons and neutrons.

This nucleus is surrounded by electrons.

The number of protons in the nucleus is known as the atomic number or the proton number Z.

The atomic number defines the chemical identity of the element e.g. All sodium atoms have 11 protons

Mass number, AThe total number of protons and neutrons in

the nucleus is called the mass number A.

It is the nucleons that make up the mass of the atom because electrons weigh virtually nothing.

Mass number A = no. protons + no. neutrons

What’s the number?

IsotopesWhat are isotopes?

Isotopes are atoms of the same element that contain different numbers of neutrons.

Is the reactivity of an isotope different to another isotope of the same element?The reactivity of different isotopes of an element is

identical because they have the same number of electrons

Are the physical properties of an isotope different to another isotope of the same element?The different masses of the atoms means that

physical properties of isotopes are slightly different.

Isotopes of chlorineAbout 75% of naturally-occurring chlorine is chlorine-35 (35Cl) and 25% is chlorine-37 (37Cl).

17 protons

18 neutrons

17 electrons

17 protons

20 neutrons

17 electrons

Isotopes of carbonCan you complete the table for the different

isotopes of carbon?

Carbon-12

Carbon-13

Carbon-14

NeutronsProtonsIsotopeExtension – Who was

Willard Libby and why is he

important?

Explain briefly how his

technique can be used

Summary questions1. Isotopes of hydrogen have been given their

own names. Hydrogen-2 is known as deuterium and hydrogen-3 tritium. State how many p, n & e the atoms of the following have.

a. Deuteriumb. Tritium

2. 3115W, 14

7X, 168Y and 15

7Z Identify which of these atoms (not their real symbols) is a pair of isotopes.

3. For each element in question 2, state:a. The number of protonsb. The mass numberc. The number of neutrons

Arrangement of electronsDescribe how electrons are arranged in an

atom (Grade C)

Describe how the structure of an atom developed from Dalton to Schrödinger

Recognise that the electron can behave as a particle, a wave, or a cloud of charge

Electron arrangementRead the page on the link attached

Electron shellsThe electron shell model is still used at A-level

particularly for describing the bonding of simple compounds

Electron shells First shell holds up to two electronsSecond shell up to eight electrons Third shell holds up to eighteen electrons

Electron diagramsDraw the electron arrangement for the

following atoms:CarbonSulfurSodium

Draw the electron arrangement for the following ions:Na+

O2-

Mg2+

Summary questions1. Draw the electron arrangement diagrams of

atoms that have the following numbers of electrons

a. 3b. 9c. 14

2. State, in shorthand, the electron arrangement of atoms with:

a. 4 electronsb. 13 electronsc. 18 electrons

3. Identify which of the following are atoms, positive ions, or negative ions. Give the size of the charge on each ion, including its sign. Use the Periodic Table to identify the elements A-E

Number of protons

Number of electrons

A 12 10

B 2 2

C 17 18

D 10 10

E 3 2