Atomic structure

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Why

All substances consist of atoms. If you know how an atom is built, you can explain why elements differ and how the periodic table is ordered.

Rule

Nucleus

The nucleus is in the centre of the atom. It consists of protons and neutrons.

Proton

A proton is a particle in the nucleus with a positive charge.

Neutron

A neutron is a particle in the nucleus with no charge.

Electron

An electron is a particle in the electron shell with a negative charge.

Atomic number

The atomic number tells how many protons an atom has. Every element has its own atomic number.

Mass number

The mass number is the number of protons and neutrons together.

An atom is neutral from the outside: it has as many protons as electrons. The electrons move on shells around the nucleus. The first shell (K) holds at most 2 electrons, the second (L) and the third (M) up to 8.

A=Z+N

Here A is the mass number, Z the atomic number (number of protons) and N the number of neutrons.

Example

The carbon atom

Building blocks of a carbon atom (C)
Protons6 (the atomic number)
Neutrons6 (12 − 6)
Electrons6 (as many as protons)
ShellsK: 2, L: 4

Practice

Task no. 1

Build a neutral atom of Hydrogen (H) with mass number 1. Set the number of protons, neutrons and electrons.

Solution

Hydrogen has 1 proton, 0 neutrons and 1 electron.

Exercise

Three worked tasks, each with its solution:

  1. Task no. 1

    How many neutrons does an atom of Phosphorus have with mass number 31 and atomic number 15?

    Hint

    Neutrons = mass number minus atomic number.

    Solution
    1. N=A−Z
    2. N=31−15=16

    Right answer: 16

  2. Task no. 2

    How many neutrons does an atom of Sodium have with mass number 23 and atomic number 11?

    Hint

    Neutrons = mass number minus atomic number.

    Solution
    1. N=A−Z
    2. N=23−11=12

    Right answer: 12

  3. Task no. 3

    How many neutrons does an atom of Fluorine have with mass number 19 and atomic number 9?

    Hint

    Neutrons = mass number minus atomic number.

    Solution
    1. N=A−Z
    2. N=19−9=10

    Right answer: 10