Edexcel IGCSE Physics: Double Science 复习笔记：7.1.1 Atomic Structure

Atomic Structure

• Atoms are the building blocks of all matter
• They are incredibly small, with a radius of only 1 × 10-10 m
• This means that about one hundred million atoms could fit side by side across your thumbnail

• Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus
• The radius of the nucleus is over 10,000 times smaller than the whole atom, but it contains almost all of the mass of the atom

Diagram showing the structure of a Lithium atom. If drawn to scale then the electrons would be around 100 metres away from the nucleus!

Parts of the Atom

• The nucleus contains:
• Protons - positively charged particles with a relative atomic mass of one unit
• Neutrons – no charge, and also with a relative atomic mass of one unit

• Almost all of the atom is empty space, but moving around the nucleus there are:
• Electrons – negative charge with almost no mass (1/2000 the mass of a proton or neutron)

• The properties of each of the particles are shown in the table below:

Charge in the Atom

• Although atoms contain particles of different charge, the total charge within an atom is zero
• This is because the number of electrons is equal to the number of protons

• The following table sets out the calculation of the total charge in the Lithium atom in the diagram above:

Total Charge Calculation Table

• If an atom loses electrons, then it is said to be ionised
• Symbols are used to describe particular nuclear by their element symbol, atomic number and mass number
• This notation is called nuclear notation

Carbon 12 in nuclear notation

Worked Example

A nucleus of carbon-12 is shown below.

How many electrons are there in an atom of carbon-12?

Step 1: Count the number of protons in the carbon nucleus

• There are 6 protons in the carbon atom

Step 2: Determine the number of electrons

• Remember, the number of electrons in an atom is equal to the number of protons
• Therefore there must be 6 electrons in the carbon atom

Exam Tip

You may have noticed that the number of electrons is not part of the mass number. This is because electrons have a tiny mass compared to neutrons and protons. We say their mass is negligible when compared to the particles in the nucleus.

Atomic & Mass Number

Atomic Number

• The number of protons in an atom is called its atomic number (it can also be called the proton number)
• Elements in the periodic table are ordered by their atomic number
• Therefore, the number of protons determines which element an atom is

• The atomic number of a particular element is always the same
• For example:
• Hydrogen has an atomic number of 1. It always has just one proton
• Sodium has an atomic number of 11. It has 11 protons
• Uranium has an atomic number of 92. It has 92 protons

• The atomic number is also equal to the number of electrons in an atom
• This is because atoms have the same number of electrons and protons in order to have no overall charge

Mass Number

• The total number of particles in the nucleus of an atom is called its mass number (it can also be called the nucleon number)
• The mass number is the number of protons and neutrons in the atom
• The number of neutrons can be found by subtracting the atomic number from the mass number

Number of Neutron = Mass Number – Atomic Number

• For example, if a sodium atom has a mass number of 23 and an atomic number of 11, then the number of neutrons would be 23 – 11 = 12

Nuclear Notation

• The mass number and atomic number of an atom are shown by writing them with the atomic symbol
• This is called nuclear notation

• Here are three examples:

Examples of nuclear notation for atoms of Hydrogen, Sodium and Uranium

• The top number is the mass number
• This is equal to the total number of particles (protons and neutrons) in the nucleus

• The lower number is the atomic number
• This is equal to the total number of protons in the nucleus

• The atomic and mass number of each type of atom in the examples above is shown in this table:

Number of Protons, Neutrons & Electrons Table

Worked Example

The element symbol for gold is Au. How many protons, neutrons and electrons are in the gold atom?

Step 1: Determine the atomic and mass number

• The gold atom has an atomic number of 79 (lower number) and a mass number of 197 (top number)

Step 2: Determine the number of protons

• The atomic number is equal to the number of protons
• The atom has 79 protons

Step 3: Calculate the number of neutrons

• The mass number is equal to the number of protons and neutrons
• The number of neutrons is equal to the mass number minus the atomic number

197 - 79 = 118

• The atom has 118 neutrons

Step 4: Determine the number of electrons

• An atom has the same number of protons and electrons
• The atom has 79 electrons