Struture of the atom

• Atoms consist of two main components:

  • The nucleus is the dense center of an atom, made up of protons and neutrons.

  • Electons, that orbit the nucleus.

• Atoms are incredibly small, with a typical radius of about 1×10-10 meters.

• The nucleus, which is at the center of the atom, is even smaller, with a radius less than 1/10,000 of the overall atom's radius. 

• Despite its tiny size, it contains almost all the mass of the atom.

The nucleus

• Protons:

  • Are positively charged particles (charge of +1). 

  • Have a relative mass of 1. 

  • The number of protons in the nucleus determines the identity of the element.

  • All atoms of a particular element have the same number of protons. 

  • Atoms of different elements have different numbers of protons. 

• Neutrons:

  • Are neutral, meaning they have no charge (charge of 0).

  • Have a relative mass of 1.

• The nucleus holds nearly all the mass of an atom because both protons and neutrons are much heavier than electrons.

Electrons

• Electrons:

  • Are negatively charged particles (charge of -1).

  • Have a very small relative mass, approximately 1/2000th the mass of a proton or neutron.

  • Orbit the nucleus in specific energy levels (or shells).

• The positive charge of protons and the negative charge of electrons balance each other out, making an atom electrically neutral overall.

The structure of an atom

Charge of the nucleus and relative mass

• The atomic number determines the charge of the nucleus since protons are positively charged. For example, a nucleus with 5 protons has a charge of +5.

• Since protons and neutrons each have significant mass compared to electrons, the nucleon number is directly related to the relative mass of the nucleus.

Atomic number and mass number

• The Atomic Number, Z, is the number of protons in the nucleus of an atom. This number defines the element (e.g., hydrogen has 1 proton, helium has 2). 

• The Mass Number, A, is the total number of protons and neutrons in the nucleus. 

• To find the number of neutrons, you can use the formula:

Number of neutrons = A − Z

• For example, if an element has a mass number (A) of 23 and a proton number (Z) of 11, then it has:

23 − 11 = 12 neutrons

Nuclear notation

Nuclear notation is a standardised way to represent information about an atom.

• It is often written as:

Where:

• A is the Nucleon Number (total number of protons and neutrons).

• Z is the Proton Number (number of protons).

• X is the chemical symbol of the element.

For example, the nuclear notation for carbon-12 is: