Structure of the Atom

The atom

An atom is the smallest particle of an element that can take part in a chemical reaction. In the Kenyan Senior School CBC (KICD) Chemistry curriculum, you use models of the atom to explain properties of elements and how they react.

Subatomic particles

Atoms are electrically neutral because the number of protons equals the number of electrons.

Atomic number and mass number

• Atomic number (Z) = number of protons.
• Mass number (A) = protons + neutrons.
• Neutrons = A - Z.

An atom of sodium is written as 23/11 Na: mass number 23, atomic number 11, so it has 11 protons, 11 electrons and 12 neutrons.

Development of atomic models

• Dalton: atoms are indivisible solid spheres.
• Thomson: atoms contain electrons embedded in a positive sphere.
• Rutherford: a small, dense, positive nucleus with electrons around it.
• Bohr: electrons move in fixed energy levels (shells).

Electron arrangement

Electrons fill energy levels from the nearest the nucleus. The first holds up to 2, the second up to 8, and the third up to 8 for the first 20 elements. Examples: Lithium (3) 2.1; Oxygen (8) 2.6; Magnesium (12) 2.8.2; Argon (18) 2.8.8. The outermost electrons are valence electrons; they decide how an atom reacts. Elements with full outer shells, like neon and argon, are unreactive noble gases.

Isotopes

Isotopes have the same atomic number but different mass numbers. Chlorine has two common isotopes, chlorine-35 and chlorine-37. Carbon-14 is used to date ancient fossils, and iodine-131 is used in medicine.

Relative atomic mass

Relative atomic mass is the weighted average mass of isotopes.

Worked example 1

Find the number of protons, neutrons and electrons in 27/13 Al. Answer: Protons 13, electrons 13, neutrons 27 - 13 = 14.

Worked example 2

Chlorine is 75% chlorine-35 and 25% chlorine-37. Find its relative atomic mass. Answer: (75 x 35 + 25 x 37) / 100 = (2625 + 925) / 100 = 35.5.

Try it

• Write the electron arrangement of potassium (19) and phosphorus (15).
• State the number of neutrons in 16/8 O and 18/8 O.
• Draw a Bohr diagram of magnesium.
• Explain why noble gases are unreactive.