Atoms of different elements have different numbers of protons.

The idea

The proton count is the distinguishing feature between elements: hydrogen atoms have 1 proton, helium atoms have 2, lithium atoms have 3, and so on. Because each proton carries relative charge 1⁺, changing the proton number changes the nuclear charge and therefore the element's identity. This is why the modern periodic table is ordered by increasing atomic number rather than by atomic mass. For example, argon and potassium are placed by proton number even though their mass numbers reverse the order. In the nuclear model, the nucleus contains the protons and neutrons, and the proton number defines which element the atom is. Different proton numbers mean different elements with different chemical and physical properties.

What earns the marks

• State that different elements have different proton numbers.
• Link proton number to the identity of the element and its position in the periodic table.
• Use examples such as hydrogen with 1 proton and helium with 2 protons to illustrate the difference.
• Explain that changing proton number changes the nuclear charge and therefore the element.
• Recognise that the periodic table is ordered by increasing atomic number.

Common mistakes and how to put them right

• Confusing proton number with neutron number when comparing elements.

  Put it right by focusing on the positive nuclear charge and the atomic number.

• Assuming elements with similar mass numbers must be the same element.

  Put it right by comparing proton numbers instead.

• Thinking a change in electron number creates a new element.

  Put it right by stating that electron changes produce ions, while proton number defines the element.

Exam tips

• When comparing two elements, quote their proton numbers explicitly rather than saying they are different.
• Use a periodic table to check atomic numbers before making a comparison.
• If asked why the periodic table order changed, refer to increasing proton number rather than atomic mass.