Atomic Mass and Diameter (Grade 10 NSC Matric Physical Sciences): Revision Notes

Atomic Mass and Diameter

What is atomic mass?

When scientists want to measure how heavy an atom is, they face a problem. Atoms are incredibly tiny! A carbon atom has a mass of about $1.99 \times 10^{-26}$ kg, and a hydrogen atom weighs about $1.67 \times 10^{-27}$ kg. These numbers are so small that they become very difficult to work with and compare.

To solve this problem, scientists created a special unit called the atomic mass unit (abbreviated as amu or simply u). This makes it much easier to compare the masses of different atoms.

The carbon standard

Scientists use carbon as their reference point, called the carbon standard. They decided that carbon should have an atomic mass of exactly 12.0 u. Using this standard, hydrogen has an atomic mass of 1.0 u, making it easy to see that carbon is 12 times heavier than hydrogen.

Atomic masses of common elements

Here are the atomic masses of some elements you'll encounter frequently:

Element	Atomic mass (u)
Carbon (C)	12.0
Nitrogen (N)	14.0
Bromine (Br)	79.9
Magnesium (Mg)	24.3
Potassium (K)	39.1
Calcium (Ca)	40.1
Oxygen (O)	16.0

Converting atomic mass units to actual mass

If you need the actual mass, 1 atomic mass unit equals:

• $1.67 \times 10^{-24}$ g
• $1.67 \times 10^{-27}$ kg

Rutherford's discovery of the nucleus

In the early 1900s, scientists weren't sure what atoms looked like inside. Ernest Rutherford designed a brilliant experiment to find out.

The experiment setup

Rutherford knew that radioactive elements give off positively charged particles called alpha particles. He decided to fire these alpha particles at thin sheets of gold foil to see what would happen.

Behind the gold sheets, Rutherford placed a screen made of zinc sulphide. This special screen would flash whenever an alpha particle hit it, allowing him to track where the particles went.

Rutherford's conclusions

These surprising results led Rutherford to conclude that:

1. The nucleus is positively charged - only something with a concentrated positive charge could deflect or bounce back the positively charged alpha particles
2. The nucleus is very small - most particles passed straight through, showing that most of the atom is empty space
3. The nucleus contains most of the atom's mass - it had to be dense enough to bounce back the fast-moving alpha particles

How small is the atomic nucleus?

The nucleus is incredibly tiny compared to the whole atom. To help you visualise this, imagine the entire atom was the size of a large football stadium. In this comparison, the nucleus would be only the size of a pea sitting in the centre of the stadium!

Relative atomic mass

This is the number you'll find for each element on the periodic table. It's called "relative" because it compares the mass to the carbon-12 standard.

The relative atomic mass takes into account that most elements exist as mixtures of different isotopes (atoms with the same number of protons but different numbers of neutrons). The periodic table value represents the weighted average of all these isotopes.