An atom has a positive nucleus and negative electrons
Definition
Atom
The smallest part of an element that can exist.
Definition
Nucleus
The tiny, dense centre of an atom, which carries its positive charge and almost all of its mass.
- The nucleus is made of protons and neutrons packed tightly together.
- The nucleus has a positive charge because protons are positive and neutrons have no charge.
- Electrons move around the nucleus in shells, and each one carries a negative charge.
- The positive nucleus attracts the negative electrons, and this attraction holds the atom together.
- An atom has no overall charge, because the positive charge of its nucleus is balanced by the negative charge of its electrons.
Common Mistake
- Do not place electrons inside the nucleus, because only protons and neutrons are found there.
- Do not describe the nucleus as neutral, because its protons give it an overall positive charge.
The nucleus is tiny compared with the atom
Definition
Atomic radius
The distance from the centre of an atom's nucleus to its outermost electrons.
- The nuclear radius is less than a ten-thousandth of the atomic radius.
- The electrons move in the large region around the nucleus, so almost all of an atom's volume is empty space.
- This is why most alpha particles passed straight through the gold foil in Rutherford's experiment.
- An atom drawn to scale would show the nucleus as a dot far too small to see.
- Textbook diagrams draw the nucleus much larger than its true size so that it shows up at all.
Example
- If an atom were enlarged to the size of a football stadium, its nucleus would be about the size of a pea on the centre spot.
- The electrons would be moving around out in the stands, with empty space in between.
An atom's mass is concentrated in its nucleus
- Protons and neutrons are much heavier than electrons.
- The electrons add so little that more than 99.9% of an atom's mass is in the nucleus.
- Because so much mass sits in so little space, the nucleus is extremely dense.
- The electrons take up nearly all of the atom's volume but contribute almost none of its mass.
- The mass of an atom is therefore set almost entirely by the number of protons and neutrons in its nucleus.
Key Idea
The nucleus holds almost all of the mass but very little of the volume, while the electrons fill almost all of the volume but add almost no mass.
The typical size of atoms and small molecules
Definition
Order of magnitude
The power of ten closest to a value, used to give its approximate size.
- A typical atom has a radius of about 1×10−10 m1 \times 10^{-10}\,\text{m}1×10−10m, which is 0.1 nm0.1\,\text{nm}0.1nm.
- The diameter of that atom, straight across its centre, is twice the radius, about 2×10−10 m2 \times 10^{-10}\,\text{m}2×10−10m.
- Small molecules, such as water, carbon dioxide or methane, contain only a few atoms, so their size is also of the order of 10−10 m10^{-10}\,\text{m}10−10m.
- Different atoms have different radii, but nearly all of them fall within the same order of magnitude.
- Using 1×10−10 m1 \times 10^{-10}\,\text{m}1×10−10m as the radius lets you estimate how many atoms fit across an everyday object.
Example
- Estimate how many magnesium atoms fit across a strip of magnesium ribbon 3.0 mm3.0\,\text{mm}3.0mm wide.
- Convert the width to metres: 3.0 mm=3.0×10−3 m3.0\,\text{mm} = 3.0 \times 10^{-3}\,\text{m}3.0mm=3.0×10−3m.
- Double the typical atomic radius to get the width of one atom: 2×(1×10−10 m)=2×10−10 m2 \times (1 \times 10^{-10}\,\text{m}) = 2 \times 10^{-10}\,\text{m}2×(1×10−10m)=2×10−10m.
- Divide the ribbon width by the width of one atom: 3.0×10−3 m2×10−10 m=1.5×107\dfrac{3.0 \times 10^{-3}\,\text{m}}{2 \times 10^{-10}\,\text{m}} = 1.5 \times 10^{7}2×10−10m3.0×10−3m=1.5×107 atoms.
Self review
- What charge does the nucleus carry, and why?
- How does the radius of a nucleus compare with the radius of its atom?
- Where is almost all of an atom's mass found?
- What is the typical radius of an atom, in metres?
- Roughly how many atoms would fit across a line 1 mm1\,\text{mm}1mm long?
