Atomic Structure

The big idea

Everything in chemistry starts with the atom. An atom is the tiny building block of an element, but it is not a solid ball. It has a small dense center and electrons moving around it. The center holds almost all the mass.

Think of an atom like a stadium. The nucleus is a pea in the middle, and the electrons are in the seats around it. The picture is not to scale, but it helps you remember where the heavy part and the light part are.

What you'll learn

• What protons, neutrons, and electrons are, and where each one lives
• How atomic number and mass number are different
• How isotopes and ions form
• How electron shells fill for the first 20 elements

Meet the atom: the nucleus and the electrons

1. Nucleus: The nucleus is the center of the atom. It contains protons and neutrons, so it is positive overall and very dense. Nearly all the atom's mass is here.
2. Proton: A proton has a charge of +1 and a relative mass of 1. The number of protons decides which element you have.
3. Neutron: A neutron has no charge and a relative mass of 1. It helps add mass and can change from isotope to isotope.
4. Electron: An electron has a charge of -1 and a tiny mass. Electrons take up most of the atom's volume, but not its mass.

This Bohr model shows a sodium atom.

The center shows the nucleus. The text inside it gives the sodium atom's 11 protons and 12 neutrons. The three circles are the first, second, and third electron shells. Sodium has 2 electrons in the first shell, 8 in the second, and 1 in the third.

1. Why this matters: The nucleus decides the element. The electrons decide how the atom behaves in reactions. The shells are a simple way to track where the electrons are.

Counting particles: atomic number and mass number

1. Atomic number: The atomic number is the number of protons. It is written on the periodic table and tells you which element the atom is.
2. Mass number: Mass number = protons + neutrons. It counts the particles in the nucleus that matter for mass.
3. Finding neutrons: If you know mass number and atomic number, subtract: neutrons = mass number - atomic number.
4. Quick example: Sodium has atomic number 11 and mass number 23, so it has 11 protons and 12 neutrons. A chlorine-35 atom has 17 protons and 18 neutrons.

Atomic number identifies the element, and mass number tells you the total in the nucleus.

Isotopes and ions

1. Isotope: Isotopes are atoms of the same element with different numbers of neutrons. They have the same number of protons, so they are the same element.
2. Why isotopes matter: Different neutrons change the mass, and some isotopes are unstable. That is why chlorine has chlorine-35 and chlorine-37, and carbon has carbon-12 and carbon-14.
3. Ion: An ion is an atom with a net charge because it gained or lost electrons. Losing electrons makes a positive ion; gaining electrons makes a negative ion.
4. Charge rule: charge = protons - electrons. If protons and electrons are equal, the atom is neutral. If sodium loses one electron, it becomes Na⁺.

Isotopes change the nucleus, but ions change only the electrons.

Electron shells for the first 20 elements

1. First shell: The first shell is closest to the nucleus and fills first. It holds 2 electrons.
2. Second shell: After the first shell is full, electrons go to the second shell. For the first 20 elements, it holds up to 8.
3. Third shell: For the first 20 elements, the third shell also fills to 8 before the fourth shell starts. This is the simplified Bohr model you use in high-school chemistry.
4. Read the pattern: Hydrogen has 1, helium 2, lithium 2,1, and by neon the second shell is full at 2,8. From sodium through argon, the third shell fills to 8, and then potassium and calcium start the fourth shell: 2,8,8,1 and 2,8,8,2.

So for the first 20 elements the shells fill 2, then 8, then 8 — and calcium, the 20th, finishes at 2, 8, 8, 2.

Putting it together

The key rules are: $Z = \text{number of protons}$ $A = \text{number of protons} + \text{number of neutrons}$ $\text{charge} = \text{protons} - \text{electrons}$

Read it left to right: atomic number gives protons; mass number gives protons plus neutrons; charge tells you whether the atom has lost or gained electrons.

Worked example: A sodium atom has atomic number 11 and mass number 23.

1. Protons = 11.
2. Neutrons = 23 - 11 = 12.
3. A neutral sodium atom has electrons = 11.
4. If sodium becomes Na⁺, it has lost one electron, so electrons = 10 and charge = 11 - 10 = +1.

The picture and the rules match: the sodium nucleus has 11 protons and 12 neutrons, and its electrons sit 2, 8, 1 in shells.

Check yourself

1. Why does atomic number identify the element? Answer Because it equals the number of protons, and proton count defines the element.
2. An atom has 16 protons and 18 neutrons. What is its mass number? Answer 34.
3. A neutral atom has 12 protons. How many electrons does it have, and what happens if it loses 2 electrons? Answer It has 12 electrons; after losing 2, it becomes a 2+ ion with 10 electrons.

Common mistakes

• "Atomic number means protons + neutrons." No. Atomic number is protons only.
• "If an atom gains a neutron, it becomes a different element." No. It becomes a different isotope of the same element.
• "Ions form because the nucleus changes." No. Ions form when electrons are gained or lost.

Remember this

• Atomic number = protons = the element's identity.
• Mass number = protons + neutrons.
• Neutral atoms have the same number of protons and electrons; ions do not.