Atomic Structure
Key Facts
- All matter is made of atoms — the smallest part of an element that can exist.
- Atoms have a positively charged nucleus surrounded by negatively charged electrons.
- The radius of an atom is about 1 x 10⁻¹⁰ m. The nucleus is about 10 000 times smaller.
- Most of the atom is empty space.
Sub-atomic Particles
Every atom contains three types of sub-atomic particle. Protons are found in the nucleus; they have a relative charge of +1 and a relative mass of 1. Neutrons are also in the nucleus; they have no charge (0) and a relative mass of 1. Electrons orbit the nucleus in shells; they have a relative charge of -1 and a negligible mass (approximately 1/1836 of a proton). In a neutral atom the number of protons equals the number of electrons, so the charges balance out to zero.
Atomic Number and Mass Number
The atomic number (also called proton number) is the number of protons in the nucleus. It defines which element an atom belongs to — every atom of carbon has 6 protons, every atom of oxygen has 8. The mass number is the total number of protons and neutrons. To find the number of neutrons, subtract the atomic number from the mass number. For example, sodium has an atomic number of 11 and a mass number of 23, so it has 23 - 11 = 12 neutrons.
Worked Example — Finding Neutrons
Question: Chlorine has an atomic number of 17 and a mass number of 35. How many protons, neutrons and electrons does a neutral chlorine atom have?
Protons: 17 (same as atomic number).
Electrons: 17 (equal to protons in a neutral atom).
Neutrons: 35 - 17 = 18 neutrons.
Electron Shell Configuration
Electrons occupy energy levels (shells) around the nucleus. The first shell (closest to the nucleus) holds a maximum of 2 electrons. The second shell holds up to 8. The third shell also holds up to 8 at GCSE level. Electrons always fill the lowest available shell first. For example, sodium (11 electrons) has a configuration of 2, 8, 1 — two electrons in the first shell, eight in the second and one in the third. The number of electrons in the outer shell determines how an element reacts, which is why elements in the same group of the periodic table have similar chemical properties.
Worked Example — Electron Configuration
Question: What is the electron configuration of aluminium (atomic number 13)?
Step 1: 13 electrons to place. First shell: 2.
Step 2: Second shell: 8. That accounts for 10.
Step 3: Third shell: 3. Configuration = 2, 8, 3.
Isotopes
Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Because they have the same number of protons and electrons, isotopes have identical chemical properties. However their physical properties (such as mass and density) differ. Carbon-12 and carbon-14 are isotopes of carbon — both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons. Carbon-14 is radioactive and is used in carbon dating.
Connection to the Periodic Table
Elements in the periodic table are arranged in order of increasing atomic number. Elements in the same group have the same number of electrons in their outer shell, which is why they react in similar ways. Elements in the same period have the same number of electron shells. Understanding atomic structure is the key to understanding why the periodic table is arranged the way it is and why elements in the same group share similar properties.
Practice Questions
- Define the terms atomic number and mass number.
- Potassium has an atomic number of 19 and a mass number of 39. How many protons, neutrons and electrons does it have?
- Write the electron configuration of oxygen (atomic number 8).
- Explain what an isotope is and give one example.
- Why do elements in the same group of the periodic table have similar chemical properties?