Structure of an Atom

As a result of the work done by previous scientists on atomic models, scientists now have a good idea of what an atom looks like. This knowledge is important because it helps us to understand why materials have different properties and why some materials bond with others. Let us now take a closer look at the microscopic structure of the atom (what the atom looks like inside). So far, we have discussed that atoms are made up of a positively charged nucleus surrounded by negatively charged electrons. These electrons orbit the nucleus.

Before we look at some useful concepts we first need to understand what electrons, protons, and neutrons are.

The Electron

Electrons are extremely small. The mass of an electron is only about 1/2000 the mass of a proton or neutron, so electrons contribute virtually nothing to the total mass of an atom. Electrons have an electric charge of -1, which is equal but opposite to the charge of proton, which is +1. All atoms have the same number of electrons as protons, so the positive and negative charges "cancel out," making atoms electrically neutral.

Unlike protons and neutrons, which are located inside the nucleus at the center of the atom, electrons are found outside the nucleus. Because opposite electric charges attract each other, negative electrons are attracted to the positive nucleus. This force of attraction keeps electrons constantly moving through the otherwise empty space around the nucleus. In reality, it's impossible to say what path an electron will follow. Instead, it's only possible to describe the chances of finding an electron in a certain region around the nucleus. The region where an electron is most likely to be is called an orbital. You will learn more about specific orbitals in a chemistry course.

For this physical science course, it is important to know that electrons are located at fixed distances from the nucleus, called energy levels. You can see the first three energy levels in the picture below. The picture shows the maximum possible number of electrons at each energy level.

Energy Levels and Electrons
Nucleus
contains protons and neutrons
1st energy level can hold a maximum of 2 electrons
2nd energy level can hold a maximum of 8 electrons
3rd energy level can hold a maximum of 18 electrons

Electrons at lower energy levels, which are closer to the nucleus, have less energy. You can remember this by thinking about the properties of positive and negative charges. Negative charges (electrons) are attracted to positive charges (protons). It does not take much energy to have them be close together because this is what they want to do...attract.

Only when a lower energy level is full are electrons added to the next higher energy level. Electrons at higher energy levels, which are farther from the nucleus, have more energy. You can remember this by thinking about the properties of positive and negative charges just as in the above point. Since negative and positive charges attract, it takes more energy to keep them farther apart.

Important Electrons

Electrons at the outermost energy level of an atom are called valence electrons. They determine many of the properties of an element. That's because these electrons are involved in chemical reactions with other atoms. Atoms may share or transfer valence electrons. Shared electrons bind atoms together to form chemical compounds.

The Neutron

Atoms of all elements—except for most atoms of hydrogen—have neutrons in their nucleus. The mass of a neutron is about the same as the mass of a proton and is 10,000 times heavier than the mass of an electron. The mass of a neutron is about 1 atomic mass unit (1 amu) or 1.6743 × 10⁻²⁷ kg. The neutron is located inside the nucleus of an atom. The nucleus is the small, dense region at the center of an atom where protons are also found. Protons and neutrons in the nucleus make up the majority of the mass of an atom. Unlike electrons and protons, neutrons are electrically neutral meaning they have no charge.

All the atoms of a given element have the same number of protons and electrons. The number of neutrons, however, may vary for atoms of the same element. Atoms generally have about the same number of neutrons as protons, but this is not always the case. For example, almost 99% of carbon atoms have six neutrons, but the rest have either seven or eight neutrons. Atoms of an element that differ in their numbers of neutrons are called isotopes.

The Proton

Each proton carries one unit of positive electric charge. As we stated above when we looked at electrons since atoms are electrically neutral then the number of protons in an atom has to be the same as the number of electrons to balance out the positive and negative charge to zero.

Just as we discussed with a neutron, the proton is also much heavier than the electron (10 000 times heavier!). A proton is also located inside the nucleus of the atom. The mass of a proton is about 1 atomic mass unit (1 amu) or 1,6726 × 10⁻²⁷ kg.

All protons are identical. For example, hydrogen protons are exactly the same as protons of helium and all other elements, or pure substances. However, atoms of different elements have different numbers of protons. In fact, atoms of any given element have a unique number of protons that is different from the numbers of protons of all other elements. For example, a hydrogen atom has just one proton, whereas a helium atom has two protons.

Summary of Subatomic Particles

To help summarize the information above, here is a chart comparing each of the subatomic particles' properties:

image of atom

Subatomic Particle	Location in atom	Electrical Charge	Mass (amu)
Proton (p⁺)	Inside nucleus	Positive	1
Neutron (n⁰)	Inside nucleus	Neutral (no charge)	1
Electron (e^-)	Outside nucleus orbiting on energy levels	Negative	1/2000

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