Lesson Video: The Atom Science

Video Transcript

In this video, we will learn how to describe the composition of the atom, determine its atomic number, and calculate its mass number.

Our world is made up of matter. The air we breathe, the water we drink, and the clothes we wear are all matter. If we could zoom in on the matter around us, we would find that all of this matter is made up of atoms. Atoms are the smallest unit of ordinary matter that forms a chemical element. We sometimes refer to atoms as the building blocks of matter.

Atoms are incredibly small, many, many times smaller than a meter. To describe the size of an atom, scientists typically use the unit angstroms. There are 10,000 million angstroms in one meter. The radius of an atom is only about 0.3 angstroms. To put this in perspective, mechanical pencil lead ordinarily has a diameter of 0.5 millimeters. If we could line up atoms right beside each other across the tip of the pencil lead, over eight million atoms would fit in a row.

Even though atoms are so small, there are even smaller pieces of matter that make up atoms. The smaller pieces of matter that can form an atom are called subatomic particles. There are three types of subatomic particles. The first two subatomic particles are protons and neutrons. Protons are positively charged, while neutrons are neutrally charged. Protons and neutrons are found in the central region of an atom. We call this central region the nucleus. As the nucleus contains positively charged protons and neutrally charged neutrons, the overall charge of the nucleus is positive.

The third type of subatomic particle is the electron. Electrons are negatively charged. In an atom, electrons are usually found in areas outside of the nucleus called energy levels. The negatively charged electrons remain in the atom because they are attracted to the positively charged nucleus.

Despite containing both positively charged particles and negatively charged particles, an atom is neutral overall. In order for this to be true, atoms must contain the same number of protons and electrons. So if this atom contained two positively charged protons, it would also have to contain two negatively charged electrons in order to have an overall neutral charge.

When we compare the three types of subatomic particles, we find that protons and neutrons are relatively large and heavy, while electrons are relatively small and have negligible mass. This means that most of an atom’s mass is found in the nucleus.

While the general structure of all atoms is the same, not all atoms have the same number of protons, neutrons, and electrons. Chemists can describe the composition of atoms using a notation that looks like this. The letters are the chemical symbol of an element. This indicates the type of atom.

The bottom-left number is called the atomic number. The atomic number is the number of protons in the atom. This value is unique for each element. For example, all atoms of helium have two protons and therefore an atomic number of two. But all atoms of lithium contain three protons and have an atomic number of three.

The number to the top left is called the mass number. The mass number is the sum of the number of protons and neutrons in the atom. In other words, the mass number is equal to the number of protons plus the number of neutrons in the nucleus of an atom.

Using the given notation, we can determine the number of protons and neutrons in this atom of helium. We know from the atomic number that this atom of helium contains two positively charged protons. We can use the mass number equation to determine the number of neutrons by substituting the mass number and number of protons into the equation. The mass number is four, and the number of protons is two. If we subtract two from both sides of the equation, we find that the atom of helium contains two neutrally charged neutrons.

We can also determine the number of electrons by remembering that atoms contain the same number of protons and electrons. As this atom contains two protons, it must also contain two electrons.

Now let’s take a look at three atoms of hydrogen. Since all three atoms are of the same type of element, they all contain the same number of protons. This means that all three atoms have an atomic number of one. As each atom contains one proton, it must also contain one electron in order to be neutrally charged overall.

But despite being the same type of element and containing the same number of protons and electrons, each of these atoms contains a different number of neutrons. This means that each of these atoms has a different mass number. So, atoms of the same type of element will have the same number of protons and the same number of electrons. But the number of neutrons could be the same as the number of protons, less than the number of protons, or greater than the number of protons.

Before we summarize what we’ve learned in this video, let’s take a look at a few questions.

Which two types of particles can be found in the nucleus of an atom?

An atom is the smallest unit of ordinary matter that forms a chemical element. There are two main regions in an atom: the nucleus in the center of the atom and energy levels outside of the nucleus. These regions of the atom are where we can find subatomic particles. Subatomic particles are smaller pieces of matter that can form an atom. There are three types of subatomic particles. The three types are positively charged protons, neutrally charged neutrons, and negatively charged electrons. The protons and neutrons are found in the nucleus in the center of the atom, while electrons are ordinarily found outside of the nucleus in energy levels.

We now know that the two types of particles that can be found in the nucleus of an atom are the protons and neutrons.

Which of the following best describes an electron? (A) A positively charged particle with a much smaller mass than that of the nucleus. (B) A negatively charged particle with a much smaller mass than that of the nucleus. (C) A positively charged particle with a much greater mass than that of the nucleus. (D) A negatively charged particle with a much greater mass than that of the nucleus. (E) A neutral particle with a mass equal to that of the nucleus.

An electron is a subatomic particle. Subatomic particles are pieces of matter that can form an atom. There are three types of subatomic particles: protons, neutrons, and electrons. Protons are positively charged, neutrons are neutrally charged, and electrons are negatively charged. This question asked, which of the statements best describes an electron? Knowing that an electron is negatively charged, we can eliminate answer choices (A), (C), and (E).

In comparison to one another, protons and neutrons are relatively large and heavy, while electrons are relatively small and have a negligible mass. Protons and neutrons are found in the central region of the atom called the nucleus, while electrons are ordinarily found outside of the nucleus in energy levels. As the nucleus contains heavy protons and neutrons, it contains most of an atom’s mass. When compared to the nucleus, an electron has a much smaller mass.

In conclusion, the statement that best describes an electron is answer choice (B), a negatively charged particle with a much smaller mass than that of the nucleus.

Which of the following equations can be used to calculate the mass number of an atom? (A) Mass number equals number of protons plus number of neutrons plus number of electrons. (B) Mass number equals number of neutrons divided by number of protons. (C) Mass number equals number of protons plus number of neutrons. (D) Mass number equals number of protons plus number of electrons. (E) Mass number equals number of protons times number of neutrons.

An atom is a very small piece of matter that consists of even smaller pieces of matter called subatomic particles. The subatomic particles found in the center of the atom, known as the nucleus, are positively charged protons and neutrally charged neutrons. The negatively charged subatomic particles, ordinarily found outside the nucleus in energy levels, are called electrons.

Not all atoms have the same number of protons, neutrons, and electrons. We can describe the makeup of an atom using atomic number and mass number.

The atomic number is equal to the number of protons in the nucleus of an atom. Since atoms are neutrally charged overall, the number of positively charged protons must equal the number of negatively charged electrons. So for atoms, the atomic number also equals the number of electrons in the atom. The atom shown in the diagram has one proton and one electron and therefore an atomic number of one.

The mass number of an atom is equal to the number of protons plus the number of neutrons in the nucleus of an atom. The atom shown in the diagram has one proton and one neutron. Therefore, it has a mass number of two.

The question asked, which of the equations can be used to calculate the mass number? The mass number is the sum of the number of protons and neutrons. So, the equation that can be used to calculate the mass number of an atom is the equation shown in answer choice (C): mass number equals number of protons plus number of neutrons.

An atom of phosphorus has the chemical symbol 31 15 P. What is the mass number of this atom?

Let’s take a closer look at the provided chemical symbol.

The capital P is the chemical symbol of the element phosphorus. In this notation, the number written to the bottom left of the chemical symbol is the atomic number. The atomic number indicates the number of protons in the nucleus of the atom. So this atom of phosphorus has 15 protons.

The number written to the top left of the chemical symbol is the mass number. The mass number is the number of protons and neutrons in the nucleus. We can express this as the equation mass number equals number of protons plus number of neutrons. This atom has a mass number of 31 and 15 protons. By subtracting 15 from both sides of the equation, we find that this atom of phosphorus has 16 neutrons.

In conclusion, the mass number of the phosphorus atom given is 31.

Now let’s wrap up this video by reviewing what we’ve learned. Atoms are the smallest unit of ordinary matter that form a chemical element. Atoms are usually measured in angstroms, a unit 10 billion times smaller than a meter. Atoms are composed of positively charged protons, neutrally charged neutrons, and negatively charged electrons. The nucleus in the center of the atom contains the protons and neutrons, while electrons ordinarily stay outside of the nucleus in energy levels.

Atoms do not have an overall electric charge. This is because they contain the same number of positively charged protons and negatively charged electrons. The atomic number represents the number of protons in an atom. The mass number is equal to the sum of the number of protons and neutrons in an atom. The mass number and atomic number can be included as part of an atom’s chemical symbol.