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Lesson Explainer: The Atom Science

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

All atoms are divisible. They contain a combination of different particles called subatomic particles. Scientists spent a lot of time determining the properties of these subatomic particles and their positions in atoms.

Definition: Subatomic Particle

Subatomic particles are pieces of matter that can form the atom of an element.

Scientists showed that there are three types of subatomic particles in the atoms of elements. The first two are the positively charged proton and the neutrally charged neutron. These particles stay in the central section, or the nucleus, of an atom. They are relatively large and heavy. So, the atomic nucleus has almost all the mass of an atom.

Figure 1: The nucleus of an atom with its positively charged protons and neutrally charged neutrons.

Definition: Atomic Nucleus

The atomic nucleus is in the center of the atom and has most of the atomic mass and an overall positive charge.

Example 1: Determining Which Two Types of Subatomic Particles Are in the Nucleus of an Atom

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

Answer

Scientists used advanced scientific equipment about one hundred years ago to show that atoms ordinarily contain three types of subatomic particles. The first two are the proton and neutron, and these form the central section, or the nucleus, of an atom. The other subatomic particle tends to stay in the space outside the atomic nucleus. Hence, the answer to this question must be protons and neutrons.

The atomic number is the number of protons in the nucleus of an atom. Chemists usually show the atomic number as a single number on the bottom-left side of a chemical symbol. The mass number is different, as it equals the total number of protons and neutrons in the nucleus of an atom. Chemists usually show the mass number on the top-left side of a chemical symbol.

Definition: Atomic Number

The atomic number is the number of protons in the nucleus and it is ordinarily on the bottom-left side of a chemical symbol.

Definition: Mass Number

The mass number is the number of protons and neutrons in the nucleus and it is ordinarily on the top-left side a chemical symbol.

For example, the notation 42He is for a helium atom with an atomic number of two and a mass number of four. Similarly, the notation 168O is for an oxygen atom with an atomic number of eight and a mass number of sixteen.

Figure 2: The notation for atoms of helium and oxygen.

Example 2: Determining the Mass Number for the Chemical Element Phosphorus

An atom of phosphorus has the chemical symbol 3115P. What is the mass number of this atom?

Answer

The notation for an element has a chemical symbol with atomic and mass numbers. The numbers are ordinarily on the left side of the chemical symbol, and the mass number tends to be above the atomic number. We can use this information to determine that phosphorus must have a mass number of 31. So, the correct answer is 31.

We can express the mass number with the following relatively simple mathematical equation. The mass number is simply the sum of two quantities. The first is the number of protons, and the second is the number of neutrons.

Equation: Mass Number

Massnumbernumberofprotonsnumberofneutrons=+

We can usually use this equation to determine the number of neutrons for an atom of any element. Let us use this equation to show that an atom has eight neutrons if it has the notation 168O. We will first write the mass number equation with an additional bracketed atomic number term for clarity: massnumbernumberofprotonsatomicnumbernumberofneutrons=()+.

We can then replace the term mass number with sixteen and the term number of protons with eight. We know that these are the mass and atomic numbers from the notation 168O: 16=8+.numberofneutrons

We can then rearrange the equation to show that there are eight neutrons in the nucleus of this oxygen atom: numberofneutronsneutrons=168=8.

Let us use this equation to determine the number of neutrons for the atoms of other elements.

Table 1: The number of neutrons for the atoms of different chemical elements.

Element SymbolAtomic NumberMass NumberNumber of ProtonsNamber of Neutrons
11H1110
4020Ca20402020
2412Mg12241212
126C61266
3517Cl17351718
2311Na11231112

The table shows that some atoms have an equal number of protons and neutrons, while others do not. Some chemical elements like magnesium and carbon tend to have an equal number of protons and neutrons. Other chemical elements like chlorine and sodium ordinarily have more neutrons than protons.

Example 3: Identifying the Equation for Calculating the Mass Number of an Atom

Which of the following equations can be used to calculate the mass number of an atom?

  1. Massnumbernumberofprotonsnumberofneutronsnumberofelectrons=++
  2. Massnumbernumberofneutronsnumberofprotons=
  3. Massnumbernumberofprotonsnumberofneutrons=+
  4. Massnumbernumberofprotonsnumberofelectrons=+
  5. Massnumbernumberofprotonsnumberofneutrons=×

Answer

The mass number is the number of protons and neutrons in the nucleus of an atom. So, we can find the mass number if we add the number of protons and the number of neutrons of an element. Choice C must be the answer, as it states that the mass number equals the sum of the number of protons and the number of neutrons of a chemical element. Choice C is the answer to this question.

The third type of subatomic particle is the electron. These negatively charged particles have negligible mass, and they tend to stay in areas around the nucleus called energy levels. The atoms of elements have no overall electric charge because they have one negatively charged electron for each positively charged proton. For example, an oxygen atom has eight negatively charged electrons for its eight positively charged protons.

Figure 3: The subatomic structure of an oxygen atom.

Example 4: Identifying the Most Accurate Description of an Electron

Which of the following best describes an electron?

  1. A positively charged particle with a much smaller mass than that of the nucleus
  2. A negatively charged particle with a much smaller mass than that of the nucleus
  3. A positively charged particle with a much greater mass than that of the nucleus
  4. A negatively charged particle with a much greater mass than that of the nucleus
  5. A neutral particle with a mass equal to that of the nucleus

Answer

Scientists showed that atoms contain relatively heavy protons and neutrons and much lighter electrons. The protons and neutrons stay in the atomic nucleus, and the electrons tend to remain in energy levels. The low-mass electrons have a negative charge. We can use this information to determine that choice B is the answer.

Example 5: Determining the Name of an Unidentified Subatomic Particle

The image shows a simplified representation of an atom. What structure is indicated by X?

Answer

The atoms of elements have no electric charge because they have an equal number of positively charged protons and negatively charged electrons. The protons stay in the central section, or the nucleus, of the atom. The electrons tend to remain in areas around the atomic nucleus called energy levels. So, the answer to this question must be that the structure is an electron.

An angstrom is a unit that describes length. The angstrom is incredibly small; there are tenthousandmillion angstroms in one metre. Most scientists use angstroms to describe the sizes of atoms since it can be awkward to talk in terms of metres for something so small. For example, scientists generally choose to say that a hydrogen atom’s radius equals 0.3 angstroms rather than 0.00000000003 metres.

Definition: Angstrom

An angstrom is a unit of length that equals one ten thousand millionth of a metre.

Let us summarize what we have learned in this explainer.

Key Points

  • The atom contains subatomic particles.
  • The atomic nucleus contains protons and neutrons.
  • Protons have a positive charge, and neutrons are neutral.
  • The atomic number is the number of protons in the atomic nucleus.
  • The mass number is the number of protons and neutrons in the atomic nucleus.
  • Electrons are subatomic particles with a negative charge.
  • Electrons stay in energy levels about the atomic nucleus.
  • Atoms have no overall electric charge because they have one negatively charged electron for each positively charged proton.

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