Question

As you move from left to right across a period you add more subatomic particles to the atoms, and yet the atoms become smaller. (a) What subatomic particles are you adding? (b) If you are adding particles, why don't the atoms become larger?

Short answer

(a) As we move from left to right across a period, the number of protons and electrons in the atoms increases. (b) The size of the atoms decreases due to an increase in the effective nuclear charge experienced by the electrons, causing them to be pulled closer to the nucleus despite the addition of more subatomic particles. The increase in protons' attractive force is more significant than the repulsion among added electrons, resulting in smaller atomic sizes.

Step-by-step solution

(a) Identifying the added subatomic particles

As we move from left to right across a period in the periodic table, the number of protons and electrons in the atoms increases. This is because the atomic number, which denotes the number of protons in the nucleus of an atom, increases by one unit for each element in a period. Since the atoms are neutral, the number of electrons also increases by one unit.

(b) Explaining the decrease in atomic size

Despite the addition of more subatomic particles while moving across a period, the size of the atoms decreases due to an increase in the effective nuclear charge experienced by the electrons in the atom. Since protons are positively charged, they generate an attractive force that pulls the negatively charged electrons towards the nucleus. As the number of protons in the nucleus increases, this attractive force (called the effective nuclear charge) becomes stronger, causing the electrons to be pulled closer to the nucleus, thus reducing the overall size of the atom. It should be noted that the increase in the number of electrons also causes repulsion between them. However, the increase in the effective nuclear charge experienced by the electrons due to the addition of protons is more significant than the repulsion among the electrons. As a result, the atoms become smaller in size as we move from left to right across a period.

Key concepts

Periodic Table Trends

Understanding the periodic table is crucial to grasp fundamental concepts of chemistry. One key aspect is periodic trends - consistent patterns observed across different periods and groups within the table. A remarkable trend reflects how atomic size changes as you move across the table.

For example, as we travel from left to right across a period, despite adding more subatomic particles—specifically protons in the nucleus and electrons in the electron cloud—the atomic size tends to decrease. This might initially seem counterintuitive since adding more particles suggests an increase in size. However, the opposite occurs not because of the amount of matter added but because of how these added particles influence the overall structure of the atom. An intensified pull from the nucleus affects the electrons increasingly, which pulls them in closer and reduces the atomic size. Simplifying these intricate interactions allows learners to better conceptualize and predict the properties of different elements based on their positioning on the periodic table.

Effective Nuclear Charge

The concept of effective nuclear charge (ENC) is vital in explaining why atoms do not expand despite the addition of more subatomic particles. ENC is the net positive charge experienced by an electron in a multi-electron atom. The closer an electron is to the nucleus, the stronger the attractive force it feels from the positively charged protons present in the nucleus.

This effective charge increases as you move from left to right across a period because the number of protons increases with each successive element. The electrons in the same energy level do not fully shield the added positive charge of the protons, thereby increasing the pull on all the electrons towards the nucleus. This greater pull compensates for the increased number of electrons and draws them closer, leading to a smaller atomic radius. Grasping ENC is pivotal as it influences various properties of an element, including ionization energy, electronegativity, and chemical reactivity.

Subatomic Particles

Atoms are composed of subatomic particles—protons, neutrons, and electrons—each contributing to the atom's properties and behaviors. Protons carry a positive charge and are found in the nucleus, neutrons are neutral and also in the nucleus, and electrons are negatively charged particles orbiting the nucleus.

Understanding these particles is fundamental when exploring why atoms become smaller across a period despite particle addition. The added particles referred to in this context are protons to the nucleus and electrons to the electron cloud. Protons increase the nucleus's positive charge, enhancing the attractive force upon electrons, which ultimately reduces the atomic size. Neutrons do not play a direct role in this trend, as they do not have an electric charge but contribute to the overall mass of the atom. In contrast, the impactful interplay between an increasing number of protons and the corresponding increase in electrons accounts for the observed trend of decreasing atomic size from left to right across the periodic table.