25. From left to right across a period in the periodic table, elements become less electropositive and more electronegative in their properties.
26. Francium has 36 isotopes, but only francium-223 occurs in nature. Francium-223 spontaneously emits particles and energy, so francium-223 is a(an) radioisotope of francium.
27. At sea level, water boils at 100°C.
28. Cooking requires continuous addition of energy to the chemical reactions that are taking place. The chemical reactions involved in cooking can be described as endothermic.
29. The Kinetic theory of matter states that all particles of matter are in constant motion.
30. In Rutherford’s experiments, some of the alpha particles aimed at gold atoms bounced back, suggesting that a solid mass was at the center of the atom.
31. You are given the melting points of three unknown substances and are asked to predict which one is an ionic compound. You would select the compound with the highest melting point.
32. A flight simulator that helps astronauts prepare for a shuttle launch is an example of a(an) _scientific model.
33. In the symbol
4/2 He, the superscript 4 is the atomic mass for helium, and the subscript 2 is the atomic number for helium.
25) Answer is: less metallic and more nonmetallic.
Electronegativity increases from left to right across a period, atoms have tendency to attract electrons.
Electronegativity (χ) is a property that describes the tendency of an atom to attract a shared pair of electrons.
26) Answer is: radioisotope.
Radioisotope has an unstable atomic nucleus and emit energy and particles when it changes to a more stable form.
Francium (Fr) is element with atomic number 87 (87 protons and 87 electrons).
27) Answer is: boils.
Boiling is the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere.
The Celsius scale was based on 0°C for the freezing point of water and 100°C for the boiling point of water at 1 atm pressure.
28) Answer is: endothermic.
There are two types of reaction:
1) endothermic reaction (chemical reaction that absorbs more energy than it releases, ΔH>0).
2) exothermic reaction (chemical reaction that releases more energy than it absorbs).
For example, the breakdown of ozone is an endothermic process. Ozone has lower energy than molecular oxygen (O₂) and oxygen atom, so ozone need energy to break bond between oxygen atoms.
29) Answer is: kinetic.
The average kinetic energy of molecules depends on the temperature.
As temperature increases, molecules gain more energy from surrounding and move faster.
Kinetic energy (standard unit is the joule J) depends on speed of the molecule.
Solid gains kinetic energy to become a liquid and then becomes a gas.
30) Answer is: alpha particles.
This was famous Rutherford's Gold Foil Experiment: he bombarded thin foil of gold with positive alpha particles.
Rutherford observed the deflection of alpha particles on the photographic film.
Alpha particles are helium atom particles, consist of two protons and two neutrons.
According to Rutherford model of the atom: atoms have their charge concentrated in a very small nucleus.
31) Answer is: highest.
For example, calcium oxide.
Calcium oxide (CaO) has ionic bonds between calcium cations (Ca²⁺) and oxygen anions (O²⁻).
Because it has strong ionic bonds, it has high melting point (around 2600°C).
Ionic bond is the electrostatic attraction between oppositely charged ions (cations and anions).
32) Answer is: a scientific model.
A scientific model is a representation of an idea, an object, a process or a system that is used to describe and explain phenomena that cannot be experienced directly.
33) Answer is: the superscript is mass number and the subscript is atomic number.
Atomic number(Z) is the number of protons, which is characteristic of a chemical element..
Mass number (A) is the total number of protons and neutrons in a nucleus.
